AMERICAN FOREST & PAPER ASSOCIATION
American Wood Council
Engineered and Traditional Wood Products
Let’s discuss IRC’s wall requirements and talk at length about bracing.
Although the program will talk in some depth about the IRC’s prescriptive
requirements, it will also address general conditions which necessitate
bracing.
Copyright © 2004 American Forest & Paper Association.
All rights reserved.
1
3
This eCourse explains different construction types and the behavior of small
structures and structural elements under gravity, seismic and wind forces.
Principles and typology of lateral load resistance systems are discussed
including prescriptive braced wall lines as addressed by the IBC/IRC. An
introduction to engineered shear wall design, location, and inspection points
is offered. Throughout, this eCourse demonstrates how AF&PA’s new Wood
Frame Construction Manual addresses these topics for one- and two-family
dwellings.
dwellings
4
This eCourse seeks to provide some answers, along with background on the
reasons why. Listed here are the topics that we’ll explore in this eCourse,
beginning with fundamentals.
5
Why does the code require bracing?
6
Loads on the framing tend to make the framing move. Even vertical loads
such as snow loads can make the framing try to rack.
7
Most loads on a building
g are applied
pp
by
y vertical p
pressure or
gravity. Roof framing bears on wall framing which bears on the
foundation.
8
Dead loads is another term used often in the code and is typically used to
indicated the weight of the building materials themselves. However, it can
have wider meaning with static loads.
9
Live loads – another common term – are loads resulting from the use of the
building. This includes people in the building, furniture, and other non-fixed
features.
10
It snows in various regions of the country, and 30 to 70 psf ground snow
load provisions are included in the 2001 WFCM (more about that document
later). Snow load span tables automatically reflect the consideration of
unbalanced snow loads.
11
Lateral loads will also be applied
pp
to a structure. The most
common is wind. Seismic forces will also occur in many parts of
the country.
12
The Federal Emergence
g
Management
g
Association ((FEMA))
developed these charts for wind and seismic zones in the United
States (although they’re similar to ASCE 7 maps they aren’t).
Notice on this wind chart that while the coastal areas around the
Gulf of Mexico and Atlantic Ocean are in a high wind zone, so is
most of the central part of the country. This apparently is a
reflection of severe thunderstorms in the middle of the country
which can generate high straight line winds and tornados.
13
Wind loads can be resisted by
y design.
g Although
g most tornados
can’t be resisted by an economically feasible design, proper
bracing can lessen damage to structures that see a near-miss
from a tornado.
14
The Fujita
j scale is used to rate the intensity
y of a tornado by
y
examining the damage caused by the tornado after it has passed
over man-made structures. F-0 has light damage while F-5 has
incredible damage. Over 80% of all tornados are classified at F-3
or below.
The reason for this slide is to illustrate that different tornados pack
different punches. The majority of tornados are not the incredible
ones. A word of caution, however, these lower strength tornados
can cause horrific damage. We don’t intend to imply otherwise.
15
This g
graphic
p
shows the impact
p
of the tornado. Homes in the
direct path were completely destroyed while those a block or two
away received a variety of damage. The home about three
blocks away received minimal damage.
No one will argue that it doesn’t matter how we build a structure
that is going to receive a direct hit from an F5 tornado. It’s the
b ildi
buildings
on th
the periphery
i h
off such
h an eventt and
db
buildings
ildi
involved in less severe storms that will perform differently
depending on the construction.
16
This g
graphic
p
shows the impact
p
of the tornado. Homes in the
direct path were completely destroyed while those a block or two
away received a variety of damage. The home about three
blocks away received minimal damage.
No one will argue that it doesn’t matter how we build a structure
that is going to receive a direct hit from an F5 tornado. It’s the
b ildi
buildings
on th
the periphery
i h
off such
h an eventt and
db
buildings
ildi
involved in less severe storms that will perform differently
depending on the construction.
17
One other feature of high wind events is wind-borne debris that driven by the
wind can cause moderate to severe impact damage. During the La Plata,
MD, Tornado, blankets and sleeping bags were found tossed into trees and
power poles. In this example, a 3”x6”x 8’ long lumber section skewered the
second story exterior wall of a building, penetrating it by several feet.
18
Here is a p
photo of the p
path of an F5 tornado that went through
g
Oklahoma in 1999. Many homes close to the tornado path
survived this tornado. Was this by chance or luck, or did the
method of construction have something to do with it?
