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[SEAOC] SEISMIC ANALYSIS/RETROFIT OF CAP BEAMS

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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)
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