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RE: F1554-55 anchor rods repair

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Charlie,

I assume that the AISC table for cold bending plate is for bending the material one time.  Is there, or should there, be a reduction since the anchors bolts will now be bent twice (first by the loader then second to straighten them out)??

Thomas Hunt, S.E.
ABS Consulting



"Carter, Charlie" <carter(--nospam--at)aisc.org>

09/07/2002 08:33 AM
Please respond to seaint

       
        To:        "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
        cc:        
        Subject:        RE: F1554-55 anchor rods repair



>...a loader ran over several F1554-55 anchor rods
>at two footings for a fixed base moment frame.  Two
>of  the rods were bent and a third was broken off.
>The anchor rods apparently have the weldable supplement,
>although we waiting on confirmation from the supplier.
>The contractor fixed them in the manner that they have
>used in the past.  The two bent rods were "slowly" bent
>back to vertical.  At the broken anchor rod, the concrete
>was chipped away from the bolt, the bent portion was cut
>away about 8 inches below the top of footing and the rod
>was threaded.  A coupler and threaded rod extension was
>then added.  We are trying to get information on the coupler
>and threaded rod extension from the supplier.
>Are the rods that have been rebent still usable?
>The inital bending may have formed cracks that will limit
>the strength of the rods and reduce ductility.

Possibly. I would look at the curvature of the bend and compare to the table
in the 3rd Edition LRFD Manual for bent plates (page 10-175). Note that
these radii tabulated differ from previous listing in older Manuals and were
updated based upon a relatively recent study done for AISI, FHWA and AASHTO.
You know the bend is in the direction of rolling for the rod, so that makes
for the best possible case. If the curvature is larger than the recommended
limit for a comparable plate strength grade (like A572 grade 55), which is
1.5t for up to 1 in., 2.5t over 1 to 2 in., and 3t over 2 in., it is good
evidence that cracking will not be a concern.

If the rdius was tighter, unknown or you are still concerned, you could have
the rods pull-tested to a load level that is comfortably beyond the
anticipated load (but below the predicted failure load) to ensure what you
have is OK.

>Do the couplers have the same ductility as the anchor rods
>or are they more susceptable to brittle failure although they
>may have been designed at 125% the strength of the rod?

Assuming the coupler used is sized to develop the strength of the rod, it
should be acceptable. Couplers will not provide ductility, but also, should
not be expected to. A coupler is a short device and ductility comes from
elongations over larger lengths. Said another way, even the most ductile
material has zero ductility in a zero gage length. But as long as the
coupler is stronger than the rod, the rod is capable of prviding the
ductility you had before it was broken.

Again for the coupled rod repair, you could pull test if you think you need
to.

Hope this helps.

Charlie