Copyright © 1999, 2004 by American Forest & Paper Association, Inc., Washington, D.C.
NDS Commentary
VIII: BOLTS
8.1-GENERAL
8.1.1-Quality of Bolts
-
-
1/4
for
bolted
inch
1- 1/2
or 1connections
sole the is
responsibility of the designer.
8.1.2-Fabrication and Assembly
In
the
previous
three editions, the bolt quality
standard referenced
in
the
Specification
was
ASTM
8.1.2.1 The range of
allowable bolt holes
of
1/32Standard A307,Low
Carbon SteelExternally
and
inch to 1/16-inch larger than the bolt diameter has been
Internally Threaded Fasteners. The current standard of
a provision of theSpecificationsince
1948.Designathis designation, A307-88a,
is
now
titled,
Standard
tionoftheselimits
as minimum and maximum
overSpecification for Carbon SteelBolts and Studs, 60,000sizeswas
added in the 1986 edition.
psiTensile Strength. The scope of thepresentASTM
Generally, the smaller diameter bolts will use the
A307is too narrow for it to beused as a reference
standard as bolt provisions in the 1991 edition of thesmalleroversizeholes
and the larger bolts thelarger
Specification
provide for the use of bolts of any
oversize. The sametargetoversizeis
to beused for all
in the Same connection.
bending yield strength (see
Appendix
I). ANWASME
Standard B18.2.1-1981, Square
and
Hex
Bolts
and
8.1.2.2 Centering
of
holes
and avoidance of
Screws (Inch Series),whichisbeingused
as thebolt
forcibledrivinghavebeen
good practiceprovisions of
qualityreferenceinthe1991
edition, isthereferenced
the Specificationsince the 1944 edition.
standard for dimensions inA307.
8.1.2.3 Use ofwashers or equivalentmetal parts
Bolt design values given in previous
editions of the
under thehead and nut to preventlocalizedcrushing
Specification werebasedon
a bendingyield strength,
of the wood at boltholes has been a requirement of
FUb, of45,000psi.
This valueisapplicable
to A36
theSpecificationsince the 1960 edition.
steelhaving an ultimate tensile strength of 58,000 psi.
8.1.2.4 Designvalues for bolted joints have been
Bolt designvalues
tabulated in Tables 8.2A-Dand
applied to connections having both tight and loose nuts
8.3A-D of the 1991 edition are based on bolts made of
since the 1944 edition. This provisionisbasedonthe
steelhavingthese
properties.
original bolted joint tests used to establish design values
Maximum bolt diameter. Boltdesignvalues
for
inwhich the nuts were intentionally not tightenedin
bolts up to 1-1/4inches in diameter were tabulated in
order to simulate the additional shrinkage that can
the
1968
and earlier editions of
the
Specification.
It is to be noted that
occur during service
(183).
Values for 1-1/2inch bolts were added tothedesign
8.1.2.4addressesonly the loosening of nuts that may
loadtablesbeginningwiththe
1971 edition. In the
occur from shrinkage and not theeffectsofmoisture
1991 edition, boltdesignprovisions and tabulated bolt
on bearing strength or the effects of checks and cracks
design loads apply only to bolts having diameters of 1
that may
occur
from seasoning after fabrication.
inch or less. This conservative
change
was
made
Reduction of boltdesignvalues
for these factors is
followingseveral reported fieldproblemswithconnecrequired when connections are assembledwithwet
or
tions involving large diameter bolts ingluedlaminated
partiallyseasonedwood(see7.3.3).
timber
members
and the
results
of
new
research
8.1.2.5 The requirement that the threaded portion
(40,173). The latter showed drying inservice,workmanship variables and perpendicular to grain load
of bolts bearing on woodmembers
be keptto
a
components could interact to affectthecapacityof
practical
minimum
was
added as a good
practice
recommendation in the 1986 edition. Allowance
of
connections made with multiple large diameter, relative1y stiff bolts.
some thread bearing on wood without modification of
boltdesignvaluesis
long standing practice supported
It isrecognized
that theexperiencerecordshows
by field experience. N~~~ should be made that when
most
large
diameter bolted
connections,
particularly
threads occur in the
shear
plane, the effect of such
those
only one Or two bolts, have performed
threads on the design shear strength of the
bolt
itself
it is to be
is to be taken into account (see 7.2.3).
satisfactorilyovermanyyears.However,
in the 1991
understood that use
of
theprocedures
edition of the Specification to establish allowable loads
NDS Commentary
8.2-DESIGN VALUES FOR SINGLE SHEAR
CONNECTIONS
Boltdesignvaluesperpendicular
to grain:
2 = c2F2Ldr2
(C8.2-2)
8.2.1-Wood-to-WoodConnections
where:
Background
Boltdesignvalues
tabulated in the Specification
prior to the 1991 edition werebased on early research
(183) which related the bearing strength of wood under
bolts to the compression parallel and perpendicular to
grain properties of the woodmembers and the resistance of the bolt to bending as measured by the ratio
of the length of bolt in the main member to its
diameter.Except
for adjustments made to reflectthe
change in lumber designvaluesin1970resulting
from
the introduction of new clear wood strength properties
and species grouping criteria in ASTM D2555, and the
provisionsof the new softwood lumber product standard PS 20, the general methodology for establishing
bolt design values remained essentially unchanged
from
the 1944 through the 1986 editions. Under this
methodology, allowable bolt designvalueswere establishedin accordance withthe equations shownbelow.
Bolt design values parallel
to grain:
2 = cIFILdrl
(C8.2-1)
where:
z = nominal bolt
Fl =
L =
d
=
-
design value, pounds
adjustment for differencebetween proportional limit load of bolted joints and proportional limit under uniform compression load
0.80
unseasoned clear woodmaximumcompression design value parallel to grain 5 percent
exclusionvalue for each species or species
group (based on ASTM D2555), reduced
1.OA.9 for load duration and factor of safety
based on ASTMD245, and increased1.20
for seasoning based on early research (183),
psi
length of bolt in main member, inches
bolt diameter, inches
adjustment depending on L/d ratio of bolt
and FI (ranging from 1.00 at L/d of 2.00 to
0.310 at L/d of 13)
Although the proportional limit joint load factor
(cI) for hardwoods was found to be 1.00(183), and
the ASTM D245 load duratiodfactor of safety adjustment for hardwoods is2.1, the softwoodvalues for
thesetermswereused
for allspecies for simplicity.
98
BoJts
- width of bearingincreasebased
on bolt
diameter (183) (ranging from 2.50 for
1/4-inch bolt to 1.2,7 for 1-inch bolt)
Fz = clear wood unseasoned average proportional
limit stress for species or species group
based on ASTM D2555, reduced 1.0/1.5 for
ring
placement
based
on ASTM
D245,
increased
1.10
for normal loading, and
increased1.20 for seasoningbasedonearly
research (1 83)
- adjustment depending on L/d ratio ofbolt
r2 and F2 (ranging from 1.00 at L/d of 5.00 to
0.375 at L/d of13)
c2
other symbols as previouslydefined
The bolt design values established by the foregoing
equations were applicable to three member joints in
which the side members were one-half the thickness
of
the mainmember.Allowable
bolt designvalues for
twomember
or four or more member joints were
established as proportions of the three memberbolt
designvaluesin accordance withspecifiedrules.
1991 Edition. In the 1991 edition, lateral design
values for bolts are based on a yield limit model which
considersthedifferentwaystheboltedconnectioncan
deform under load (see Commentary for 7.2.1 and
Appendix I). The capacity of specific
a
joint is
determined for eachyield mode and thelowestdesign
valuecalculated for the differentmodesisselected
as
the nominal bolt designvalue, 2, for the joint. The
yield mode equations are entered
with
the
dowel
bearing strengths and thicknesses of the wood members
and the diameter and bendingyield strength ofthe
fastener.
Wood dowelbearing strengths usedintheyield
mode equations are based on a load representinga 5
percent diameter offset on the load-deformation curve
obtained from a bolt embedmenttest.
This loadis
intermediate betweenthe proportional limit and ultimate loads obtained from such a test (see Commentary
section7.2.1).
Although the yield limitmodelrepresentssignificantly different methodology than that used previously
to establishboltdesignvalues,therelativeeffects
of
various joint variablesshown by both procedures are
generallysimilar(166).
Short-term boltdesignvalues
NDS Commentary
obtained from application of the yield model equations
havebeenreduced
to the average bolt designvalue
levels published in previous editions of the Specifications for connections madewith the samespecies and
membersizes(seeCommentary
for 7.2.1).Asnoted
above, these previous tabulated bolt designvalues are
indexed to nominal proportional limit bolt design values
rather than 5 percentoffset bolt designvalues.
I
Previous Methodology for SingleShearBolted
Connections. Boltdesignvalues tabulated in the 1986
and earlier editions applied to three member joints in
which the side memberswereeach
one-half the thicknessof
the mainmember.
For twomember joints,
those with a single shear plane, aproportion ofthe
bolt design value for the three member connection was
used. Prior
to
the 1977 edition, the two-member
proportion was taken as one-half the tabulated three
member bolt designvalue fora piecetwice the thickness
of
the thinner piece. This early practice for
establishing single shear bolt designvalues, introduced
in 1935 (127), was considered appropriate application of
the results of the original boltresearch(58).
