Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Anchoring to concrete

[Subject Prev][Subject Next][Thread Prev][Thread Next]

        I'm not sure that I completely understand your question but I'll give it a try.  As I understand it you have a series of loaded base plates supporting fixtures (which could be light poles, or any of a number of other similar but independent devices) where the base plates are not connected together to form a statically indeterminate problem.

        My approach would be to analyse each one as if it were a reinforced concrete column with the gross concrete cross section equal to the fixture base plate and the reinforcing steel equal to the anchor bolt layout.  This would give (as I understand it, but not necessarily) a rectangular concrete area with two anchor bolts taking tension, T, and a concrete area resembling a Whitney stress block of dimensions equal to the width of the base plate, b, and the depth of the compression area, a, taking compression, C.  The plate is loaded with a load, W, and a moment, M.

        First, I would transform the loading, W, and M, into new loading, W' and M' such that W' was located at the centre of the compression loading, C, and the moment M' was adjusted as required (probably reduced slightly) to give the same structural effects as before the transformation.

        Now for the math:

W' = W, only the location is changed.

e = M/W

x = distance W was moved to be collinear with C

M' = M*(e/(e+x))

C = T + W

T = M'/jd (where jd is the distance from the anchor bolts to the centre of concrete compression, d-a/2)

        I would use ultimate concrete strength = 0.85f'c. I would not increase this based on partial area loading unless I was really desperate.  Also, I would not increase the allowable loading on anchor bolts due to concrete compression due to the Poisson effect of the load, W, (I think I would not do this even if I were desperate, besides, some codes may not even permit it).

        I'm a little bit paranoid about using using short headed anchor bolts to anchor important components to concrete.  If a component is really important I like to ensure that the anchor bolt load is actually transferred to the reinforcing steel not just to the concrete.

        I hope this is helpful, Nels.


H. Daryl Richardson

"Nels Roselund, SE" wrote:

A fixture anchored to concrete with 4 anchor bolts in a rectangular pattern is subject to overturning: two bolts are subject to tension, two are in compression.  Using ACI 318-02, Appendix D to determine the tension capacity of the bolts of a single fixture in tension, with the distance between bolts being 5-1/2", hef being 10", would you base the value of AN on 1.5 x hef each side of the bolts in tension, ignoring the bolts in compression? The installation I am working on has several identical fixtures in a line at 16" spacing sharing in connecting the lateral force to the top of a concrete shear wall.  Thus, the bolts in tension are at 16" spacing, with an intervening compression zone.  One approach that I have in mind, in figuring the tension capacity of the bolts in a fixture: to use 1/2 x 16" = 8" as the longitudinal dimension factor [instead of hef] in determining AN. However, this approach seems to ignore that the compression due to overturning will supplement the strength of the concrete in resisting breakout of the tension bolts, and it provides for no benefit for anchors of greater depth, which is counter-intuitive. Perhaps it would make more sense to treat this like a single installation with a line of tension anchors at 16" spacing.  This is better, but still allows no benefit for anchors longer than slightly over 10" How would you approach this? Regarding a previous question to the List, the effect of grout under a plate on the strength of an anchor bolt in shear, Section D.6.1.3 says, "Where anchors are used with built-up grout pads, the nominal strengths  of [a steel anchor in shear] shall be multiplied by a 0.80 factor".  I find nothing related to surface preparation or grout quality, or gap thickness, etc. Nels Roselund
Structural Engineer
South San Gabriel, CA