19
Proper design of wood structures to resist high wind loads requires the
correct use of wind load provisions and member design properties. A
thorough understanding of the interaction between wind loads and material
properties is important in the design process.
20
Wind-structure interaction is highly complex. Wind can induce a variety of
structural responses as a whole building, and on individual components and
assemblies, as seen here. Each of these responses needs to be checked
for structural integrity as part of the wind design process.
21
When wind is applied
pp
to one side of a structure,, it wants to p
push
the end wall and roof in the prevailing direction. In addition, the
wind wants to pull the opposite end wall. While this is occurring,
the foundation acts to hold back the walls. Hence, the wall is
subjected to most of the force.
22
Seismic loads arise from ground shaking which can be a result of many
causes. Earthquakes by far are the most serious and unpredictable seismic
load type. Other more predictable seismic loads arise from human-induced
activities.
23
The primary feature of seismic motion is that its magnitude varies with time.
The time variation can be very short, such as a sharp jolt, or longer, such as
a slow rumble. Moreover, the motion direction is typically random and
constantly changing. Such a behavior can be described in terms of a wave.
Using recorded seismic data, we can describe a seismic load mathematically
in terms of a wave function of distance and time.
24
Waves have three primary characteristics:
•amplitude (the magnitude of the wave),
•frequency (the number of complete wave cycles per second) or inversely its
period (the number of seconds per complete wave cycle), and
•duration (the time lapse of the wave).
25
Damping describes the decay rate of the wave amplitude as the wave “dies”
out. Friction in the wave generating system is an example that causes
waves to damp.
26
With respect to dynamic response, buildings offer three primary
characteristics:
•mass of the building or sub-assemblies,
•stiffness of the building structural system,
•and damping inherent in the building construction.
These can be simply modeled as the “lollypop” shown here. If the stick of
the lollypop is sufficiently thin (low stiffness)
stiffness), and the mass is pulled back
and released, the mass will swing back and forth in free motion. The free
sway motion can be described by the mass displacement wave shown here,
with measurable frequency. This simple sway mode is known as the natural
frequency of vibration. Mathematically, the sway motion equation takes the
form of a second order differential equation with respect to time.
27
In the motion equation, all the components of the dynamic structural
behavior are evident. The equation here is written in terms of linear
displacement, x, although angular displacement terms (not shown) and other
directional displacements may be present. Solution techniques for this
equation exist mathematically.
28
Solution of the motion equation can lead to a modal result: a series of
frequencies at which the structure will freely vibrate if disturbed. An example
of this solution can be heard when a guitarist uses fret harmonics (octave
pitches) to tune a guitar. In a building, the sway shape takes different forms
that correspond to the modal frequencies in the solution. Thus, a structure
can have a number of sway modes with associated frequencies of vibration.
29
Let’s put the whole seismic problem together now. Ground shaking occurs
with a certain acceleration, a, moving the soil under the building. As the soil
moves, the building mass wants to stay put due to its inertia, putting a force
on the structure equal to the mass times the exciting acceleration.
Eventually the mass moves, lagging the exciting acceleration, causing
further inertial forces to develop on the structure. This gets even more
problematic when the exciting acceleration changes direction, as the mass
wants to keep moving (through inertia) in the the original direction of mass
movement. This is sometimes referred to as the whipping force.
30
If the exciting wave characteristics match any of the building’s modal wave
characteristics, then resonance results when the exciting and response
systems vibrate in unison. Resonance is very dangerous since the response
system normally self-destructs due to its inability to cope materially with the
exciting wave. Hence, it is very desirable from a building design perspective
to separate building modal response frequencies from any potential exciting
frequencies.
31
WFCM Chapter 2 Engineering provisions are based on the ASCE 7-98
Equivalent Lateral Force procedure. Building masses/weights are calculated
and collected at floor plane levels. The seismic event base shear is
calculated from the seismic loads and distributed on the basis of weights at
each story as story forces. Finally, story shears are determined for each
floor by adding all the story forces above the floor of interest. The story
shears are the forces applied to the top of the lateral force resisting system
at each floor level
level.
32
Buildings move under dynamic conditions. Two principle movements are:
racking and twist.
33
Structures have horizontal surfaces that can be used to transfer loads
applied laterally to the structure. An inertial mass load can originate in the
surface and transfer the same way. Resulting shear forces develop across
the surface, with maximum values occurring at the supported edges of the
surface. These maximum “reaction” forces are the lateral forces that are
transferred into the vertical building elements below.
