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# Re: L in deflection criteria

• To: seaint(--nospam--at)seaint.org
• Subject: Re: L in deflection criteria
• From: <ErnieNSE(--nospam--at)aol.com>
• Date: Fri, 17 Jul 1998 21:30:46 EDT

```I have been using twice the cantilever span in my L/240(or L/360 for LL) since
I feel that this will make the criteria for allowable deflection for simple
span equivalent to allowable deflection at the end of the cantilever. And I do
this only to satisfy Code requirements and also plan checkers. But the real
issue is what this deflection criteria means or what is its purpose. We have
to use our judgement when we apply these deflection limitations.

For example, for a cantilever floor joist with a 20 foot interior span and a
10 foot cantilever with a  bearing wall in the interior span portion.
Depending on load combinations(DL+LL both spans or the combination that will
give us the worst cantilever span deflection, upward or downward), it is
posible to get a zero deflection (or a very small one). Does this mean that
this is OK per Code and go on to the next joist? I don't think so.

When I see long span members(interior and cantilever) I do the typical Code
allowable deflection check. But I don't stop there. Like what Roger said, I
check the curvature of the full length of the member to make sure that finish
materials at both top and bottom does not crack, or that the curvature is not
too obvious that  you can see it from the side elevation. Also, whatever
deflection you get from whatever load combination, try to get the feel of how
bouncy the joist or beam is going to be when live load is applied. I don't
think deflection criteria will give us a measure of joist bounciness.

I remember that Trus Joist have a table of allowable deflection for commercial
floor joist which depends on the span. As the span gets to about 20 feet or
more, the L/240 and L/360 could give you a false sense of security and peace
of mind but try going out to the field and walk on these long span joist.

My point is don't just follow the Code blindly. Use your judgement or other
people's experience and judgement.