Post-Installed Reinforcing Bar Guide
6.0 WHAT ’ S THE BACK STORY ?
ψ ec , N
ψ c , N
ψ cp , N
λ a
k c
h ef
Factor to account for eccentrically-loaded groups Increase factor that accounts for uncracked concrete Factor to account for splitting hoop stresses Lightweight concrete adjustment factor
Efficiency factor for concrete breakout ( characteristic value , cracked concrete ); for adhesive anchors , this value is determined by testing in accordance with ACI 355.4 .
Effective embedment depth in inches
Similarly , in accordance with ACI 318-19 Section 17.6.5.1 , the characteristic bond strength , N ag
, of adhesive anchors in cracked concrete is determined as shown in equation 13 :
N ag
where
A Nca
= ( 2c Na
) 2
A Na
c Na
d a
cr
uncr
= A Na ANao
Projected influence area based on critical anchor edge distance , c Na
= 10d a ψ ed , Na ψ ec , Na ψ cp , Na uncr
1100
( in ) cr λ a πd a h ef
( lb ) ( Eq . 13 )
Characteristic bond stress in cracked concrete per evaluation in accordance with ACI 355.4
Characteristic bond stress in uncracked concrete per evaluation in accordance with ACI 355.4
Diameter of anchor element ( threaded rod , reinforcing bar )
Other terms are analogous to the expression for concrete breakout .
Strength reduction factors ( ϕ ) given in ACI 318-19 Tables 17.5.3 ( a , b & c ) are applied to the nominal steel , concrete breakout , and bond strengths , and the minimum value is compared to the factored design load N ua
. Additional design checks are made in accordance with ACI 318-19 Section 17.5.2.2 for adhesive anchors subjected to sustained tension loads . Where anchors are used in structures assigned to Seismic Design Categories C , D , E or F , additional requirements in accordance with ACI 318-19 Section 17.10 are placed on the anchor behavior .
According to ACI 318-19 R25.4.2.4 , splitting governs the behavior of reinforcing bars placed at minimum cover with no transverse or other confining reinforcement . Bars placed with increased cover and / or provided with transverse reinforcing are governed by pullout failure , but , it is noted , “… an increase in cover or transverse reinforcement ( beyond that assumed to ensure pullout behavior ) is unlikely to increase the anchorage capacity .” Note that in no case is concrete breakout anticipated , regardless of the density of bars placed in a specific volume of concrete . This assumption is likely predicated on the relatively low bond stresses associated with the development length equation ( see Figure 45 ). For post-installed reinforcing bars designed in accordance with anchor theory , however , the full tested bond strength of the adhesive is utilized and as such evaluation of both the bond and breakout capacities in accordance with ACI 318 is required .
Post-installed reinforcing bars can be designed by recasting the concrete breakout and bond strength expressions in ACI 318 into development length equations ; that is , by equating the strength associated with concrete failure or bond failure with the yield strength of the embedded bar and solving for the embedment . This may be particularly useful where cover ( edge distance ) is large but embedment depth is limited , such as the development of bars into the face of a wall .
Note : For additional information on this approach , see Charney , et al ., “ Recommended Procedures for Development and Splicing of Post-installed Bonded Reinforcing Bars in Concrete Structures ,” ACI Structural Journal , Vol . 110 , No . 3 , May-June 2013 [ 4 ].
Per Charney et al ., when a single post-installed reinforcing bar is installed in normal weight concrete away from edges such that the concrete break out strength is not affected by edge distance , the concrete breakout-associated embedment required to achieve yield in the embedded reinforcing bar may be expressed as shown in equation 14 :
l d , breakout
= 1.2
Similarly , when a single post-installed reinforcing bar is installed away from any edges , the bond-controlled embedment required to achieve yield in the embedded reinforcing bar can be expressed as shown in equation 15 :
l d , bond
=
0.3d b f y cr
A b f y k c
2 / 3
( in ) ( Eq . 15 )
( in ) ( Eq . 14 )
The design development length for this particular case may be taken as the greater of and , l d , breakout and l d , breakout
, as shown in equation 16 :
l d , bond
= max | l d , breakout ; l d , bond |
( in ) ( Eq . 16 )
Note : Eq . 14 and Eq . 15 may also be derived using the CSA provision wherey the ratio of the resistance modification factors can conservatively be taken as 1.0 ."
29 November 2022