Subject: Re: Capcity reduction for closely spaced epoxy anchor bolts
From: ad026(--nospam--at)hwcn.org (Paul Ransom)
Date: Thu, 25 May 2000 00:25:58 -0400
> From: "Lee" <le98(--nospam--at)telusplanet.net>
> concrete slab, octagonal with 20 feet wide from side to side and 2.5 feet
> Everything sounds ok except that we have to increase the number of anchor
> bolts from 4 cast in place 3" diameter to 12 epoxy bond 1.5" diameter.
> We are planning to use epoxy anchor bolts, with a bonding strength of 1800
> psi, As the bolt circle diameter is about 5 feet, the spacing of 12 anchor
> bolts comes to about 16", which is less than recommended minimum bolt
> spacing 12*bolt diameter=18" (ACI SP130 Anchorage in Concrete).
> 1. Anchor pullout capacity = Bonding strength*contact area =
> 1800psi*3.14*1.5"*25" (embedment) = 212 kips
> Do I have to multiply this capacity by a reduction factor less than 1 as in
> the above reference?
> 2. Cone failure effective area: What is the effective area for calculating
> capacity based on failure of the concrete cone? The effective area is
> composed of 12 circles overlapped each others and some anchor bolts are in
> tension, others are in compression (self weight and moment caused by wind
> load) . Again, what is the reduction factor if i use an effective area of a
> single cone?
There is probably an ICBO evaluation (www.icbo.org) for the anchor that
you are proposing to use. It provides limiting capacities as well as
embedment and spacing reduction factors. The controlling condition may
not be "bond" strength.
Generally the ICBO evaluations require the anchor be embedded maximum
2/3 of the depth of the slab (20" max in your case) to develop full
strength as listed. This will affect your pullout cones.
Before you get concerend about overlapping pullout cones and reduction
factors, does the assembly work if you assume no overlap? How much loss
would be acceptable before it won't work? If embedment was only 18"
would it work? Bracket the solution and decide how narrow your analysis
has to be. Maybe you're barking up the wrong tree.
Paul Ransom, P. Eng.
Burlington, Ontario, Canada