Concrete Anchorage
By Jason Oakley, P.E.
New Anchorage
Design Requirements
•
•
•
•
•
•
•
Why Cracked Concrete?
Structural Requirements (2006 IBC)
Anchorage Design Provision (ACI 318 – App. D)
Design Example
Software that helps
Evolution of anchorage
Code Reports
1
Why Cracked Concrete?
Stress Concentration
2
Studies Show…
• Anchors lose capacity when in a crack
– Ex: a crack width of 0.4 mm, losses are…
are…
CIP=25%
expansion=40%
1.0
0.75
TC/TUCC 0.50
adhesives=50%
0.25
0.8
0.4
1.2
1.6
Crack Width (mm)
When & Where to Use
“Cracked Concrete” Anchors
• “Tension Zone”
Zone”
• Any application where seismic
forces are considered
– Regardless of location
(moderate/severe regions)
3
What makes an anchor work in CC?
• Tri-segmented clip design
– Grabs at least 2 sides of crack
– Distributes loads more uniformly
• redundancy
• 2 Teeth per segment
– Creates ledges when installed
– Provides ‘secondary’
secondary’ expansion as
crack cycles
The code has changed
• New Codes have adopted new
•
design provisions
The new test standard addresses
anchor performance in “cracked
concrete”
’07 CBC
’06 IBC
ACI 318 - App. D
ACI 355.2
AC 193a – mechanical/screw
AC 308 – bonded
4
Structural Requirements (2006 IBC)
CIP / Post-installed Mech. Anchors
• Per IBC 2006, Section 1911
–
–
–
–
Allowable Stress Design Approach
Applies to castcast-inin-place anchors only
Does not apply to postpost-installed anchors
Earthquake loads not permitted
5
CIP / Post-installed Mech. Anchors
• Per IBC 2006, Section 1912
–
–
–
–
Strength Design Approach
Covers CIP and postpost-installed mechanical anchor
Appendix D referenced
Does not cover screw and bonded type anchors
STRUCTURAL
CIP / Post-installed Mech. Anchors
--STRUCTURAL--
• Per 2006 IBC, Section 1908.1.16 – Concrete
– Appendix D modified for SDC C/D/E/F:
• Anchor steel must have ductile failure mode or
• Fixture must yield at anchor nominal strength or
• Design strength = 2.5 x factored forces
non-ductile
overstrength factor
STRUCTURAL
6
Steel Failure
L
Steel Failure
L
7
Steel Failure
L
Fuse
L
8
Fuse
Plastic Hinge
Mp
L
Use over-strength factor
Mu x 2.5
L
9
Anchorage Design Provision (ACI 318 – App. D)
Anchorage Design Provision (ACI 318 – App. D)
10
Anchorage Design Provision (ACI 318 – App. D)
Appendix D
Strength Based Design Approach
φNn > Nu
φVn > Vu
Design Strength > Factored load
11
ACI 318-05, Appendix D
• Design equations check 6
different failure modes
– Steel capacity
• Tension and Shear
– Concrete breakout capacity
• Tension and Shear
– Pullout/Pull-through capacity
• Tension only
– Concrete Pryout
• Shear only.
ASD method used safety factors
12
USD considers the spread of the data
13
Concrete Breakout Strength (Tension)
Concrete Breakout Strength (Tension)
100000
80000
60000
40000
20000
0
0
2
4
6
8
10
12
14
Concrete Breakout Strength (Tension)
What happens when we get close to an edge?
Concrete Breakout Strength (Tension)
15
Concrete Breakout Strength (Tension)
Not considered suppl. Reinf.!!
Concrete Breakout Strength (Tension)
Suppl.
Reinf.
(CIP)
Only 7 to 20% increase in CBS!
(PI)
16
Concrete Breakout Strength (Shear)
Concrete Breakout Strength (Shear)
Capacity doubles!
17
Concrete Breakout Strength (Shear)
Must use rational engineering judgment:
100”
100”
100”
5”
• (1) 5/8”
5/8” x 55-1/8”
1/8” emb. Î Shear = 6,800 lbs
• (2) 5/8”
5/8” x 55-1/8”
1/8” emb. Î Shear = 7,025 lbs!!
Treat as one anchor if adjacent
anchor is greater than 3 x hef away
Concrete Breakout Strength (Shear)
Flaw of ’05 ACI-318 App. D…..