34
Building forms impact how lateral forces get transferred into the vertical
supporting elements. Here’s an example. A floor has a center of mass
located somewhere in it. The structural system below provides a torsional
stiffness that can also be centered somewhere within the floor plane. If the
stiffness and mass centers coincide, then the building will simply rack in the
direction of the applied lateral load. If however, the mass and stiffness
centers are displaced, the building frame will twist. The greater the
displacement the greater the twist
displacement,
twist. The diaphragm reactions transferred to
the top of the shearwalls can also become very large. Thus, good design for
lateral performance would suggest that centers of mass and stiffness be
kept in as close proximity to each other as possible.
This subject is important for a rigid analysis where the stiffnesses of the
system components are known
known. The WFCM 2001 assumes a flexible
analysis: flexible components that lend to a tributary area approach for the
loads.
35
There is another way - a technique that dates back to early human
inhabitation some 10,000 years ago. Archaeological findings prove the
theory of light and strong in known seismically active areas of the earth. The
theory holds that humans discovered early that heavy things fall down easily
when disturbed with catastrophic results. Light things are not disturbed
nearly as easily, and are much easier to support and be made strong. Thus,
the simple tent has become a common domestic structure to many peoples
of the earth in regions that are seismically active
active, even to this day
day. Wood
frame structures tend to fit this philosophy, mainly because of wood’s very
high strength-to-weight ratio.
36
Now let’s look at some strategies used in buildings to resist lateral forces,
primarily through wall framing methods.
37
Here’s a summary of what we’ve discussed so far.
38
Without being braced in some fashion, wood frame walls tend to rack in
response to loads.
39
Bracing – whether it’s to resist the day-to-day loads on the buildings and the
typical storms or whether it’s to resist high-wind or high-seismic events – is
critical to the performance of the building.
40
Here's an example of what can happen when the lower floor of a building
doesn't have adequate bracing.
This was a building that had 9 condo units, some of them 3-stories tall,
which collapsed in 40 - 50 mph wind. A structural collapse while under
construction isn't unusual because not all of the required bracing may be in
place when a storm strikes. But as you can see in this case, the exterior
walls were being bricked and framing was almost complete. If you look at the
unit on the left, you will see that the lower floor had mostly doors and
windows and very little braced wall area, indicating poor design.
41
There are fundamentally three ways to stop a frame from racking. We’ll talk
in some detail about the application of these methodologies, but all of the
bracing materials and systems involve some version of what you see here.
42
Let’s talk in some detail about the methods of providing bracing, starting with
the use of triangles.
Diagonal tension ties create a triangular geometry within the frame that in
itself, is a stiffening element. Compression ties are rarely effective, if at all.
Diagonal board sheathing, however, works in this mode.
43
This is one of the simplest ways of providing lateral resistance to a wall
assembly. However, let-in braces require a perfect and well connected fit in
order to work properly, which is often difficult to achieve. And, they cannot
provide the same capacity as a properly constructed wood panel shear wall.
44
Proper installation and connections are key to making this method work.
45
A very efficient way to brace walls, and one that was common years ago, is
to sheath the wall in diagonally oriented boards.
46
Triangular geometry is used profusely in the truss industry for the trusses
themselves, as well as the bracing of them in the context of an entire structural roof
or floor system. For peaked roofs, trusses are braced in three planes.
47
This is a photo of a demonstration assembly that shows the different kinds of
bracing. The ground bracing is in dark blue. Notice how the ground bracing is
braced with triangles. Also notice how the lateral bracing on the top chords (in
orange) line up with the ground bracing making the truss more stable.
Photo showing continuous lateral bracing (CLB) and the diagonal bracing
needed to restrain the CLB.
Diagonals brace the bracing. It shows rows of lateral bracing being tied with
di
diagonal
l bracing.
b i
Those
Th
diagonals
di
l should
h ld not be
b more than
h 20 to 30 feet
f apart.
48
Temporary bracing is one of the most critical issues facing the wood truss
industry when it comes to construction safety. This Truss Technology in
Building shows how critical diagonal bracing is when using short spacer
pieces for lateral bracing as is typical on construction sites today.
The WTCA Warning Poster completes the educational information that is
needed at the jobsite to install and brace trusses safely. For more
information on bracing contact WTCA or visit the WTCA website at
www.woodtruss.com.
49
For long term roof system performance the proper installation of permanent
bracing is a necessity. This TTB defines the rule of thumb approach for
trusses. Where possible, however, the Building Designer should provide a
permanent bracing plan.
