Product Technical Guides : US-EN Post-Installed Rebar Guide - Page 30

Post-Installed Reinforcing Bar Guide
6.0 WHAT ’ S THE BACK STORY ?
6.6.1 DESIGN OF POST-INSTALLED REINFORCING BARS USING ANCHOR DESIGN CONCEPTS 17
The use of anchor design concepts for determining the embedment of post-installed reinforcing bars is discussed in the literature and is appropriate for systems that have also been qualified under anchor qualification provisions ( e . g ., ACI 355.4 , AC308 ). The use of anchor design concepts is relatively straight forward , but appropriate assumptions are required with respect to the embedment . For example , is achievement of bar yield required ? If so , should over strength be applied to the nominal bar yield stress ? The answers to these and other questions are situation dependent .
This approach can be subdivided into two categories :
Category 1 — Bars installed far away from edges in the face of walls , slabs , and foundations . In this case , post-installed reinforcing bars are assumed to be located sufficiently far from any edge to preclude splitting failure . Concrete breakout and bond resistances are likewise assumed to be unaffected by edge conditions .
Category 2 — Bars installed away from edges but still potentially affected by edge distance when determining the concrete breakout strength . The use of anchor design concepts is still possible in such cases , but is complicated by the edge proximity and requires iteration to find an optimal solution .
ACI 318-19 Section 17.5.1.2 specifies that the design strength for adhesive anchors in tension be taken as the minimum value of the steel , concrete breakout and bond strengths as determined for a given bar diameter and anchorage length ( embedment depth ), including the appropriate strength reduction factors , and limits the uniform bond model that is the basis for the design provisions to embedment depths between 4 and 20 anchor diameters . Beyond 20 diameters ,
120 the nonlinearity of the bond stress distribution can lead to a reduction in the usable bond stress at failure that must be assessed with engineering judgment .
The nominal steel strength in tension , N sa
, of a single anchor element ( e . g ., threaded rod or reinforcing dowel ) is determined per Eq . ( 17.6.1.2 ) of ACI 318-19 as the product of the tensile stress area of the anchor element and the anchor element nominal ultimate strength . ACI 318 notes that the ultimate strength is used because many anchor elements do not exhibit well-defined yield plateaus . As such , a limit of 1.9f y is placed on the value of the nominal ultimate stress that can be used in Eq . ( 17.6.1.2 ) in order to avoid yielding of the anchor element at service load levels . In the case of reinforcing bars , the nominal yield strength is not generally well controlled . Generally speaking , yield strengths may exceed specified values by 25 %. ASTM A706 Grade 60 bars are controlled such that the actual tested yield cannot exceed 78 ksi , and the tested ultimate cannot be less than 125 % of the actual yield . 18
In accordance with ACI 318-19 Section 17.6.2.1 , the nominal concrete breakout strength in tension of headed anchors is obtained as shown in equation 12 :
N cbg
where f ' c
120
Concrete compressive strength in pounds per square inch ( psi )
A Nco
9 ( h ef
) 2
A Nc
ψ ed , N
= A Nc ANco ψ ed , N ψ ec , N ψ c , N ψ cp , N k c λ a
( h ef
) 1 . 5
( lb ) ( Eq . 12 )
Projected area of theoretical breakout body based on critical anchor spacing of 3h ef and anchor edge distance of 1.5h ef
Factor that accounts for near-edge anchors that reflects the disturbed stress state caused by the presence of an edge
100 100
Stress [ ksi ]
80
60
# 3 bar
Stress [ ksi ]
80
60
# 5 bar
40
40
20
20
0 0 0.01 0.02 0 . 03 0.04 0.05 0 . 06 0.07 0 . 08 0.09 0.1 Strain [ in / in .]
Figure 46 — Stress strain curves for # 3 and # 5 ASTM A706 Gr . 60 reinforcing bars . 19
0 0 0.01 0.02 0 . 03 0.04 0.05 0 . 06 0.07 0 . 08 0.09
Strain [ in / in .]
17 ) Charney , F ., et al ., “ Recommended Procedures for Development and Splicing of Post-installed Bonded Reinforcing Bars in Concrete Structures ,” ACI Structural Journal [ 4 ]. 18 ) ASTM Standard A706 / A706M , 2014 , “ Standard Specification for Deformed and Plain Low-Alloy Steel Bars for Concrete Reinforcement ,” ASTM International , West Conshohocken , PA ,
2003 , DOI : 10.1520 / A0706 _ A0706M , www . astm . org . 19 ) Publication No . FHWA-HIF-13-038 , “ Laboratory Tests of Column-to-Drilled Shaft Socket Connections ,” Federal Highway Administration , June 2013 , p . 11 .
28 2021