In 1977,
based on new research and reevaluation of original test
results (82,183,200), the basis for boltdesignvalues for
two
member connections was
changed to the smaller of
(i) one-half the tabulated three member bolt design
value for a piece the thicknessof the thickestmember,
or (ii) one-half the tabulated three member bolt design
value fora piecetwice
the thicknessof
the thinner
member. This change
provided
more
conservative
designvalues for alltwomemberwood-to-woodsingle
shear bolted connections inwhich the thicker member
wasless thantwice the thickness of the thinner member, with reductions of50percent
occurring in some
joints inwhich the twomembers wereof equal thickness. The new practice for establishing designvalues
for two
member
bolted connections was continued
through the 1986 edition.
Results from application of the yield model confirm
the general relationship betweendesignvalues for two
member and three member bolted connections established in the 1977 edition (202).
-
JointMembersLoadedinDifferent
Directions.
The change in single shear joint provisions introduced
in the 1977 edition resulted in inconsistent treatment of
joints in whichonememberwas
loaded parallelto
The
grain and the other perpendicular to grain.
inconsistency occurred when the parallel loaded member
was just slightly thicker than the perpendicular loaded
member. In this case the joint wasassigned a perpendicular bolt designvaluebasedon
a piecetwicethe
thicknessof the perpendicular member rather than a
piece the thickness of this member. The deficiency was
corrected in the 1986 edition by assigning single shear
to
joints inwhichonememberwasloadedparallel
grain and the other at an angle to grain a bolt design
one-half the tabulated
value equal to the lesserof(i)
bolt designvalue
fora
piece the thickness of
the
parallel to grain loaded member, or (ii) the bolt design
value obtained from the equation for allowable bearing
at an angle to grain (Appendix J of the Specification)
using
one
half
the tabulated parallel to grain and
perpendicular to grain bolt design values for a piece the
thickness of the angle-to-grain member as P and Q,
respectively. This procedure assured that allowable
loads on joints in which one member was loaded at an
angle to grain would converge with those for joints in
which the angledmemberwas
at 90" to the parallel
loadedmember.
In the 1991 edition, the condition wheremembers
of a single shear bolted connection are loaded at
different angles to the grain isprovided
for by the
factor, Ke , in the denominator oftheyieldmode
equations which accountsfor the maximumangle of
load to grain for anymember inthe connection; and
by Equation 8.2-7 of the Specification whichusesthe
bearing angle to grain equation (Appendix J) to adjust
the dowel bearing strength ofeachmember
loaded at
an angle to grain.
Yield Mode Equations
The bolt design value equations (8.2-1 to 8.2-6) for
single shear wood-to-wood connections weredeveloped
from European research (93,104) and have been confirmed by bolt tests on domestic species (116,163,166,167). The limiting yield modescoveredbythese
equations are bearing in the main or sidemembers(Mode
I), bolt rotation withoutbending(Mode
11), developmentof a plastic hingein the bolt inmain or side
member (Mode 111) and development of plastic hinges
in the bolt in both main and side members (Mode IV)
(seeAppendix I of the Specification). The term4K8 ,
3.6K8 or 3.2K8 in the denominator of equations 8.2-1
to 8.2-6 represents the average factor relating yield
modeldesignvalue for eachmodebasedon
5 percent
offsetdowel bearing strength to the proportional limit
based bolt designvalues tabulated in the 1986 edition
(202). For bolts loaded parallel to grain, Ke equals
one. For perpendicular to grain loading, Ke equals
1.25 for a connection with one member loaded parallel
to grain and the other member loaded perpendicular to
grain (202).
Dowel bearing strengths usedin
the yieldmode
equations are tabulated in Table 8A for all structurally
graded
lumber
species.
These
values
also apply to
Bolts
99
NDS Comentary
mainmembersofglued
laminated timber. The values
in Table 8A represent 5 percent diameter offset values
determined in accordance with the following equations
(203):
Parallel to grain:
different joint designs are shown in Example C8.2-1
to
illustrate theresults
of applying equations 8.2-1to
8.2-6.
-~
Example C8.2-1
F,
=
(11,200) G
(C8.2-3)
Perpendicular to grain:
F, = (6,100) G'*45D-OS5
(C8.2-4)
Yieldmodedesignvaluesforwood-to-woodsingle
shear bolted connections:
Hem-fw two memberconnectionsmadewith1/2and
1 inchbolts,sidememberthickness
of 1-1/2inches,
main member thicknesses of 1-112, 3 and 5-112 inches,
and loadsappliedparallelandperpendiculartothe
grain of main and side members
where:
Fern, Fes = 4800 psiparallel
= 2550 psi perpendicular, 112 inch bolt
= 1800psiperpendicular, 1 inchbolt
Fyb = 45,000 psi
F, = dowelbearing strength, psi
G = specificgravitybased
on oven dry weight
and volume
D = bolt diameter, inches
in. & Bolt
grain direction Diam.YieldModeDesignValue,
SideMain
in. ZI,
ZI,
zu ZUI,
ThiCknCSS,
Effect of specific gravity on dowel bearing strength
wasestablished from 3/4-inchdowelembedmenttests
on
Douglas-fir,
southern pine,
spruce-pine-fir.
Sitka
spruce,red oak, yellow poplar and aspen. Diameter
effectswereevaluated from testsof1/4-,1/2-,3/4-,1-,
and 1 -1/2 inch dowels in southern pine using bolt holes
1/16-inch larger than thedoweldiameter.
Diameter
was found to be a significant variable only in perpendicular to grain loading.Bearingspecimens
were1/2inch or thicker such that width and number of growth
ringsdid not influenceresults(203).
The specificgravityvaluesgivenin
Table 8A for
eachspecies or species group are those used to establishthe corresponding dowelbearingstrengthvalues,
F, . Thesespecificgravityvaluesrepresentaverage
values from in-grade lumber testprograms (see Commentary for 4.2.3.2) or are based on information from
ASTMD2555. No separate specificgravityvalues are
available for dense Douglas-fir and dense southern pine;
therefore, no dowelbearing strength values nor bolt
design
values
are tabulated for thesedensespecies
combinations in the 1991 edition.
The bending yield strength, Fyb , oftheboltused
in the yield mode equations is taken as the average of
the yield and ultimate tensile strengths of the metal (see
AppendixI). For A36 and stronger steels, Fyb equal
to 45,000psiis a conservative value and is equivalent
to thebolt strength reported intheoriginalbolttest
research(183).
For each particular joint configuration, the nominal
bolt design value for each yield mode must be calculated to determinethelimitingvalue
for theconnection.
Such yield mode bolt designvalues for anumberof
100
Bolts
1/2
112
1/2
Ibs
ZIII, ZIV
900 900 414
720 382 250
382 720 21p
550 550 663
380 324 442
324 380 442
900 845
674
IIp 663
1-112 //
1-112 //
1-112 I
1-112 //
1-112 I
1-112 //
3 /I
3 I/
3 1
1- 112 // 1800
112
1-1/2
442
592
I
472
382
1440
112
1-112 // 720
765
1/2
3 I/
3 /I
3 1
1- 112 //
1-112 I
1-112 //
1
1
1
3600
1800
1359 220018842652
2880 14p 874 137811431567
1080 1440 111 111711351567
5-112 //
5-112 //
5-112 I
1-112 //
1-1/2 I
1-112 //
1
1
1
6600 1800 2451 316118842652
5280 14p 1678 202111431567
1980
1440
816 1311 11351567
x 442
380
433
Member Loaded at Angle to Grain. Equation
8.2-7 is used to calculate the dowel bearing strength for
amain or sidememberloaded
at an angle to grain.
This equation, a form ofthebearingangletograin
equation (Appendix J), has beenusedsincethe1944
editiontodetermineallowabledesignvalues
for bolts
acting in a plane inclined
to the direction of grain.
In
earlier editions, the equation was entered with allowable
boltdesignvaluesparallel
and perpendicular to grain.
In the1991 edition, the equation is enteredwiththe
parallel and perpendiculardowelbearingstrengths
for
themember and theboltdesignvalue
isdetermined
from the yield mode equations using FCe as the dowel
bearing strength for themain or sidemember.The
bolt
design
value
obtained from this
procedure
is
similar to that obtained from usingparallel to grain
and perpendicular to grain 2 valuesinthebearing
interaction formula to obtain a Ze design value for the
-
NDS Commentary
connection (202). Determining a Ze designvalueusing
this latter approach is an acceptable alternative to
calculating Fee for useineachyieldmode
equation
and allows the useof
tabulated 2 valuesfromthe
Specification.
-
Tabulated Two Member Wood-to-WoodBolt
Design Values. Bolt
design
values
for lumber to
lumber(Table8.2A)
and gluedlaminatedtimber
to
lumber (Table8.2B)connectionshave
not previously
been tabulated in the Specification,beingtaken
as a
proportion of the tabulated threememberboltdesign
valueinearlier editions (see Commentary on Previous
Methodology for Single Shear BoltedConnections).
With the added refinement of the yield mode equations,
twomember bolt designvalues are not necessarilya
fixedone-halfthethreememberboltdesignvalue.
Separate twomemberboltedconnectiondesignvalues
are therefore
given
to facilitate
designer
use.
All
tabular bolt design values are based on a bolt bending
yield strength of 45,000 psi.Twoboltdesignvalues
for perpendicular to grain loading are shown: one for
a connection with the side member loaded perpendicular to grain and themainmemberloadedparallel
to
grain (Zs1
); andonefor
aconnectionwithmain
member
loaded
perpendicular
to grain and the
side
memberloadedparallel to grain (Z, ).