WTCA also has a “Commentary For Permanent Bracing of Metal Plate
Connected Wood Trusses” that goes into more detail. For more information
contact WTCA or visit our web site at www.woodtruss.com
50
Shearwalls are a vertical building element that can resist lateral forces
applied at the top of the wall. In a wood shearwall, the panel perimeter nails
provide the bulk of the racking resistance through wood bearing and nail
deformation when the lateral external force is applied. Horizontal wall sliding
is resisted by nailing or other anchorage installed along the bottom of the
shearwall sufficient to resist the external lateral force.
51
Secondly, the perimeter-nailed panel resists racking through the resisting
action of the perimeter nails to the applied racking moment on the panel.
Here the nails do most of the work.
52
In order to make this concept work, panels must have a height-to-width
aspect ratio of less than 3.5 to 1. This ratio is sufficient to develop “racking
action” in the shearwall panel. Aspect ratio’s greater than this produce
cantilever beam action - a completely different behavior that is much less
effective in resisting lateral forces. The concept of aspect ratios is
incorporated into prescriptive bracing requirements, but isn’t specifically
addressed. It is, however, that basis for limits to minimum widths of various
bracing material that we’ll
we ll discuss shortly.
shortly
53
Although they aren’t part of the wall bracing systems, roofs and floors take
the loads from the walls and transfer them to the foundation. Aspect ratios
for roof and floors aren’t addressed prescriptively, but in engineered design
these elements, called diaphragms, have length-to-width aspect ratio limits
as wall as limits on openings.
54
In its prescriptive provisions the IRC refers to braced walls as braced wall
lines. In engineered design, however, the bracing is provided by shearwalls.
Shearwalls feature special nailing and hold-down connections designed to
resist applied lateral loads in shear and overturning. Minimum wall aspect
ratios apply in order to develop “shearwall action” as opposed to “cantilever
beam action” when the wall panel aspect ratios become very slim. Typically,
the closer to the minimum aspect ratio for a shearwall, the more dense the
nail perimeter nail spacing
spacing. In shearwalls
shearwalls, it is the perimeter nailing that is
the most effective in resolving the transferred applied forces.
A more convenient method is the use of shearwall systems: panels, or
entire walls. Shearwalls feature special nailing and hold-down connections
designed to resist applied lateral loads in shear and overturning. Minimum
wall aspect ratios apply in order to develop “shearwall
shearwall action”
action as opposed to
“cantilever beam action” when the wall panel aspect ratios become very slim.
Typically, the closer to the minimum aspect ratio for a shearwall, the more
dense the nail perimeter nail spacing. In shearwalls, it is the perimeter
nailing that is the most effective in resolving the transferred applied forces.
55
Diaphragms are usually horizontal surfaces that resist in-plane shear forces.
Nailing is more dense where the shears are highest – typically in the panels
around the diaphragm perimeter.
56
In the second general bracing methodology, the perimeter-nailed panel
resists racking through the resisting action of the perimeter nails to the
applied racking moment on the panel. Nailed horizontal boards with at least
2 nails on the same stud has the same effect, but to a lesser degree. Here
the nails do most of the work.
57
A third method absorbs the racking moment directly in the rigid joints in the
corners. This is called a moment frame since the rigid corners induce
bending moments in all the members near the rigid connections, so
indirectly, the frame members also resist the racking forces through flexure.
58
Proprietary systems such as APA Sturd-I-Frame try to utilize normal
construction
t ti – a couple
l off anchor
h b
bolts
lt and
d iintegrating
t
ti th
the h
header
d and
d
sheathing with nails. Notice the attention given to making the corners rigid.
Construction of these types of assemblies requires careful attention to
details.
Proprietary systems such as APA Sturd-I-Frame try to
utilize normal construction – a couple of anchor bolts and
integrating the header and sheathing with nails to
p the rigid
g frame jjoint.
develop
Note a few things that are important to this detail..
If the overall wall height is more than 8 feet (say you
build a cripple wall on top of the header), the panel joint
should
h ld ffallll within
ithi 2 ffeett off th
the mid-height
id h i ht off th
the b
braced
d
portion of the wall.
59
Remember this house? Notice the failure of the garage door header at the
corners. In dwelling construction, this type of wall design – an opening with
small braced wall sections on either side – is ideal for the application of a
moment frame.
60
Braced wall lines are employed on the building-level scale to develop lateral
resistance in two orthogonal directions for the structure.
61
Now let’s look at the prescriptive method in which the IRC addresses wall
bracing. The code approaches the subject by specifying where the bracing
is to be placed, what materials are acceptable, and what quantities of
bracing materials are needed.
