Jacks under beam

[jrlester]

How many jacks required under a 15' 2x8 beam with 1/2" plywood in between?

[Jerry Peck - Codeman]

Not enough information to even make a guess, but ...

First, you should not be using jacks anyway (that term implies "temporary" support columns) as you should be using full bearing support columns designed for permanent installation and for the load carried (which is some of the information which is missing).

The following example is from the 2006 IRC, Table R502.5(2) Girder Spans and Header Spans For INTERIOR Bearing Walls (note that the table is addressing double 2x8, no 1/2" plywood between them, 1/2" plywood between them would help increase the overall strength and thus allow for a longer span, however, that is not addressed in the table, that would need to be addressed by other sources than the code or by a structural engineer, and the nailing pattern would also affect the outcome of the sandwiched girder/beam with the 1/2" plywood plate in the middle of it):

Presuming that the 15 foot span is the width of the building (width is perpendicular to the ridge), and that the girder/beam is supporting ONE FLOOR ONLY, a doubled 2x8 girder/beam has an allowable span (per that table) of 5'-9", presuming #2 grade lumber of Douglas fir-larch, hem-fir, southern pine, spruce-pine-fir.

That means the maximum span for that girder/beam would be 5'-9" between supporting columns ... under the stated presumptions.

[Jerry Peck - Codeman]