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[SEAOC] RE: Seismic Analysis/Retrofit Of Cap Beams

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F CAP BEAMS

In a message dated 96-02-14 10:34:39 EST, you write:

>Is anyone familiar with the seismic analysis and retrofit of cap beams.?  I
>am referring to those typically used in beam and girder type bridges.  The
>cap is usually supported on two columns and has a length of +/- 30 ft.
>
>Our seismic analyses of these indicate possible flexural failures (negative
>bending) in the vicinity of the columns.  The moment ductility demands have
>been on the order of 1.0-1.5 (Mdem/Mcap).  Is a demand ratio of 1.5 high
>enough to be concerned about?  Would a flexural failure of a poorly
>confined cap beam be as devastating (i.e., potential for structure
>collapse) as that of a poorly confined column?
>
>Assuming a retrofit is required, what options are presently being used?  So
>far, we have decided that post-tensioning is a viable solution provided
>service load capacity is still adequate.  I would appreciate any
>information on this matter.
>
>                      Bryan Nelsen  (U6420(--nospam--at)sandy.mdt.mt.gov)

Assuming non-collapse of the structure is the controlling acceptance
criteria, a flexural Demand/Capacity Ratio (DCR) of 1.5 for the bent cap
would be considered acceptable if this yielding mechanism did not lead to the
collaspe of the structure. Our experience indicates that performing a
displacement demand-to-capacity analysis currently provides the most rational
approach to determining whether a plastic hinge will lead to the collapse of
a structure. Moment-curvature relationships can be developed for the cap beam
and then the Bent can be displaced until collapse. The displacement capacity
at the point of collapse is then compared to the displacement demand from the
seismic event to determine if retrofit is required.

While performing this displacement analysis, brittle modes of failure
resulting from such actions as beam shear, insufficient development lengths,
and joint shear must also be investigated.

If retrofitting is required, post-tensioning is frequently used. It can be
applied externally or, as is frequently done in California, internally
through cored holes in the cap beam or in bolsters applied to the sides of
the cap beam. Coring through a 30' long cap beam would not pose a problem.
Mild reinforcement is also used in place of post-tensioning when appropriate.
It can also be placed in bolsters either placed on the side of the cap beam
or at the bottom of the beam.

MIchael H. Jones, S. E.
HNTB Corp. (714) 752-6940
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