PART 4 SHEAR LOAD
Pryout Failure Mode ( Concrete Breakout )
Calculations A Nc
Calculations 318-14 Chapter 17 Provision Comments for PROFIS Engineering
A Nc
17.5.3.1 The nominal pryout strength , V cp for a single anchor or V cpg for a group of anchors , shall not exceed : ( a ) For a single anchor
V cp
= k cp
N cp
( 17.5.3.1a )
For cast-in , expansion , and undercut anchors , N cp shall be taken as N cb determined from Eq .
( 17.4.2.1a ), and for adhesive anchors , N cp shall be the lesser of N a determined from Eq . ( 17.4.5.1a ) and N cb determined from Eq . ( 17.4.2.1a ). ( b ) For a group of anchors
V cpg
= k cp
N cpg
( 17.5.3.1b )
For cast-in , expansion , and undercut anchors , N cpg shall be taken as N cbg determined from Eq .
( 17.4.2.1b ), and for adhesive anchors , N cpg shall be the lesser of N ag determined from Eq . ( 17.4.5.1b ) and N cbg determined from Eq . ( 17.4.2.1b ) ……………………………………………………
17.4 . 2.1 The nominal concrete breakout strength in tension , N cb of a single anchor or N cbg of a group of anchors , shall not exceed : a ) For a single anchor
When calculating the nominal concrete pryout strength in shear ( V cp or V cpg
), the parameter A Nc is a modification factor that accounts for the area of influence assumed to develop in concrete when a shear load applied to a single anchor or a group of anchors creates a prying ( i . e . tension ) action on the anchor ( s ). A Nc is calculated with the edge conditions and anchor spacing that have been input into the PROFIS Engineering model . The geometry for A Nc is defined by projected distances from the anchors that are in shear . The maximum projected distance from an anchor that is considered when calculating A Nc is limited to 1.5h ef , where h ef is the effective embedment depth of the anchor . Therefore , the maximum edge distance parameter used to calculate A Nc equals 1.5h ef and the maximum spacing parameter used to calculate A Nc equals 3.0h ef
. Using these limits for edge distance and spacing , and defining the parameter A Nc0 per Eq . ( 17.4.2.1c ), the value for A Nc will never be greater than nA Nc0
, where n corresponds to the number of anchors in shear . This limit is described below .
The figure below illustrates how A Nc is calculated for pryout when a shear load acts on a group of four anchors with fixed edge distances equal to c a1 and c a2 , and spacing parameters equal to s 1 and s 2
. Note that the maximum edge distance parameter used to calculate A Nc equals 1.5h ef
. Anchors spaced greater than 3.0h ef from one another would not be considered to act as a group with respect to that spacing .
N cb
= A Nc ψ ed , Na ψ c , N ψ cp , N
N b
( 17.4.2.1a )
A Nc0 a ) For a group of anchors
N cbg
= A Nc ψ ec , N ψ ed , N ψ c , N ψ cp , N
N b
( 17.4.2.1b )
A Nc0
……………………………………………………….. A Nc is the projected concrete failure area of a single anchor or group of anchors that shall be approximated as the base of the rectilinear geometrical figure that results from projecting the failure surface outward 1.5h ef from the centerlines of the anchor , or in the case of a group of anchors , from a line through a row of adjacent anchors . A Nc shall not exceed nA Nc0
, where n is the number of anchors in the group that resist tension . A Nc0 is the projected concrete failure area of a single anchor with an edge distance equal to or greater than 1.5h ef
A Nc0
= 9h ef
2
( 17.4.2.1c )
A Nc
= ( c a1 + s 1
+ 1.5h ef ) ( c a2
+ s 2 + 1.5h ef
) where : c a1 and c a2 are ≤ 1.5h ef s 1 and s 2 are ≤ 3.0h ef
For the example above , if c a1 = c a2
= 1.5h ef and s 1
= s 2 = 3.0h ef
, then
A Nc would equal ( 1.5h ef + 3.0h ef
+ 1.5h ef ) ( 1.5h ef
+ 3.0h ef + 1.5h ef
) = 36h ef2 . The parameter A Nc / A Nc0 would equal 36h ef2 / 9h ef
2
= 4 . Therefore , since the maximum edge distance parameter ( 1.5h ef ) and maximum spacing parameter ( 3.0h ef
) have been assumed , A Nc equals nA Nc0
, where n = 4 corresponds to the number of anchors in the group that resist shear .
It is important to understand that “ A Nc
” calculated for concrete breakout failure in tension is not necessarily the same as “ A Nc
” calculated for concrete pryout failure in shear since the number of anchors in tension may not be the same as the number of anchors in shear .
Reference the Variables section of the PROFIS Engineering report for more information on h ef
.
Reference the Equations section of the PROFIS Engineering report for more information on A Nc
.
276 NORTH AMERICAN PROFIS ENGINEERING ANCHORING TO CONCRETE DESIGN GUIDE — ACI 318-14 Provisions