Ψh,V (added in ’08 ACI 318)
See: “Shear Toward a Free Edge” paper by Burdette
Code fix: Ψh,V = [1.5 x ca1 / ha ]1/2
18
Concrete Breakout Strength (Shear)
Ψh,V makes a big difference......
ca1
No Ψh,V Î ca1 = 160”
Include Ψh,V Î 37.5”
Big difference?
Concrete Breakout Strength (Shear)
Btw, lightweight concrete factor is not correct…..
Should be 0.6…software uses
0.6, not 0.85
19
Simple Design Example
Strong Bolt
(ESR(ESR-1771)
Example (STB)
(8d) 5”
1 kip @ 45 deg.
hef = 4-1/2”
5/8” Strong-Bolt
CC / SDC F / STRUCTURAL
20
Example (Strong-Bolt)
TENSION
• Steel
•
•
– Ns = 1 x 0.167 x 125,000 = 21 kips
Concrete
0.5
1.5
– Nb = 17 x (4,000) 4.5 = 10.3 kips [basic]
– Ψed,N = 0.7 + 0.3 x 5 / (1.5 x 4.5) = 0.92 [edge]
– Ψc,N = 1.0 [cracked]
– Ψcp,N = 1.0 [does not apply to CC]
2
– Anc = (5 + 1.5 x 4.5) x (2 x 1.5 x 4.5) = 159 in [edge]
2
2
– ANno = 9 x 4.5 = 182 in [1 anchor; no edge]
– Ncb = 159/182 x 10.3 x 0.92 x 1.0 x 1.0 = 8.3 kips
Pullout
0.7
– Npn,cr = 5,200 x (4,000/2,500) = 7.2 kips
Example (Strong-Bolt)
TENSION
• Steel
•
•
– φNs = 0.75 x 21 kips = 16 kips [ductile steel]
Concrete
– φNcb = 0.65 x 8.3 kips = 5.4 kips [no supplementary reinf.]
Pullout
– φNp = 0.65 x 7.2 kips = 4.7 kips
Pullout governs
21
Anchor Categories
Example:
• Anchor A: Reference Test = 10000#
Reliability Test = 8600#
w = 0.012”
w = 0.020”
φ
8600#/10000# = 0.86
0.65
0.55
0.45
Example (Strong-Bolt)
SHEAR
• Steel
•
•
– Vsa = 9.7 kips
Concrete
– Vb = 7(4.5/0.625)0.2(0.625)0.5(4,000)0.5(5)1.5 = 5.8 kips
– Ψed,V = 1.0
– Ψc,V = 1.0 [cracked]
2
– AVc = 2 x (1.5 x 5) x (1.5 x 5) = 113 in [edge]
2
2
– AVco = 4.5 x 5 = 113 in
– Vcb = 113 /113 x 1.0 x 1.0 x 5.8 = 5.8 kips
Pryout
– Vcp = 2 x 8.3 = 16.6 kips
22
Example (Strong-Bolt)
SHEAR
• Steel
•
•
– φVs = 0.65 x 9.7 kips = 6.3 kips [ductile steel]
Concrete
– φVcb = 0.70 x 5.8 kips = 4.1 kips [no supplementary reinf.]
Pryout
– φVp = 0.70 x 16.6 kips = 11.6 kips
Concrete governs
Example (Strong-Bolt)
0.75 x φNcb = 0.75 x 4.7 k = 3.5 kips
0.75 x φVcb = 0.75 x 4.1 k = 3.1 kips
NDOF
2.5 x 0.707 x Pmax 2.5 x 0.707 x Pmax
+
= 1.2
3.5
3.1
Pmax = 1.1 kips > 1 kip …OK
Pmax
0.707Pmax
0.707Pmax
23
Reality Check Time
• CC/UCC = 0.65 to 0.8, call it 0.70
• Seismic: 0.75 factor (code: SDC C, D, E or F)
• Then you apply phi factor:
•
•
– 0.65 (cat. 1)
What is left?
– 0.7*0.75*0.65 = 0.34
If we assume 15% COV for CC tests N5%=0.49Navg_ult:
– 0.34/1.1*0.49 = 0.15*Navg_ult (S.F. = 6.6)
– Oh, and if it’s not ductile, divide by 2.5! (S.F. = 16)
Ultimate Strength Design (USD)
4”
6”
Anchor Software
4”
Nua
Tension
(lbf)
Shear
(lbf)
No Edge
3,000
1,900
4” single edge
3,000
900
4” double edge
3,000
550
Vuay
½” x 3-7/8” emb. STB
24
Evolution of Anchorage
Bonded Anchors
25
In the “old” days - ASD
****TENSION****
Dia.