The softconversionprocedureused
to translate
short term yield mode bolt design values to bolt design
values previously tabulated in the Specification involved
use of average adjustment factors for each mode based
on all bolt and member sizes, all species combinations,
andboth
wood and steel
side
members.
For two
memberconnections,theadjustment
factor wasbased
on joints inwhich the sidememberwasone-halfthe
thicknessofthemainmember.
Under thisindexing
procedure, new bolt design values are both higher and
lower than previous bolt design values depending upon
the bolt diameter, the thicknesses of the main
and side
member, and the particular species combination involved.
__
average level of 1986 and earlier tabulated boltdesign
values using average factors for joints in which the side
memberwasone-halfthethicknessofthemainmember. This was the configuration used for the early tests
from whichtheboltdesignvaluesinthe1986and
earlier editions were derived.Only
one configuration
(3-1/2main and 1-1/2side)inthetableabovemeets
this calibration condition. The average ratios of1991
to 1986 bolt designvalues for the joints inthetable
which have this calibration geometry are 0.97 and 1.18
for parallel and perpendicular to grain loading, respectively.
Table C8.2-1- Comparison of 1991 and 1986 NDS
Wood-to-Wood SingleShear Bolt Design Values
Thickness, in.
Bolt Design Value,
lbs
Bo1t
z,,
Main,
Side,
Diam.
Wood Wood in. 1991
1986
Ratio
1991
1986
314
1
530 470
800 710
1060 945
314
1
1-112
1-112
112
3
330 215
460 270
580 325
1.53
1.70
1.78
1.04
635
660
1270
1315
0.97
1740
1875
0.93
470 430
620 540
750 650
1.09
1.15
1.15
314
1
1.18
635
750
1690
1400
1.21
2480
2135
1.16
520 490
960 630
1360 760
1.06
1.52
1.79
1
1270
1315
0.97
2150
1875
1.15
850 540
1190 650
1.57
1.83
1
1690
1430
1.18
2870
2435
1.18
1090
1.16
940
1550
1190
1.30
112 1-112
3-112
3-112
112
5-112
1-112
314
5-112
3-112
314
.
.
1.13
1.13
1.12
Ratio
S~ruce
-h e-FK:
1-112
1-112
112
314
1
3
112 1-112
Comparison of 1991 and Earlier EditionBolt
Design Values. Differencesbetween1991
and earlier
editionwood-to-woodsingle
shear boltdesignvalues
for two
species
combinations are shown
in
Table
C8.2-1.
3-112
3-112
112
For the joint configurations compared, bolt design
valuesbased on the1991edition
for theparallel and
perpendicular to grain loading cases
averaged
11
percent and 54 percenthigher,respectively, than those
based on the 1986 edition. It is to be noted that the
short-term yield mode bolt design values on
whichthe
1991 boltdesignvalues are basedwerereduced to the
5-112
3-112
314
410
610
810
340
510
680
1.21
1.20
1.19
240 140
340 175
430 210
1.71
1.94
2.05
330 280
440 350
540 425
1.18
1.26
1.27
430 325
730 410
1000 495
1.32
1.78
2.02
314
1
540
580
0.93
1000
1015
0.99
1330
1355
0.98
314
1
660 1.14
580
1420
1155
1.23
1890
1585
1.19
1
1080
1015
1.06
1760
1355
1.30
690 350
830 425
1.97
1.95
1
1480
1310
1.13
2330
2275
1.02
900 640
1120 775
1.41
1.45
5-112
1-112
314
The yield mode equations on which the 1991bolt
design values are based provide a fully rationalized and
consistentmeasure
of theeffectsofmain
and side
memberthicknesses and boltdiameter.
Assuchthey
Bolts
10 1
NDS Commentary
are morereliableindicatorsofrelative
joint loadcarryingcapacity than theboltdesignvaluesgivenin
earliereditionswhichrepresentaconservativeapplicationoftestresults
for onlyonemain to sidemember
configuration.
Mixed SpeciesConnections.
Design
values for
bolted
connections
made with
side
a member
of
different species than the main member can be calculated usingtheyield mode equations (Eqs. 8.2-1 to 8.2-6)
andthe appropriate dowelbearingstrength
for each
species.Mixedspeciesconnectionswereprovided
for
in the 1982 and 1986 editions by assigning the joint the
lesser of the bolt design value applicable to a comparable jointmadewithmembersofthemainmember
species or a comparable joint madewithmembersof
thesidememberspecies.Inlieu
of usingtheyield
mode equations, bolt
design
values
for
connections
made with different main and side member species may
bebasedon bolt design values in Table 8.2A and 8.2B
forthespecieswiththelowerdowelbearingstrength,
Fe
8.2.2-Wood-to-MetalConnections
Background
In the 1977 and earlier editions of the Specification,
boltedconnections made withsteelsidememberswere
assigneddesignvaluesthatwere
25 percentlargerthan
tabulatedboltdesignvalues
for woodmainmembers
to grain. No increase for steel
side
loaded
parallel
plateswasrecognizedwhenwoodmainmemberswere
loadedperpendicularto
grain. Theseprovisions were
basedonearlybolttests(183)involvingtwosoftwood
andtwohardwoodspecieswhichshowed
that proportional limit bolt design values for joints made with steel
side members and the main member loaded parallel to
grainaveraged
25 percent
higher
than proportional
limitboltdesignvaluesforthecomparablewoodside
member joint.
In the 1981 edition, following additional research on
two and three member joints made with steel and wood
side plates, the adjustment of tabulated parallel to grain
bolt designvalues
for use
of
steel
side
plates
was
increasedfrom
25 to 75 percent. The new research
( I 13)was conducted as aresultoftheimpactofthe
1977 change in theprocedure
for establishingbolt
designvalues for twomemberconnections(seeCommentary for 8.2.1 - PreviousMethodology for Single
ShearBoltedConnections).
The 1977changereduced
allowable design values for singleshearboltedconnections, including those made with steel side members,
as
much as 50 percent below bolt design values previously
used successfully for manyyears.Applicationsparticu-
larlyaffectedincluded
shear walltie-downswherethe
change in the bolt design provisions of the Specification
required the number of bolts used in such hardware
to
bedoubled for thesamecodespecifiedloads.
Results from the additional parallel to grainbolt
joints made with1/2-inch
tests (113),whichinvolved
bolts,1/4-inchthicksteelsideplates
and southernpine
main and side
members,
showed
that the
average
proportional limitboltdesignvalues
for joints made
with steel side plates were 20 to 63 percent higher than
those for the matching wood side plate joints; that the
lowest proportional limit joint designvalue(wood-towoodthreemember joint) was 32 percentlowerthan
the applicable tabulated bolt design value;
and that the
slipofthemetal-to-wood
joints waslessthan that of
thewood-to-woodjoints.
The designvalues
of the
metal-to-wood joints associatedwiththeproportional
limit slip of the matching wood-to-wood joints were 75
percentlarger
on theaveragethantheproportional
limit design values for the wood-to-wood joints. These
average equivalent test slip design values
for the metal
side plate joints in turn averaged from 20 to 70 percent
lowerthanthemaximumaveragetestloads
for these
joints.
The information obtained from the newtestingon
the difference in bolt design values on joints made with
steel and woodsidemembers at the proportional limit
slipofthe
latter wasconsistentwith that reported in
theearlyboltresearchwhichformedthebasisofthe
Specifications
bolt
design
provisions
(183).
In the
earlier work, a slip of about 0.025 inches was reported
associatedwiththe proportional limitdesignvaluesof
joints made with steel side plates and 1/2 inch diameter
bolts, and theseboltdesignvaluesaveraged
about 25
percentlarger than those for comparable joints made
withwoodsideplates.However,theslipassociated
withthe
proportional limitloadofthewoodside
member joints was 0.035 inches, or about 40 percent
greater than the proportional limit slip of the steel side
memberjoints. As afirst approximation, theseresults
indicatedtheload
of thesteelsidememberjoints
associated with the proportional limit slip of the wood
side member joints was [(1.25Pw)/0.025] x 0.035, or
about 75 percent larger than the proportional limit load
of the wood-to-wood joint; the same as observed in the
newtests.
30 years of successfulperforOnthebasisofthe
mance oftwomemberboltedconnectionsmadewith
steel side plates at bolt design values up to twice those
established in the 1977 edition, and recognizing that the
structural serviceability
of
a
joint is relatedtoits
stiffness or slip;a
75 percentadjustmentfactorfor
NDS Commentary
-
-
joints loaded parallel to grain and made with steel side
plates, as shown by theslipequivalentloads,was
This revision had the
introduced in the1982edition.
effect of increasing design values
for steel side member
bolted connections loaded parallel to grain approximately 40percentoverthoseprovided
by1977provisions.
In the1986edition,
more conservativeprovisions
for establishing design values for connections made with
steel
side
members
and large
diameter
bolts
were
introduced. This further revisionfollowedunsatisfactory
field
experience
with
a long span truss
design
involvinggluedlaminatedtimbertensionchords
and
large
diameter
bolts
designed
in accordance
with
provisions of the1977edition.
In responsetoquestions raised about design procedures for all joints made
with1-inchandlargerbolts,
a specialstudy(97)was
conducted of double-shear connections made with glued
laminated 4-1/2 by 4-1/2 inch main members, 1-1/4-inch
bolts
and
2-1/4-inch
wood
and
1/2-inch
steel
side
members.Althoughtheaverage
ratio of ultimatetest
loadto 1982 NDS boltdesignvaluewas2.7forthe
joints madewithsteelsideplates,the
ratio waslower
thanthe comparable ratio of3.9obtainedforconnectionsmadewith1/2-inchboltsandsteelsidemembers
that were
tested
earlier
(113).