62
Braced wall lines are walls made up of a series of unbraced sections and
sections of walls that are braced with acceptable materials in the required
amount. These braced sections are called braced wall panels.
The braced wall lines are required to be placed in both directions of the floor
plan and are required in each story.
Often the exterior walls will provide the required braced wall lines. However,
when the distance between exterior walls is too large, an interior wall is
required to be a braced wall line. The IBC is specific about the maximum
distance between braced wall lines being 35 feet. However, for some
reason the 2000 IRC is vague. The only mention on spacing is in Section
R602.10.11, which addresses spacing
g in high
g seismic areas. In the ’03
edition, the IBC’s 35 feet provision has been added.
Braced wall lines are permitted to have an out-of-plane offset of no more
than 4 feet in one direction, with a provision saying that total offset can be no
more than 8 feet. This would allow a wall to have a 4 foot offset in each
direction.
63
The concept in the IRC is to break the structure of the building into boxes
with a limited aspect ratio.
64
This is a very rough schematic illustrating the general code provisions for the braced wall
li concept.
line
t All exterior
t i walls
ll mustt b
be b
braced
d wallll lilines. IInterior
t i b
braced
d wallll lilines are
required if the spacing between exterior walls is excessive (greater than 35’ in the 2000 IBC
and in the 2003 IRC).
Individual portions of wall that contain bracing material are called braced wall panels.
An offset in the overall braced wall line of 4’ is p
permitted. As illustrated in the lower left
portion of the floor plan, the code will allow an offset of up to 4’ in each direction, meaning
that the bottom exterior wall could have been offset as shown by the dotted line.
Openings are permitted at the corners of the braced wall line as shown on the exterior wall
on the left. There is some confusion in the code, however, on this account. Section R
602.10.1 permits the first braced wall panel to be 12-1/2 ft from the corner. Then it says that
when the bracing begins more than 12’ from the end of the wall a designed collector is
required.
i d IIt’s
’ unclear
l
whether
h h this
hi was iintended
d d to require
i a collector
ll
off some sort ((what’s
h ’
termed a drag strut in seismic design) if the braced panel is placed in that permitted 6”
beyond 12’-0”, or whether the intent is for a collector to be required in all instances in which
the first braced panel isn’t at the end of the wall.
65
The code lists 8 general materials that are acceptable as bracing, and we'll
talk more about those in a moment. Not all of those materials, however, are
acceptable as bracing materials in all instances.
The seismic category or wind speed zone, as well as which story is being
braced, will determine whether a specific material can be used and how
much of the wall must be braced using that material.
66
Here are the 8 general bracing materials. The numbers that you see here
are the numbers that you'll see on a table that we'll talk about in a moment.
We’ll also talk in some detail about the various materials.
These are the materials that the code accepts outright. That's not to say
that there aren't other materials that will provide adequate wall bracing.
However, any other material must be addressed under the alternate
methods and materials provisions of the code.
67
Add sheathing
N ili pictures
Nailing
i t
Attaching the bracing materials properly is critical; none of the materials will
function properly if fasteners are improper (either too few or of the wrong
size). It’s important that care be given to compliance with these
requirements.
68
This is a relatively formal way to say that we’re not sure exactly what
resistance to lateral loads are being provided by prescriptive bracing. We
know from experience that it works under the limitations of conventional
construction. However, since the wall isn’t formally designed and it lacks
elements of a shearwall, such as connections to the foundation or floor, it’s
hard to quantify the resistance to any exact degree.
69
In a formal shearwall design, we can quantify the shear resistance in bracing
material; in fact, the code provides those numbers for everything but let-in
bracing. But because the overall resistance to racking in conventional
construction isn’t completely understood, we don’t know exactly what shear
resistance is being provided by the bracing material itself. Here are some
estimates of the shear strength of the 8 allowed bracing materials applied
according to the IRC. The widely varying numbers explain why different
materials must be provided in different amounts
amounts.
70
Now,, lets talk about the details of the various methods of bracing.
g
71
One of the bracing methods accepted in the code is 1x4 let-in bracing such
as you see here.
72
R602.10.3(1) calls for the let-in bracing to be no less than 45 degrees from
the horizontal and no more than 60. It also calls for the 1x4 to be let-in to
the top and bottom plates as well as the studs.
73
But let-in bracing will be effective only if it's installed properly. Here you see
an example of poor installation. The notches in the studs are so wide that
the 1x4 isn't being held tightly. If the wall tries to move the wide notches are
going to permit some racking before the 1x4 comes into contact with the
edge of the notches.