Emb.
Edge
5/8
5”
7-1/2”
1/2”
6,700 lbs 2,100 lbs
Bonded
TCA
5/8
5”
4-1/2”
1/2”
1-3/4”
3/4”
2-1/2”
1/2”
3,300 lbs
3,700 lbs 1,400 lbs
Bonded anchor was dominant!
Bonded anchor evolution
(5/8” dia. x 5” emb. in 4 ksi NWC)
Old adhesive
New adhesive
TCA
(CC) **
ASD
TENSION
“prev.”
prev.” ESR
(UCC)
LARR
(UCC)
New Code/ESR
(CC)**
Critical Edge
8.9 k @ 7.5”
7.5”
6.4 k @ 7.5”
7.5”
2.1 k @ 12.5”
12.5”
1.3 k @ 9”
9”
Min. Edge
4.3 k @ 1.75”
1.75”
3.1 k @ 11-7/8”
7/8”
750 lbs @ 33-1/8”
1/8”
1.3 k @ 5”
5”
50% loss
19-D or 1911.2
**2.5 NDOF included
26
Double anchor example (AC 308)
What’s the ASD value for a “dry”,
“structural” connection in CC and SDC C
– F for (2) ½” ATR x 9” emb.?
ASD
= 0.65 x [(105 / 74) x 0.98 x 0.95 x 1.0 x
0.19] x 15,200 / 1.1 / 2.5
= 0.65 x 0.25 x 15,200 / 1.1 / 2.5
= 0.059 x 15,200
= 900 lbs
[bond strength governs]
Cracked Concrete
……compared with single anchor (AC 58)
1.75”
Take (1) ½” ATR x 6” emb. @ 1-3/4” –
What do we get?
Live pull test = 10 to 12 kips (steel fails 90% of
the time – witnessed by almost 1,000
eng./arch./plan checkers, contractors, etc.)
18,556 x 0.59 = 11 kips (coincidence?)
ASD (old) = 11 / 4 x 1.33 = 3,650 lbs
3650 lbs vs. 900 lbs.
@ ½ the edge
@ 3” less emb.
Cracked Concrete
@ ½ the anchors
27
28
Shear walls (ASD / 2006 I.B.C.)
4’
5/8” dia.
8’
“a”
Wood
“b”
CIP
Max Tension
8” emb
SETSET-XP
41.5”
8” emb
IXP
6-5/8”
5/8” emb
**ductile
a
b
Tension
3-3/4”
3/4”
12”
12”
8”
12”
12”
2.7 k
8.9k
1-3/4”
3/4”
10”
10”
8”
10”
10”
1.2k
2.7k
6-5/8”
5/8”
10”
10”
8”
10”
10”
1.7k
2.5k
**
Shear walls (ASD / 2006 I.B.C.)
4’
5/8” dia.
8’
“a”
Wood
“b”
CIP
Max Tension
8” emb
SETSET-XP
41.5”
**ductile
8” emb
IXP
6-5/8”
5/8” emb
a
b
shear
3-3/4”
3/4”
12”
12”
8”
12”
12”
290 plf
960 plf
1-3/4”
3/4”
10”
10”
8”
10”
10”
130 plf
295 plf
6-5/8”
5/8”
10”
10”
8”
10”
10”
190 plf
280 plf
**
29
Shear walls (ASD / 2006 I.B.C.)
Titen HD
2,500 psi NWC / CC / USD
1-3/4”
567 plf
(140 lbs/bolt!)
½” THD x 3-1/4” emb.
5”
3” typ.
Wood allowable = 1,040 lbs / bolt
30
2006 IBC Compliant Code Reports
• Current
– ESR 1771 (Strong(Strong-Bolt)
• Concrete / LWCMD
– ESR 2138 (pins)
• Concrete / Steel / CMU /
LWCMD
• Pending
– SET XP
• Concrete
– Titen HD
• Concrete
– Torque Cut
– ESR 1772 (SET)
• Concrete
• CMU / URM
– ESR 1056 (Titen HD)
• CMU
– ESR 1396 (Wedge(Wedge-All)
• CMU
Questions
31