Wood-to-wood
1/2
inch
and1-1/4inchbolted
joints both had testtodesign
load ratios exceeding4.0.
it wasconsidered appropriBasedontheseresults,
ate toreducethedesignvalues
for connectionsmade
with steelsidemembers and largediameterboltssuch
thattheload
ratio factor for the1-1/4inchbolttests
was comparableto
that for the1/2inchbolttests.
This was accomplished in the 1986 edition by providing
a variable adjustment for connections loaded parallel to
grainandmade with steelsidememberswithlimitsof
75 percent for bolts 1/2 inch or less in diameter and 25
percent for bolts1-1/2inch
in diameter,with proportionateadjustmentsforintermediatediameters.
-
1991 Edition.
Bolt
design
provisions
in the
currentedition are limited to bolts 1 inch or lessin
diameter. This new limitationreflectsconcern
about
of workmanshipvariablesanddrying
in
theeffects
serviceontheperformanceoflargediameterbolted
connections(seeCommentaryfor8.1.2).
Further, all
boltdesignvalues
are nowbasedontheyieldlimit
modelwhich does not account for degreeofdeformation or slip(seeCommentary for 8.2.1 - Background).
Short-termboltdesignvaluesbasedondirectapplicationoftheyieldmodel
equations havebeenreducedto
thenominalaverageboltdesignvaluelevelspublished
in earliereditionsforconnectionsmadewiththesame
species and member
sizes,
with
the
exception
that
conversion
factors
for joints made with
steel
side
memberswerereferenced
to 1977editionboltdesign
a 25 percentincrease for joints
valueswhichutilized
loadedparallel
to grain and made withmetalside
plates.This approach wasused on thebasisthatbolt
designvaluesinthe1977
and earliereditions
were
related to proportional limit joint loads, yieldmodel
boltdesignvalues
are basedonloadsassociatedwith
an offsetof 5 percent of fastenerdiameter, and both
are independentof a specificsliplevel.
In addition,
the same conversion factors are applied to connections
madewithsteelsidemembers
asto woodsidemembers.
It is recognized that relating short-term yield model
boltdesignvalues for paralleltograinloadedconnections made with steel side members to 1977 edition bolt
designvalues for theseconnectionsresults in a reduction in boltdesignvalues from 1982 and 1986 edition
levels. The advantages
of
consistency
of
treatment
across
all
dowel
fastener
types,
and the
ability
to
determine by fully rationalized methodology the effects
ofmemberthickness,member
strength, boltsize,bolt
strength and numberofmembers,
both singlyand in
combination, were consideredtooutweightheimpact
of the reduction in bolt design values for those connections
employing
steel
side
members.
Differences
for suchbolted
between199 1 and 1986designvalues
joints are illustrated in the Commentary for 8.2.2.1.In
that designvalues for
thisregard, it is tobenoted
boltedconnectionsinvolvingproprietarytie-downs
or
similarhardwaremay
be established by other procedures than thosegiveninthe1991edition(see7.1.1.4
and 1.1.1.4 of the Specification). However, use of such
alternate methodologies is the sole responsibility
of the
manufacturer and ofthedesignerutilizingthedesign
values so derived.
8.2.2.1 The same yield modeequations used for
single shear wood-to-wood bolted connections
are used
for wood-to-metalboltedconnectionsexceptequation
8.2-2 for modeI,whichis
for uniformbearing inthe
metal side member. This condition
ischeckedindependently in accordancewith8.2.2.2
and 7.2.3(seeCommentary for this latter section).
The yield mode equations of8.2.1maybeentered
for
with a dowelbearingstrength, Fcs , of58,000psi
the side member when A36 or higher strength steel side
plates are used. This valueisequivalenttothe
ultimatetensionstrengthofthesteel(seeAppendix1.2).
A nominalboltdesignvalue,
2, iscalculated for
eachofthefiveapplicableyieldmodeequationsand
thelowestvalueisselected
as thedesignvalue for the
bolted
connection.
The effects
different
of main
Bolts
103
NDS Comentwy
memberthicknesses, bolt diameters and loading direc2 that can be obtained from each
tionsonvaluesof
yield mode equation, and on thelimitingload for the
connection, are illustrated by the example joint design
calculations shown in ExampleC8.2-2.
Tabulated boltdesignvalues for lumber and glued
laminatedtimbersingle shear connections with1/4in.
steel side plates given in Tables 8.2C and 8.2D assume
A36 steelhaving a dowelbearing strength of58,000
psi, and a boltbending yield strength of45,000psi.
Comparison of 1991andEarlier
Edition Bolt
Design Values. Differences in boltdesignvalues
for
lumber-to-steelsingle
shear connections betweenthe
1991 and the1986
editions are illustrated in Table
C8.2-2. For the species and joint configurations
Example C8.2-2
Yield mode design values for wood-to-metal single
shear bolted connections:
Hem-fir two member wood-to-metal connections
made with 112 and 1 inch bolts, steelside member
thickness of 1/4 inch, main member thicknesses of 11/2, 3, 3-1/2 and 5-112 inches, and loads applied
parallel and perpendicular to the grain of mainand
side members
F,,,
= 4800psi parallel
= 2550psi perpendicular, 1/2inch bolt
= 1800psi perpendicular, 1 inch bolt
F,, = 58,000psi
Fub = 45,000psi
Wood
Thickness,
in.
1- 112
Bolt
Diam.
Direction in.
I
I/
3
I
1/2
I1
3-
I
5- 112
I1
I
104
Bolts
1/2
1
112
1
The 1986 edition bolt design values in Table C8.2-2
representone-half the bolt designvalue for a three
member joint made withwoodsidemembersone-half
thethickness of themainmember
and then, for the
case ofparallel
to grain loading only, increased 75
percent, 62-1/2 percent and 50 percent for 1/2 inch, 3/4
inch and 1inch bolts, respectively,whensteel
rather
As previously
thanwoodsidememberswereused.
discussed
in
the Background
commentary
this
to
section,the1991
bolt designvalues are basedon a
conversion of short term yield mode bolt design values
Table C8.2-2 - Comparison of 1991 and 1986 NDS
Wood-to-Metal SingleShear Bolt Design Values
Thickness,
in.
Bolt Design Value, lbs
Bo1t
ZIl
Main, Side, Diam.
z*
Wood Steel
in. 1991
1986
Ratio
1991
1986
Ratio
112 114
1-112
3112
Z I ~
900
1800
382
540
1800
829
1040
1/2
1 3604
2577
2798
3600
765
112
1310
1280
1080
1 1809
1/2 2100
957
1174
2577
2979
4200
1 3604
112
893
1310
1294
1260
1 1809
ZII
ZflI,
21.1,
780
1111
0.70
1560
2136
2090
2812
0.74
314
1
1
901
112 114
1-112
314
1
535
428
495
706
3112
114
901
422 535
428
546
1/2 3300
1486
1741
1
6600
2971
3902
535 775
1/2 6461402
1310
1461
1980
11809
3/4
1
314114
5-112
940 2557
2577
3604
238 535
428
384
381 1367 1310
1809
375
570
822
860
1154
1140
1418
0.80
112 114
3-112
ZN
470 700 901
706
706
314
1
114
Yield Mode
Design Vaiuc, Ibs
Load
I1
compared, the parallel to grain design values for single
shear steel-to-woodbolted connections based on the
provisions of the 1991 edition average 25 percent lower
than thosebasedonprovisionsin
the 1986 edition.
Perpendicular to grain bolt designvaluesaverage
30
percenthigherinthenew
edition than the perpendicular to grain boltdesignvaluesinpreviouseditions
which contained no increase for use of steel side plates.
314
1
3-1/2112 114
706 901
2577 3604
114
5-112
310 215
390 270
480 325
1.44
1.44
1.48
0.73
500 360
640 540
750 650
1.39
1.19
1.15
780 1111
1650
2275
2420
3202
0.70
0.73
0.76
500 490
730 630
850 760
1.02
1.16
1.12
1650
2324
0.71
2880
3802
0.76
460 595
690
829
920 1020
700
1015
0.69
1220
1649
1630
2032
0.80
1
700
1015
0.69
1410
1877
0.75
1880
2378
3/4
1
1470
2129
2550
3412
0.75
314
0.69
0.75
950
940
1.01
1290
1190
1.08
0.77
0.83
0.90
230 140
300 175
370 210
1.64
1.71
1.76
0.74
360 280
460 350
540 425
1.29
1.31
1.27
410 325
520 410
610 495
1.26
1.27
1.23
770 640
900 775
1.20
1.16
0.79
0.69
A?DS Commentary
I
to the average levels of previously tabulated bolt design
values increased only 25 percent for steel side members.
The fact that the 1991 bolt
design
values
for the
parallel to grain load cases in Table C8.2-2 average 25
percentlower than the 1986 boltdesignvaluesreflects
the average effect of this difference (I -1.25/1.625 or 23
adjustment. (For addipercent) in steelsidemember
tional discussion see Commentary for 8.2.1 - Comparisonof1991
and EarlierEditionBoltDesignValues).
8.2.2.2 (See Commentary for 7.2.3)
8.2.3-Wood-to-ConcreteConnections
A specificprovision for establishingdesignvalues
for a single shear connection involving a wood member
attached to concrete or masonry through an embedded
bolt in the latter was introduced in the 1982 edition.