And it also appears that some of the nails may not be properly driven into
the studs. Notice that the nail head in the yellow circle appears to fall right
on the outside edge of the stud, meaning that probably only one of the nails
is providing any resistance.
74
As mentioned before, diagonal boards provide a very efficient bracing
material, but that method has fallen out of favor because of the availability of
panel products.
75
Wood structural panels are very desirable bracing materials, particularly
when used in lower floors of multi-story buildings or in buildings subject to
high lateral loads. Additionally, the use of these panels to completely sheath
a building will offset some other problems that we’ll talk about in a moment.
76
Keep in mind that structural fiberboard and particleboard must be
manufactured in accordance with standards referenced in the code. While
they might look like the materials used in cheap, short lived furniture,
compliance with those standards assures a long lasting structural product.
77
Gypsum wallboard is acceptable as a bracing material, but because it’s
brittle and easily crushed, it’s values are limited and longer lengths of it are
required to provide racking resistance.
78
79
Keep in mind that the prescriptively permitted stucco cited in code is
traditional portland cement stucco and not EIFS or other stucco-looking
material.
80
Because it has a rather soft surface, it’s important to prevent overdriving of
fasteners. If over driven, two problems are created:
1. If driven far enough, the amount of the nail shaft that bears on the wood
fiber to resist shear is less, lowering the shear resistance value.
2. Broken surface permits moisture get into the panel around the fastener,
creating decay
decay.
81
Table 602.10 in the IRC specifies the amount of bracing material that is
required and where the bracing is to be applied.
Here is the first of the table. Notice that it’s predicated on the SDC and/or
wind speed for the location in question and the story to be braced. Then the
acceptable types of bracing, identified by the numbers on the previous slide,
are listed. The final column talks about the amount of bracing panels
required and the location of the panels.
The lower seismic zones – A, B & C -- require the least amount of bracing.
Zones D1 and D2 require much more because of the intensity of seismic
forces. Where wind speeds exceed 110 mph, engineered shear walls are
required.
82
The site of the building must be reviewed for applicable Seismic Design
Category and Design Wind Speed.
The lower seismic zones – A, B & C -- require the least amount of bracing.
Zones D1 and D2 require much more because of the intensity of seismic
forces. Where wind speeds exceed 110 mph, engineered shear walls are
required.
83
Here we see the acceptable types of bracing which are based on which story
is being braced.
Notice that all 8 bracing methods are acceptable in 2-story buildings and in
the top 2 stories of a 3-story building. But the let-in 1x4 is not acceptable in
the bottom story of a 3-story building.
84
The table specifies the amount of bracing material that is required and where
the bracing is to be applied. Notice that for lower stories the overall amount
of bracing varies with the type of bracing material and with loads carried from
floors and walls above.
The higher floors require less bracing because they carry less load.
85
The table contains a provision that is in conflict with Section R602.10. That
section permits the first braced panel to start at a point 12 (or 12-1/2) ft from
the corner. The table says that the bracing is required at the end of the wall.
This appears to be a correlation error. The intent of the IRC drafting
committee was to permit the bracing panel to be placed some distance from
the end of the wall. This table was, in all likelihood, borrowed from some
other source and this “at the corner” statement was erroneously retained.
86
And there’s a more minor problem with the text of the table. As you see
here, the code seems to require both the use of Method 3 and the other
Methods. That’s obviously not the intent, but the use of “and” rather than
“or” makes it somewhat confusing for people who tend to read literally.
87
As previously stated, alternate bracing methods and materials may be
accepted. However, it’s important that their acceptance be based on the
knowledge that they will perform adequately when loaded.
88
The point to make here is that the provisions of Table R602.10.3 and
Section R602.20-4 -- which addresses minimum width of bracing panels -- is
going to mean that often narrow wall sections won't be acceptable as braced
panels in braced wall lines.
89
The code stipulates a minimum length of braced wall panel for most of the
materials. This means that the short sections of walls, even if they’re wood
structural panels (Method 3), can’t be counted toward the required amount of
bracing. This is an example of a built-in aspect ratio limitation similar to the
1:3.5 aspect ratio we talked about for engineered shearwalls.
90
However, the IRC permits narrow all sections if the building is sheathed
overall with wood structural panels.
91
Here’s a summary of that provision. This will permit narrower braced wall
panels, but keep in mind that there will still be some limitation on how narrow
they can be.
