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Unformatted text preview: up of anchors
far from an edge. They include ec for
non-uniform loads that would result in
eccentric load on a group of anchors,
ed for close edges, c for cracking and cp
anchor that are not cast-in-place. The
second subscript covers the type of anchor R
S resistance mechanism, N for tension, P
for pullout or V for shear. Here is the
good news, for a concentrically loaded
group of cast-in-place bolts located away
from the edge of a slab that is assumed
to be cracked, all eight factors are equal
The least understood of the ψ factors
is ψec,N, the modification factor for
anchor groups loaded eccentrically in
tension (Appendix D Section D.5.2.4).
An engineer who his designing the
anchor boltsofor r a column base plate
that carries an overturning moment
and axial tension typically thinks of
eccentricity as the distance from the
centroid of the anchor group to the
resultant axial force. Unfortunately this
is not the eccentricity that is being
addressed in Section D.5.2.4.
Figure 1 shows an example of an
eccentrically loaded base plate. To
calculate the concrete capacity for this
example, apply equation D-5 of ACI
318 Appendix D. C
Equation D-5 Note that the 10 kips uplift load is
applied at a twelve inch eccentricity
from the centroid of the anchor bolt
group. Twelve inches is not the e´N to
be used in equation D-9 of ACI 318.
Also note that by assuming concrete is
cracked, all of the ψ factors except for
ψec,N are equal to 1.0.
If we assume that for a thick base plate
the applied load will cause the plate to
rotate as a rigid body about the left edge,
then a simple hand calculation can be E
R 3” EMBED 2” 4” e
g a m ® U
T Ncbg = (ANc/ANco) ψec,N ψed,N ψc,N ψcp,N Nb Figure 1: Eccentrically Loaded Plate. performed to determine the loads on
the anchor bolts (Figure 2). The 120
in-kip moment applied to the centroid
of the plate will result in 5.77 kips per
bolt on the right side of the attachment,
and 1.15 kips per bolt on the left side of
the attachment. The 10 kip tension load
is assumed t...
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This document was uploaded on 01/28/2014.
- Spring '14