Such connections wereassigned
a boltdesignvalue
equal to one-half the tabulated bolt design value for a
piecetwice thethickness of thewoodmember.
This
procedurewasbased
on theconservative assumption
that the concrete or masonry wasprovidingbearing
support and fixity at leastequivalent to that provided
by a wood member twice the thickness of the
attached
wood
member.
The 1982
provision
for connections
involving concrete and masonrywascontinued
in the
1986 edition.
The 1991 edition continues the approach established
in earlier editions ofconsidering a wood-to-concrete
bolted connection equivalent to a single shear wood-towood boltedconnectionmade
with a mainmember
twice thethickness of the sidemember. To determine
theallowableboltdesignvalue
for such a connection,
theyield mode equations of 8.2.1can be enteredwith
tm equal to twice t, and Fern equ.alto Fes ; or the
designvalue from Table 8.2A or 8.2B for the applicable main and side member relative thicknesses and bolt
diameter can be used.
It is thedesigner'sresponsibility to assure that the
concrete or masonry has sufficientembedment
and
dowel bearing strength to resist loads imposed through
the embedded fastener.
8.2.4-Load at Angle to BoltAxis
I
Two member connections in which the load acts at
anangle to theaxis of thebolt are checkedusingthe
component of theload acting at 90" to theaxis and
in
memberthicknesses equal to thelengthofthebolt
eachmembermeasured at thecenterline of thebolt(see
SpecificationFigure8B).
This methodology has been
a provisionoftheSpecificationsincethe1944
edition.
Prior to the1977
edition, theallowable bolt design
values for such joints were taken as one-half
the
tabulated bolt
design
value
for a member
whose
thickness was twice the bolt length in the thinner piece.
This waschangedin the 1977 and subsequent editions
to the lesser of one-half the tabulated bolt design value
of thethickermember
or one-halfthe tabulated bolt
designvalue
for a piecetwicethethicknessofthe
thinner member(see Commentary for 8.2.1 - Previous
Methodology for Single Shear BoltedConnections),
where the length of the bolt in each member was used
as the thickness of that member. The centerline of the
boltwas made the reference for measuringthebolt
length ineachmemberin
the 1986 edition.
In the 1991 edition, allowable bolt design values for
connections inwhich the load acts at an angle to the
boltaxis are based on the yield mode equations of
8.2.1. The lowestvalueof
2 obtained, using tq and
ts equal to the length of
bolt in each member, divided
by thecosineof
the angleofintersectionofthe
two
members is the maximum nominal design value
for the
bolted connection.
The adequacyof the bearing area under washers
and platestoresist
the component offorceacting
parallel to the bolt axis can be checked using tabulated
compression design values perpendicular to grain, FcL,
adjusted as appropriate by the bearing area factor, Cb ,
(see 2.3. IO); and bearing on the angled member should
be evaluatedusing
tabulated and allowablebearing
designvaluesparallelto grain, Fg and Cb FcL , inthe
interaction equation of 3.10.3.
8.3-DESIGNVALUES FOR DOUBLE SHEAR
CONNECTIONS
8.3.1-Wood-to-WoodConnections
Background
YieldMode
(See Commentary for 8.2.1)
Equations
The yield mode equations for three member, double
shear bolted connections parallel those for two member,
single shear bolted connections in 8.2.1 except that two
of the modes for the latter configuration are not
applicable:bolt
rotation without bending,Mode 11;
and development of a plastic hinge in one of theside
members, Mode 111,. The equations for the remaining
modes (Im, Is, IIIs and IV) are the same as those for
the single shear configuration except for three ofthe
conversion factors, n KO , usedtorelate
short-term
yield
model
bolt
design
values
to averagenominal
proportional limit
bolt
design
values
tabulated in
previous editions of the Specification, as shown below.
Bolts
105
NDS Commentary
Allowable bolt designvalues, 2,for sawnlumber
and glued laminated timber double shear connections
are tabulated in Tables 8.3A and 8.3B. The latter table is applicable to glued laminated timber
main
members and sawn lumber side members.
~
Single
Mode Shear
Shear
Im
IS
IIIs
IV
4Ke
4K*
3.2 K,
3.2 K,
Double
4K,
2Ke
1.6 K,
1.6 K,
The angle factor KO has thesamevalues for the
double shear as for thesingle shear case:1.00
for
parallel to the grain loading and 1.25 for perpendicular
to grain loading of either the main or side member.
The four yield mode equations for double shear
bolted connections are solved for 2 and the lowest
value obtained is the allowabledesignvalue
for the
joint. The effectsofmemberthickness,bolt
diameter
of loading onthe
2 values for each
anddirection
mode and the limiting value are illustrated in Example
C8.3-1.
4
Comparison of 1991 and Earlier Edition Bolt
design
in values
for
Design Values. Differences
double shear bolted connections inthe1991 and 1986
editions are illustrated for two species in Table C8.3-1.
For the joint configurations comparedinthistable,
bolt designvaluesinthe199
1 edition average 16
percent and 35 percenthigher for parallel and perpendicular to grain loading, respectively, than boltdesign
valuesbased on 1986 provisions.Averagedifferences
for the one configuration in which the thickness of the
Table C8.3-1 - Comparison of 1991 and 1986NDS
Wood-to-Wood Double Shear Bolt Design Values
Thickness, in.
Bolt Design Value, lbs
Bolt
z//
Main, Side, Diam.
Wood
Wood
in. 1991
1986
Ratio
1991
1986
Ratio
z,,
1-1/2 1150
1-112
1/2
Example C8.3-1
Yield mode design values forwood-to-wooddouble
shear bolted connections:
Hem-fir three member connections made with 112 and
1 inch bolts, side member thickness of 1-1/2 inches,
main member thicknesses of 1-1/2, 3 and 5-112 inches,
and loads applied parallel and perpendicular to the
grain of main and side members
Fe,n,Fes = 4800 psi parallel
= 2550 psi perpendicular, 112 inch bolt
= 1800 psi perpendicular, 1. inch bolt
= 45,000 psi
Fub
Thickness, in. &
grain direction
Main
Side
Bolt
Diam.
in.
ZI,
112
1/2
1/2
3 I/
3 /I
3 1
1-1/2 //
1-1/2 I
1-1/2 //
1/2
1800
112884 7651440
112
1440
765
3 I/
3 /I
3 1
1-112 //
1-1/2 I
1-112 //
1-112 //
1-112 I
1-112 //
1
1
1
106
Bolts
314
1
2550
2630
0.97
3750
4310
1.22
1.22
3 1.04
1270
1320
112
1-112
3-1/2
3-112
1500
1270
112
5-1/2
1-1/2
314
4270
53801
3380
2800
3/4
1
2550
2630
4310
3750
1.15
1
1.13
5090
5740
112
314
1
680
880
1320
1020
1360
1760
550 430
660 540
770 650
1.15
940
860
1.09
1330
1080
1.23
1530
1300
1.18
1.18
1.21
1.26
1040
980
1.06
1550
1260
1.23
1790
1520
1.18
0.97
1690
1080
1.56
2700
1300
2.08
21801870
36802950
7-1/2
3-1/2
3380
3/4
2860
1.18
21,
ZIII,
ZIV
1-1/2
1-1/2
!&)Q 1800 1 100
1326
1-112 //
1-112 I
1-112 //
1
1
1
1730
1420
1890
2310
1.28
1.22
1.18
1.17
1.25
Yield Mode
Design Value, lbs
1-1/2 //
1-112 //
1-112 I
5-1/2 //
5-1/2 //
5-112 I
1.22
940
314
1
765720
182
884
884
1440
760
JJQQ 1326
261(1
884
5303
3600
3768
2880 Ja8a 2286 3 133
2269
3 133
J 080 2880
5303
3768
6600
3133
2286
5280
J 980
2880
2269
133
3
1-1/23
3-1/2
3-1/2
1/2 0.93
1160
1080
2160
3/4
2030
1.06
1
1.30
2710
3530
112
1310
1160
1.13
3/4
2950
23
10
1
3170
4110
1.28
1.30
370 280
450 350
530 420
.32
.29
.26
740
560
900 700
1050 850
1.32
1.29
1.24
860 650
1050 820
1230 990
1.32
1.28
1.24
1.97
2.27
1
1.30
2710
3530
1380 700
1930 850
314
1
2950
2550
1.16
1.00
4650
4660
1820
1360
1.34
2630
1960
1.34
5-1/2
1-1/2
2160
3/4
2030
1.06
7-1/2
3-1/2
1.29
1.29
1.29
A
NDS Commentary
-
sidemembersisone-halfthethicknessofthemain
members(3inchmain
and 1-1/2
inch
side)
are 8
percent and 22 percent for parallel and perpendicular
grain
to loading,
respectively.
This
main
side
to
member
thickness
ratio was
used
to
establish
the
conversionfactorsbetween short termyieldmodebolt
design
values
and bolt
design
values
tabulated
in
previous editions of the Specification (see Commentary
for 8.2.1 - Comparisonof
1991 and EarlierEdition
BoltDesignValues).
Mixed Species Connections. Where
the
side
memberspeciesdiffersfromthemainmemberspecies,
in Tables8.3Aand8.3B
for the
boltdesignvalues
specieswiththelowestdowelbearingstrengthmaybe
Species
used
(see
Commentary for 8.2.1 - Mixed
Connections).
Loads at Angle to Grain. When themain
or
side members are loaded at an angle to grain, the yield
mode equations of8.3.1may
be enteredwithdowel
bearing strengths, Fee , determined in accordancewith
Equation8.2.7.