92
While we all know intuitively
y that a fullyy sheathed house
outperforms one with only minimal bracing units, we also know it
by experience. The house fully-sheathed with plywood in the
foreground performs much differently than this foam sheathed
homes in this 2003 high-wind event. This type of performance
difference was also confirmed by NAHB testing in 1998.
93
Do the braced wall line provisions apply to garage walls? Yes. If the narrow
walls on the sides of the garage opening can’t comply with the minimum
widths required for the bracing material chosen – even if the building is fully
sheathed – there are options permitted by the code. However, they are very
restrictive.
94
Here’s a summary of the alternate braced wall system provisions of the code
that were intended to be used in the narrow sections of wall on either side of
a garage opening.
95
But it may be that there are proprietary methods of bracing that will work in
the narrow walls by garage door openings.
96
APA – The Engineered Wood Association recently had a code change
proposal approved that will revise the alternate braced panel section of the
code to permit wall construction similar to what you see here. This will
permit a more user-friendly and less expensive alternative in lieu of what’s
currently in the code.
97
Although the prescriptive bracing requirements in the IRC address
construction in some high-seismic areas, wind is limited to areas with a
design wind speed of less than 110 mph.
If the building is located in a higher wind area it must be designed according
to ASCE 7, must comply with the provisions of SSTD 10, or it must comply
with the engineering-base prescriptive provisions of AF&PA’s Wood Frame
Construction Manual.
98
Diaphragms and shear walls are engineered systems designed to resist
lateral loads typically of higher magnitudes. These systems typically
require involvement of a design professional. In this segment, we’ll review
important features of these.
99
Shear walls are engineered wall bracing that are designed to withstand high
forces. Often these are used with design professional input.
100
Here are other conditions that will trigger the use of an engineered solution
such as shear walls.
101
In the design of irregular structures, guidance for designing irregular
structures as separate structures or inscribed is provided. Typically, the
inscribed technique can only be used for wind design. Splitting into separate
structures can be used for either wind or seismic design, and often is the
preferred method.
102
Now we come to Section 2305, Lateral Force Resisting System.
103
This section applies to the ASD and the LRFD methodologies.
104
Section 2305.1 contains these provisions.
105
The use of diaphragms to withstand lateral forces is an integral part of both
ASD and LRFD design methodologies. Section 2305.2 contains a deflection
formula that can be used for diaphragms.
106
Similarly, a deflection formula for shear walls, and other provisions for shear
walls, are found in 2305.3.
107
This is one of the helpful diagrams in the chapter, which helps to define the
height and width of shear walls and shear wall segments in walls with
openings. These definitions are used in the determination of the aspect
ratios for the shear wall
108
A shear wall (wall brace) is essentially a deep, thin cantilevered beam
projecting from the foundation that is subjected to one or more lateral forces,
such as those due to wind or seismic activity. Listed are the primary
components of a complete shear wall design.
109
The code provides two alternative methods for the design of shear walls.
There is the traditional method, in which force transfer around wall openings
is actually designed.
The other alternative is the perforated shear wall method, which is the result
of recent research about the shear capacity of shear walls where there is no
specific provision for transfer of force around the openings
110
The idea behind the perforated shear wall method is that even though
overturning restraints are not provided at the solid panels between openings,
there is still inherent overturning resistance which contributes to the capacity
of the shear wall overall. The openings, of course, do cause a reduction in
capacity. . . and this table gives the reduction factors based on the amount
and size of openings.
111
Reassessment of the inherent shear capacity of shear walls under wind
loads led to a 40% increase in shear capacity of Wood Structural Panels
(WSP) for wind design only.
112
The shear capacity of a shearwall segment sheathed on both sides to resist
wind loads only is additive.
113
In proof, APA tested 3 walls: WSP only, gypsum only, and a combination of
the two.
114
The results showed the shear capacity of each wall using traditional shear
capacities and procedure, compared to adjusted shear capacities and
addition of interior sheathing. Walls tested showed 2-3 times greater
capacity than the earlier methodologies allow for.
115
Perforated shear walls have reduced shear capacity from the traditional
segmented wall, but interior holddowns have been eliminated – a usually
beneficial feature.
116
Three sources were used in development and substantiation of perforated
shear walls, including Sugiyama, APA-The Engineered Wood Association,
Virginia Tech, and NAHB Research Center tests.
117
From these tests, correlation is very good and conservative.
118
Shear capacity is reduced using these effective shear capacity factors,
adopted into the Standard Building Code. Critical features in this table are
percent of full height sheathing, and maximum unrestrained opening height.
