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Re: anchor bolts in concrete

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The action and failure mode for rebar and anchor bolts are quite different.  For a headed anchor bolt the assumption is that at significant loading the bond along the shaft is lost, since it is a smooth bar, and the load is completely transferred to the embedded head.  The load at the head is now transferred into the concrete as shear radiating up to the surface at approximately 35 degrees (old school was 45 degrees).  All things being equal with no edge distance effects or overlapping shear cones the failure will be a perfect (slightly truncated ) cone.  Rebar is deformed its entire length which creates significant bond along the surface.  For rebar placed into a slab like an anchor bolt (for comparison purposes) the initial load is transferred into the concrete along the entire shaft (not necessarily equal).  Near failure due to elongation the concrete near the surface will start to spall and work its way down the shaft as more load is applied (basic unzipping effect).  This is partly why the edge effects (etc.) are not as critical for rebar near the surface as for headed anchor bolts.

Thomas Hunt, S.E.
ABS Consulting


11/08/2002 07:38 AM
Please respond to seaint

        To:        seaint(--nospam--at)
        Subject:        anchor bolts in concrete

the code and pca equations for anchor bolt embedment for tension in concrete
seem to address shallow embedment (less that 24") in unreinforced concrete.  
these anchors are all affected significantly more by edge distance, group
affects and anchor spacing than rebar.  is the action of a deeply embedded
anchor bolt with a nut or deformed head at the embedded end significantly
different than a rebar and if so why?  if a 1" diameter bolt is embedded 36"
i am at the lap splice length for a #8 rebar.  if the bolt is embedded in a
wall or pilaster, laps vertical reinforcing and is enclosed by ties, it would
seem to act similarly to rebar.  does anyone have information on tis subject?

thanks in advance

paul franceschi s.e.