Example C8.3-2
Yieldmodedesignvaluesfor
shear bolted connections:
Hem-firdoubleshearboltedconnectionsmadewith
1 inchbolts,steelside
metalsideplateswith1/2and
memberthickness of 1/4 inch,main memberthickloadsapplied
nesses of 1-112, 3 and5-112inches,and
parallelandperpendicular
tothegrain of mainand
side members
Fern = 4800psiparallel
= 2550 psi perpendicular, 1/2 inch bolt
= 1800psiperpendicular, 1 inchbolt
F,, = 58,000psi
Fub = 45,000 psi
wood
Background
ground)
(See Commentary for 8.2.2 - Back-
8.3.2.1 Yield
mode
equations used for double
shearwood-to-woodboltedconnections
are used for
double shear bolted connections made with wood main
membersandmetalsideplatesexceptequation8.3-2
for mode Is, whichis for uniform bearing in the metal
is
side
members,
not
is
applied.
This
condition
checkedindependentlyinaccordancewith8.3.2.3and
7.2.3.
A nominalboltdesignvalue,
2, is calculatedfor
each of thethreeapplicableyieldmodeequationsand
thelowestvalueisselected
as thedesignvalue for the
2 andthelimitingmode
bolted connection. Values of
boltdesignvalue
for example joint configurations are
illustrated in ExampleC8.3-2.
Where the wood member of a double shear woodto-metalboltedconnection
is loaded at an angleto
grain, adowelbearing strength, Fes , based on Equation8.2.7may beused.
I
Tabulated bolt design values
for lumberandglued
laminated timber double shear bolted connections made
and
with1/4 in. steelsideplatesgiveninTables8.3C
8.3D are basedonA36steelhavingadowelbearing
strength of 58,000
psi,
and
bolt
abending
yield
strengthof45,000psi.
Bo1t Mode Yield
Diam.
Design
Value,
Thickness,
Direction
in. in.
Load
1-112
II
112
I
1
1/2
1
II
112
ZIm
8.3.2-Wood-to-MetalConnections
-.
wood-to-metal double
3
1
3-112
I
112
1
II
1/2
1
1/2
1
I
5-1/2
II
1412
5154
857
2.Q262 1
ZIV
1802
1sM
xi2
7208
1070
3619
5154
857
lQsa 262 1
1802
7208
1070
3619
1412
1802
1800
m
m
2 100
1412
42M 5154 7208
892
852 1070
LZQ
262 1
u . 2 1802
1/2
3300
6600
1402
1
lesn
262 1
1/2
I
I
Ibs
Z1IIs
3619
7208
852 1070
3619
Comparison of 1991 and Earlier Edition Bolt
Design Values. Differencesinboltdesignvaluesfor
lumber-to-steeldoubleshearconnectionsbetweenthe
1991 andthe
1986 editions are illustrated in Table
C8.3-2. For the joint configurationscompared,the
1991paralleltograinboltdesignvalues
for double
shear joints made withsteelsideplatesaverage
23
percentlowerthanthosebased
on provisions in the
is aresultoftheproce1986 edition.Thisdifference
dure used to convert short-term yield mode bolt design
values
to
the
level
of
previous
edition
bolt
design
valueswhereinametalsideplateadjustmentof1.25
rather thanfrom1.75 to 1.50 as providedinthe1986
edition (see Commentary for 8.2.2.1 - Comparison of
1991 and EarlierEditionBoltDesignValues).Also,
for the joint configurations compared, the 1991 perpen-
Bolts
107
de,
NDS Commentary
Table C8.3-2 - Comparison of 1991 and 1986 NDS
Wood-to-Metal Double Shear Bolt Design Values
Design
Value,
lbs
Thickness,
Boltin.
Main,
Wood
Bolt
Diam.
Steel
in.
ern
314
1
3112
1991
1986
Ratio
1/4
3-112
1150
1645
0.70
1730
2308
0.75
2310
2835
0.81
0.71
550 430
660 540
770 650
1.28
1.22
1.18
1000 860
1330
1080
1.23
1530
1300
1.18
1.16
1
1570
2222
3300
4274
0.77
4610
5625
0.82
112
314
1
1570
2222
0.71
3300
4550
0.73
5380
6405
0.84
1
3300
4648
5750
7605
0.76
314
1
880
1190
0.74
1320
1658
0.80
1760
2040
0.86
314
1
1400
2030
0.69
2640
3299
0.80
3530 4065
0.87
370 280 1.32
450 350 1.29
530 420 1.26
1802 1800
112
740 560 1.32
900 700 1.29
1050 850 I .24
314
1
1400
2030
0.69
2940
3754
0.78
41 10 47660.86
840 650
1050 820
1230 990
1
2940
4258
0.69
5110
6825
0.75
1590
1280
1.24
1930
1550
1.25
114
314
314 114
5-112
112 114
1-112
3112
Ratio
m:
112 114
1-112
114
112 1143-112
314114
5-112
-
z,
Zll
1991
1986
Boltdesignvaluesdetermined
from theapplicable
yield mode equations for several different configurations
ofsteelmainmember
joints are shown inExample
C8.3-3.
Example C8.3-3
Yieldmodedesignvaluesforsteelmain
double shear bolted connections:
Hem-fir three member connections made with 1/2 and
1 inchbolts, 1/4 inchsteelmainmember,
wood side
memberthicknesses of 1-112, 3 and 5-112 inches,and
loadsappliedparalleltothegrain
of thesidemembers
1000 1.02
980
1550
1260
1.23
1790
1520
1.18
0.71
member
Fern = 58,000 psi
Fes = 4800 psi
Fub = 45,000 psi
1910
1880
1.02
2810
2380
1.18
Thickness,
in.
1.29
1.28
1.24
diculartograinboltdesignvaluesaverage
30 percent
higher than similarboltdesignvaluesbasedonthe
1986 edition. In previous
editions,
no
increase
in
perpendicular to grain bolt design values
was made for
useofsteelside
plates.
8.3.2.2 In
the
1982 and 1986 editions,
double
shearboltedconnectionsmadewithmetalmainmembers and wood side members were assigned bolt design
valuesequaltothetabulatedparalleltograinbolt
designvalue for apiecetwicethethicknessofoneof
the side members increased by the applicable steel plate
adjustment
factor.
In the 1991 edition,
bolt
design
values for such joints are determined from yield mode
equations as for joints withmetalsidemembers(see
8.3.2.1) except that Equation 8.3-2 for bearing in side
membersreplacesEquation
8.3-1 for bearing in main
of the
bolt
in the
metal
main
member.
Bearing
parts
memberiscovered in thedesigncheckformetal
(see 8.3.2.3 and 7.2.3).
Main,
Side,
Stee1
1-112
Bo1t
Dfam.ln.
wood
Yield
Mode
Design
Value, lbs
ZZ
IIV
I
S,
114
1I4
3
1I4
112 5-112
1
3600
112
1
3600
2080
7200
5597
7208
1
8.3.2.3 (seeCommentary
6600
3482
13200
7804
51
14
7208
lljQ2
W
for 7.2.3)
8.4-DESIGN VALUES FOR MULTIPLE SHEAR
CONNECTIONS
Background
Evaluatingboltedconnectionsmadewith
four or
moremembersofequalthickness
on thebasisofthe
sum of the allowable bolt design value
for each shear
plane has beenaprovisionoftheSpecificationsince
1944. In the 1977 edition,provisions were addedto
cover allowable bolt design values for joints of four or
more members that were not ofequalthickness.Such
of
joints were
resolved
into themaximumnumber
contiguousthree-member joints and one-halfthebolt
design value applicable to each such joint was assigned
to
each
shear
plane
in the
connection.
For shear
planesassignedtwodifferentboltdesignvalues,the
lowestvaluewasassignedtheplane.Wheretheloads
on eachmember were known,theboltdesignvalue
for
anymember in the joint wasthesumofthebolt
design
values
for eachshearplaneactingon
that
member.Wheremembers
in the joint carriedequal
-
NDS Commentary
and the grain orientation of member 2 and 4
isthedifferencebetween
8,
and thegrain
orientation of member 3;
t
loads or the loads carried by each were unknown, the
design value for the bolted connection was taken as the
lowestboltdesignvalue
for anyshearplanetimesthe
numberofshearplanes.Thesenewprovisionswere
carried
forward
unchanged
to
the
1982 and 1986
editions.
1991 edition.
The procedures
of
the
current
edition require evaluation of each individual shear plane
usingtheyieldmode
equations of 8.2.1 or 8.2.2 and
then assigning the connection a bolt design value equal
to thelowestvalue
for anysingleplanetimesthe
number
of
planes
in
the
joint. This
methodology,
which encourages use of symmetrical member thicknesses,presumes
that theconnectionload
is shared in
proportion to member thickness and that members are
loaded in nomorethantwodifferentanglestograin.
Wheremorecomplexconnectionconfigurationsoccur,
evaluation of the adequacy of the bolt design value
for
eachindividualshearplane
maybe required (179).
