Joists span for falsework

[morganjoe]

My falsework question is for the design span length for 4x4 joists. The plywood will sit on top of these joists, and these joists will be setting on top of steel beam stringers (HP14x89). The flat slab bridge is fairly thick, and the stringers are fairly close together, so the joists are continuous over multiple stringers. It seems a little conservative to use the center-to-center distance between the stringers as the span length for the joists. You would be ignoring the entire flange of the stringer. It seems correct for larger timbers that are used as stringers, but the simple span rule seems more appropriate for short span joists.

The Beam Span section for Timber Beams in the Cal-Trans Falsework Manual states:

"For simple beams the span length is the clear distance from face-to-face of supports, plus one-half the bearing length required at each end.

For continuous beams the span length is the center-to-center distance between supports over which the beam is continuous. For end spans of continuous beams the span length is the distance between the center of bearing at the continuous support and the point of end support determined in accordance with the simple beam rule stated in the preceding paragraph."

Any thoughts on the correct span length for checking the bending stress and shear in the 4x4 joists?
Thanks

 

[SlideRuleEra]

IMHO, falsework has so many variables that it is best to be conservative. Here are a few reasons:
1. Workers may not space the members precisely.
2. Some members may be installed as simple spans because of job conditions or material availability.
3. Members may have been reused several times and not be as good as they were when new.
4. Variations in loading during concrete placement.

I suggest that you assume simple span, center-to-center span length. This can be compensated for by taking advantage of the 7-Day Construction Load Duration Factor that allows a 25% increase in allowable stresses.

Even if you use a few more members than the absolute minimum, it should not be a big cost item since they are normally reusable.

http://www.SlideRuleEra.net

 

[RWF7437]

Slide Rule Era has given you excellent answers to some of your technical questions. Now, pose yourself some liability questions:

Are you, or your employer, responsible for the design in case of failure ?

Can you sue CalTrans for giving you "bad" design criteria ?

How good is your E&O insurance ?

How smart is your attorney?

Even Perry Mason lost one case.

good luck