119
To demonstrate the effects of each of these three design methodologies.
First look at an example of the traditional shearwall using the unadjusted
shear capacities. Note that a dozen holddowns are required along the length
of the first floor.
The WFCM uses 60% of the dead load to resist wind uplift
uplift. In most cases
cases,
the wind uplift is much greater than 60% of the dead load, and will require
more than the dead load alone to offset the wind force component. For
seismic loads, the holddowns are conservatively sized in the WFCM to meet
the shear capacity of the shearwall.
120
WFCM 1997 Supplement Table 3B is used to determine shearwall capacity.
Assuming SPF framing with G=0.42, 8d nails, 15/32” Structural sheathing,
and nailing of 6”/12” and 4”/12” along the panel edges.
121
With increased shear capacities (40% increase) and summation of dissimilar
materials (100 plf for gypsum) the required sheathing has been reduced,
however there are still numerous interior holddowns required.
122
By using the perforated shearwall method the amount of sheathing has been
increased, however holddowns are only required at each end of the
shearwall.
The WFCM uses 60% of the dead load to resist wind uplift. In most cases,
the wind uplift is much greater than 60% of the dead load, and will require
more than the dead load alone to offset the wind force component. For
seismic
i i lloads,
d th
the h
holddowns
ldd
are conservatively
ti l sized
i d iin th
the WFCM tto meett
the shear capacity of the shearwall.
123
Using the perforated shearwall method, shear capacity is reduced using
effective shear capacity factors. Assuming an 8’ wall height, window
openings are H/2 or 4’ and door openings are 5H/6 or 6’8”. Interpolation is
permitted based on percent full-height sheathing in the wall.
124
Here are the wall assembly assumptions used for the development of the
preceding methodology. A cooler nail is also known as a drywall nail.
125
Finally, overturning may be a problem, especially for high aspect ratios.
These skinny panels usually develop high overturning forces at the bottom
corners of the walls that need to be resisted with the installation of special
hold-down hardware.
126
Back to the 2003 Missouri storms which fell into the F3 or less
category, our damage assessment team, which was made up of
members of the American Association of Wind Engineers,
estimated that 50% of the failures initiated with poor lateral
capacity of walls enclosing two-car garages, such as this one.
These are the types of storms where how a home is built can
make
k a bi
big diff
difference.
127
…so don’t forget the holdowns. You’ll find a handy table in 2001 WFCM
Table 3.17F.
128
Here’s a handy list for referencing design aids for anchorage against wall
sliding and/or uplift. Roof/truss anchorage are also included in the Table 3.x
list, among other assembly connections. Many of these tables will give a
connector capacity, or a connector spacing as a result.
129
Hold-downs are required to prevent a wall panel from overturning. Holddowns may also be used elsewhere to prevent uplift, and to tie the structure
load path together to the foundation. Typical calculations are provided for
hold-down connections in AF&PA’s LRFD Manual, Example 7.7-1.2.
130
The conventional construction bracing
g requirements
q
of the code
are required to handle lateral loads as determined by the
building’s site location.
Handling lateral loads in California is a whole different ballgame,
as can be seen from this residential construction detail. We have
lateral loads and the code rightly stipulates that our structures
need
d tto h
handle
dl th
them appropriately.
i t l
131
Floor diaphragm members must be properly secured at the diaphragm
periphery to properly transfer shear forces. These figures show details at
the foundation...
132
…while these apply to intermediate stories.
133
….and when the forces get large, the connecting hardware gets more
interesting.
134
Here is a glulam beam tension connection good for up to 75 kips.
135
136
When we get to the larger structures or higher loads, naturally engineered
construction will be referenced.
137
…and here are the referenced standards that will apply.
138
The IBC and IRC both reference engineered construction. The IRC
references the WFCM, which is engineered construction. The IBC also
references engineered construction, but much of the prescriptive provisions
of the IBC are still based on conventional construction. Some of the tables in
both the IBC and IRC have an engineering base. To clarify the scope of the
two documents: IRC = detached and attached one- and two-family; IBC =
multifamily, although some designers have used the lateral bracing
provisions for business and office occupancies
occupancies, as well
well. The limiting factor is
the 40 psf floor loading.
139
To aid designers, the WFCM 2001 Workbook (free from www.awc.org) has
been developed. It is a real design example using a real house in a real
location and the workbook provides tabulated calculations with complete
references to WFCM and blank worksheets for future designs of your own.
140
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