Whereamultiplememberconnectionconsistsof
membersloaded at three or more differcntanglesto
grain,thefollowingproceduresmaybe
used todetermineallowableboltdesignvaluesonindividualshear
planes:
Determinethe loads ineachmember
membersenteringtheconnection;
or pairof
"i" members in the
Number
consecutively
the
connectionfromoutsidetowardthecenter
as I , 2,
3 ...i;
Enterthe yield mode equations of 8.2.1 or 8.2.2
with Fee based
on
the
angular
difference
(8)
betweenthedirectionoftheresultantforceinthe
shear plane and the grain direction of the member,
andwith Bmax equaltothelargest
8 for thetwo
membersadjacenttotheshearplanebeingconsid8 for eachmember
ered.Calculatetheapplicable
adjacent to each shear plane and the allowable bolt
designvalue for that plane by thestepsbelow:
(i)
Determinetheallowableboltdesignvalue
for
the plane between members 1 and 2 based on
aload (P,) acting in thedirection of the
stress in member1,where
8, = 0" or 90"
and 8, isthedifferencebetweenthegrain
orientations ofmembers 1 and 2;
(ii)Determinetheresultant
(P,,2 ) oftheforces in
members 1 and 2 and thedirection (efe2
)
thisresultant is acting.Determinetheallowableboltdesignyalue
for theplanebetween
members 2 and 3 basedonaloadacting
at
, where 8, is thedifferencebetween
(iii) Determine the resultant
(P,2-7) of the forces
in members 1, 2 and 3 and' the direction (0,2-3 ) thisresultantisacting.Determinethe
allowable
bolt
design
value
for the
plane
betweenmembers 3 and 4 based on aload
acting at 8,-2-.7, where 8, isthedifference
between 8,,,
and thegrainorientationof
member 3 and 8, isthedifferencebetween
Bl-2-.? and thegraindirectionofmember
4;
(iv)Continue
as in(iii)untiltheallowablebolt
design value in the plane between members
1 and i has beendetermined;
(v)
i-
For symmetrical joints inwhichmemberson
either side of the center member
are oriented
in the same direction and carry equal load to
the connection, only bolt design values for the
shearplanesbetweenthe
outer memberand
the center member on one side of the connection need to be evaluated.
(3) Determinetheadequacyoftheconnectiondesign
by checking the resultant loads in each shear plane
(P,, PfA2, Pl,4.t ... P,2 - 7 i) against the allowable
boltdesignvaluedetenhided
for that plane.
The foregoingprocedure is illustrated inExample
adapted from an earlier
reference
C8.4-1, which
is
(179).
8.5-PLACEMENT OF BOLTS
8.5.1-Terminology
8.5.1.2 For a joint in which one member is loaded
at an angle to theboltaxis(seeSpecificationFigure
8B), enddistancerequirements
are expressed in terms
ofsheararea.Shear
area for sucha joint isdefined
as thetriangular area in thethicknessplaneofthe
member
which
is
enclosed
between
the
tip
of
the
member and the centerline of the bolt (see Figure
8B).
This shear area for the angled member is compared to
theshearareaofa
joint inwhichbothmembersare
loaded perpendicular to the bolt axis (members parallel
toeachother)
and whichmeetenddistancerequirements. The equivalent
shear
area for the
parallel
member joint is
the
product
of
the
required
end
distanceandthelengthofthebolt
in themember.
The useofequivalentshear
areas to checkend
distance requirements in members loaded at an angle to
theboltaxiswasintroduced
in the 1986 edition.The
Bolts
109
NDS CommentHy
methodologyis used to checkenddistances for joints
loadedparallel to grain in both tension(boltbearing
towardmember
end) and compression(boltbearing
away from member end) (see Specification and Commentary for 8.5.4.3 and ExampleC8.S-1).
8.5.2-GeometryFactor,CA
The geometry factor expresses the provisions in the
previousedition
for proportionate reduction of bolt
design value for less than full end distance or less than
fullspacing distance givenin the equation format of
the1991 edition. It should be noted that thelowest
geometry factor for anybolt in a joint applies to all
other bolts in that same connection, notjust totheend
bolt or a pair of bolts in a row. This is a continuation of the provisioninthe1982
and 1986 editions
whichrequiredreductionof
the full bolt designvalue
on a joint when less than full end or spacing distances
wereused.
The requirement of 8.5.2 that bolt design values for
multiple shear plane connections or asymmetricthree
member connections be based on the application of the
lowest geometry factor for any shear plane to all bolts
in the joint presumes that members are loaded in only
one or two angles to grain and that total joint capacity
is proportional to the number of shear planes. For
Example C8.4-1
Procedures to determine allowable designvalues
multiple shear bolted connection:
for a
Assume a seven member connection consisting of two compression diagonal web members (members 1 and 7), two
main bottom chord members (2 and 6), two tension
diagonal web members (3 and 5 ) , and a single vertical web
member (4)with total loads and orientations as shown in
the member force and configuration diagram of Figure A.
Themagnitude and direction of the resultant totalshear
force on each pair of shear planes (1-2 and 6-7, 2-3 and 5-6,
and 3-4 and 4-5)are shown in the load vector analysisof
Figure B.
Assume all members are Southern pine 1-1/2 inch thick and
that a 314 inch diameter bolt isbeingused. The angle the
resultant load in each shear plane is acting relative to the
grain direction of the members (from Figure B), the dowel
bearing strengths applicable to each member for each shear
plane (Table SA and Equation 8.2-7 of the Specification),
the allowable bolt design value for each shear plane (Equa-
tions 8.2-1 to 8.2-6) and the applied loadon each plane
(one-half the resultant vector loadsfromFigure
B) are
tabulated below.
Dowel
Bo1t
Angle of Bearing
Design Applied
Load
Shear Strength
to
Value, Load,
Plane Member (Fc@),
psi Ornu K8
lbs
lbs
1-2
8 , =O"
8, =39"
6150
4302
39"510
608
1.108
2-3
8, ~ 2 9 " 4900
8 , = 7'
6052
29" 658
1.081
656
3-4
8 , =54"
8,=0"
3597
54" 1.150
Assuming a
quate for all
544
C'' of 1.15 applies, a
3/4 inch bolt is adeshear planes in the connection.
n
Q)
.?
Y
0
'
8
=I
Y
-
1942 Ib s
4,390
"
U
F~gureA
1 10
Bolts
Member Fomm and Conflgumtion
96
6150
P, 11942
zyl
F~gureB Load Vactor Analysis
-
ADS Commentary
those connections in which members are loaded in three
or more differentangles to grain and allowablebolt
designvalues for each shear plane are evaluated (see
Commentary for 8.4), the geometry factor for each
member adjoining a shear plane
may
be
used to
determine the allowable design value on that plane for
an
individual
bolt. Although shear planes
may
be
assigned different geometry factors, all bolts intersecting
thesameplanes are assignedthelowest factor applicable toanybolt
in that plane.
8.5.3-Edge Distance
8.5.3.1 Minimumedge distance requirements in
Table 8.5.3 for parallel to grain loading of 1 . 5 0 or the
greater of 1 . 5 0 or 1/2 thespacingbetweenrows
for
@/Dgreater than 6, and for loaded edge - perpendicular to grain loading of 4 0 are based on early research
(183) and have been provisions of theSpecification
sincethe 1944 edition. The unloadededge - perpendicular tograinminimum
of 1.5D wasintroduced in
the 1971 edition as a goodpracticerecommendation.
Section 8.5.3.1 does not providespecificguidance
onedge
distance requirements for loads applied at
angles other than 0" and go", and on edgedistance
requirements for loadededge - perpendicular to grain
under less than full allowable bolt design value. Where
these conditions are encountered, thefollowingprocedures, based on long standing split ring and shear plate
connector design provisions (see Part 10 of the Specification), maybeused.
( 1 ) For angles of loading between 0" and 45", minimum edge distances (loaded edge) shall be based on
linear interpolation between 1 . 5 0 and 4 0 , or
EDLL= 1 . 5 0 + (A)(2.5D/45)
(C8.5-I)
where:
EDl,
A
0
= loadededge
- minimumedgedistance
= angle of load
= bolt diameter
For angles of loading greater than 45", a loaded
edge - minimum edge distance of 4 0 shall apply.
(2) For angles of loading greater than 15", a reduced
loaded edge - minimum edge distance not less than
2 0 may be used if the bolt design value is reduced
proportionately tothereduction inedgedistance.
For perpendicular
grain
to
(90") loading, (2)
provides for a loaded edge - minimum edge distance of
notlessthan
2 0 when the full boltdesignvalueis
reduced 50 percent or more. For a load acting 30" to
the grain, the required edge distance for full bolt design
value from (1) is 3.20. This edgedistancemay
be
reduced to not less than 2 0 if the fullboltdesign
valueisreduced 37-1/2 percent or more.
8.5.3.2 The @/Dequations for determiningminimumedgedistancerequirements
for parallel to grain
loading have been added to the Specification to clarify
thespecific ratio beingreferenced.
It is to be noted
that the ratio of thelength of bolt insidemember
material to bolt diameter, tS/D,is based on the total
thickness of both wood side members when connections
ofthree or more woodmembers are involved. For
connections involving metal main or side members, only
the P / 0 ratio for the woodmembers are considered
for determination of edge distance requirements.
8.5.3.3 Avoidance ofheavy or mediumsuspended
loads below the neutral axis of a beam was introduced
as a good practice recommendation in the 1982 edition.
This recommendation was added to the Specification as
a resultofseveral
reported fieldproblemsinvolving
gluedlaminatedtimberbeamssubject
to a line of
concentrated loads applied through boltedhangers or
ledger strips attached in the tensionzone orat the
bottom edgeofthebeam.
Concentrated loads less
than 100 pounds and spaced more than 24 inches apart
maybeconsidered a lightload condition.
It should be noted that anyboltedconnection
which transmits a transverse load to a bending member
is required to be checked for shear in accordance with
3.4.5 of theSpecificationusing
a reduced depth, d e ,
equivalent to the beam depth (d)
less the distance from
theunloadededge
of thebeamto
the center ofthe
nearest bolt. When a connection is within 5d from the
end of the member, the actual shear stress based on de
is further increased by the ratio d/de.
8.5.4-End Distance
8.5.4.1 Enddistancerequirements
in Table 8.5.4
for full bolt design value parallel to grain are based on
early recommendations (183) and have beenprovisions
of the Specification since the 1944 edition. For tension
loads (bolts bearing toward member end), the minimum
enddistances for fullboltdesignvalueof
7 0 for
softwoods and 5 0 for hardwoods were established by
test. For compression loads (bolt bearingawayfrom
member end), the minimumend distance for fullbolt
design value of 4 0 was based on the minimum spacing
for fulldesignvalue
for bolts in a row (183). The
limit for fullboltdesignvalue
for perpendicularto
grainloadingof
4 0 was introduced in 1962. Earlier
editions dependedontherequirements
for checking
BoJts
11 1
NDS Commentmy
shear at connections (3.4.5) to provide for appropriate
joint designs for perpendicular loading. Minimumend
distances for reduced bolt design values, limited to onehalfthose for fullboltdesignvalues,
were introduced
in the 1982 edition to provide for designflexibility.
theedges) that occur as the bolt bends(183).
The
practice of
basing
spacing
requirements
for joint
members loaded perpendicular to grain on the requirements for the other attached members also has been a
provisionsince1944.
Special note should be made of the requirements of
3.4.5 when bolted connections produce perpendicular to
grain loading in bendingmembers.
Use ofreducedspacingbetween bolts in a rowin
proportion to the ratio of the applied load to the bolt
design
value
for the attached members has been
recognized
since
1944.
The lowerspacinglimit
for
reduceddesignvalueof
3 0 for both parallel and
perpendicular to grain loading was introduced in1982.
End distances for angle to graintensionloadings
may be linearly interpolated from those for perpendicular to grain and tension parallel to grain tabulated bolt
designvalues.
8.5.4.2 The provisions introduced in1982 for use
ofreducedend
distances for boltedconnections when
proportionate reductions (geometry factors) are made in
designvalues are supported by earlyresearch(57,62,183)whichshowed
a linear relationship betweenend
distance and joint proportional limit strength. A subsequent study showed that a minimumenddistanceof
only 5D was sufficient to develop the full
proportional
limitload of Douglas-fir joints made with metalside
plates and loaded in tensionparalleltograin(162).
Other recent research further substantiates the adequacy
of the
end
distance requirements for bolted joints
loaded in both compression and tensionparallel
to
grain (144,15 1).
Reduced end distances less than 50 percent of those
required for full bolt design value (geometry factors less
than 0.50) are notallowed. The reducedenddistance
provisions for boltedconnections are similartothose
that havebeenusedwith
timber connectors since1944.
8.5.4.3 For membersloaded at an angletothe
boltaxis,end
distance requirements are expressedin
termsofequivalent
shear areas (seeCommentary for
8.5.1.2). Aswithend distance requirements for parallel
member connections, reduced shear areas less than 50
percent of those required
for full bolt design value are
not allowed. It isrecommended as goodpractice that
the distance between the bolt
axis
and the
inside
juncture of the angledsidemember
and themain
member (see Figure 8B of the Specification)
be at least
1.5D. ExampleC8.5-1illustratesthe
use of end and
edgedistancerequirements.
8.5.5-Spacing Between Bolts in aRow
8.5.5.1 The minimumspacingrequirement of 4 0
for bolts in a rowloaded at full designvalueparallel
tograin has been a requirement of theSpecification
since1944. This requirementissufficienttocoverthe
effects
of
nonuniform distribution of shear stresses
throughthethickness of themember (concentrated at
8.5.5.2 Reducedspacingsless than 75 percent of
those
required
for the full design
value
(geometry
factors lessthan0.75) are not permitted.
8.5.6-Spacing Between Rows of Bolts
8.5.6.1 Minimumdistancesbetweenrowsofbolts
for perpendiculartograin
loading in Table 8.5.6are
based on early research (183)
and have been provisions
of
the
Specification
since
1944.
These
requirements
relatethetendency
of thebolttobend
and cause
nonuniform bearingstresses and theresistance of the
woodbetweenrows
to resist splitting. It is for this
reason that staggering of bolts loaded perpendicular to
grainisdesirable(see8.5.7.2ofSpecification).
The minimum
spacing
between
bolt rows for
parallel to grain loading of 1 . 5 0 wasaddedtothe
Specification in1971 as a goodpracticerecommendation. Prior to the revision, this distance was considered
sufficientlycontrolled bynet sectionrequirements.
8.5.6.2 In computingthe @/Dratio for determining the appropriate minimum spacing between rows for
perpendicular
to
grain loading, the ratio for side
(@)of
membersisbasedonthecombinedthickness
both side members where three or more wood member
joints are involved(see Commentary for 8.5.3.2).
8.5.6.3 In the 1960 edition, a maximumlimit of
bolts paralleling the member
was introduced. The provisionwas made more restrictive
in
the 1991 edition by limiting the maximum
distance between outer rows of bolts on the same splice
with greater
plateto 5 inches.Althoughconnections
distancesbetweenthe
outer rows of bolts have been
used successfully in the past, the new criterion has been
added as a goodpracticerecommendationtoavoid
splitting that could occur in members at connections as
a result of restraint of shrinkage associated with drying
inservice.
5 inches between rows of
The limitation onrowspacingappliestometal
as
well as wood side plates, to members loaded perpendic-
NDS Commentary
Example C8.5- 1
A No. 3 Hem-Fir 2x4 tension web intersects a No. 2 HemFir 2x4 chord at a 30" angle, connected by a single 1/2-in.
bolt as shown. Check edge and end distance requirements
in the web and check the adequacy of the web to resist the
resultant tension force based on the allowable bolt design
value.Assume
C, = 1.0.Alsoassume
that theboltis
centered on the members 1.5" dimension.
actual shear area:
= (1'2)(xend
)
where xmd = 'Q /tan a
= (1/2)(3.5/tan 3OOX3.5)
= 10.61 in2 c 12.25 in2 ng
> 6.125 in2 ok
Since the actual shear area isbetween the minimums for
reduced and fulldesignvalues,thegeometry
factor, C, ,
must be calculated:
-+ + O S "
c,
actualshearm a
mioimum shear m a forfull deHgn
= 10.61/12.25 = 0.866
=
(8.5.4.3)
Allowable Bolt Design Value
For a1/2-in.boltinsingle
and Hem-Fir lumber:
2'
Check Distance
Edge
.-
Requirements
(8.5.3)
For parallel to grain loading
minimum
edge
distance:
= 1.5D = 1.5(0.5)
= 0.75in. = 0.75 in. ok
CheckDistance
End
Requirements
(8.5.4)
For the loaded end of the web use an equivalent shear area
for a parallel member with 'Q = 3.5 in.
minimum end distance (tension):
=70
for full
design
value
minimum shear area:
= (70x9 )
for full
design
value
= (7)(0.5)(3.5) = 12.25 in2
for reduced design value = (1/2)(12.25) = 6.125 inL
ular as well as parallel to grain, and to three or more
member connections occurring at truss panel points.
8.5.7-Multiple Bolts
8.5.7.1 It is to benoted that wheneveraconnectioninvolvestwo
or more bolts inarow,the
group
action factor, Cg, is applied to the design value for a
singleboltinaccordancewith
7.2.2 and 7.3.6 when
establishing the total allowable design value on the bolt
group.
8.5.7.2 (See Commentary for 8.5.6.1.)
shear with
fm
= $ = 3.5in.
= 660 lb/bolt
8.2A) (Table
= Z,/CDCgC,= (660)(
1 .O)( 1.0)(0.866)
(7.3.1)
= 572 lb
(loadactsperpendicular to bolt)
(3.8.1)
Tension in Web Based on
Allowable Bolt Design Value
= Z'/wsa
= (572)/(cos 30") = 660Ib
Check net section at bolt (critical)
Anef = (3.5)(1.5 - (112 + 1/16)) = 3.28 in2
For No. 3 Hem-Fir 2x4:
Ff = 300 psi CF = 1.5
F; = FtCDCF = (300)(1.0)(1.5) = 450
psi
(2.3.1)
fi = P/Anet= 660/3.28 = 201 psi c F; = 450psi
resultant tensionforce
Boltedwebconnectionsatisfies
distance and strength criteria
NDS edge and end
8.5.7.3 The difficultyofprescribinggeneralrules
for theplacement
and spacingofbolts
that would
coveralldirectionsofboltloading
and number of
members in the connection was specifically noted in the
1986 and all earlier editions. For such connections, the
Specificationimposedtherequirement
that thegravity
axisofallmemberspass
through thecenter ofresistance of the bolts in the connection if uniform stress in
main members and uniform distribution of load to all
boltsis to beassumed.
This criterion iscontinued in
the 1991 edition. If it is not possible to achieve
intersection of member gravity axes with the center
of
resistanceofthebolt
group, thedesignerhasthe
responsibility to fully evaluate and account for the
Bolts
_ _ ~~~.
~
ok
113
NDS Commentary
effects of theresultanteccentricloadingonboththe
load carrying capacity of,the members and the capacity
of theconnection (see SpecificationandCommentary
for 3.1.3).
114
BoJts