Deflections in Steel Joists Under DeadLoad

[Saadat Umer]

I have composite slabs supported by steel joists. The slabs have been casted and steel joists are loaded with dead load right now. We were told by the Client's Engineer to survey existing floors joists. The survey revealed joists are deflected more at the sides and less in the middle. More deflections occur at halfway from the middle to the end and we have bolted splice plates at these locations. Now theoretically, the deflections should be highest in the middle and decrease towards the ends. Can anyone tell me some code or commentry which advise how much can be the difference if deflections at sides are more than the middle.

Regards.

 

[CBEngi]

If your floor joists are simply supported and your slab is one way, I find it almost impossible to believe the deflection isn't maximum in the middle. Even with splice plates at the quarter points the slab can't deflect back up after the splice.

Was the survey completed on the finished concrete surface and not underside of steel? I'm assuming so as top of concrete would be much easier. With this assumption I think the deflection you see is caused by the concrete finish not being "flat" and not some weird deflection in the steel below.

Something to consider when looking at your deflection results, is the the floor sloped for drainage.

 

[JAE]

There may be significant floor thickness variations that don't reflect the actual joist deflected shape across the span.

You should survey the bottom of the joist chords to get the actual joist deflections.

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[BAretired]

If the joists were cambered, a survey of the bottom of the joist chords will not reveal the true magnitude of deflections.

BA

 

[KootK]

Is the middle segment cambered? That's about the only way I can think to produce the profile in the joist that you've described.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[MotorCity]

If the max deflection occurs at the bolted splice plates, the bolts may not have been designed or installed as slip critical. If the bolts slip and the splice separates, that could be the reason for deflection.

However, since the slab has been poured and presumably troweled level, does it really matter what the deflection of the joists is as long as the slab is level? In other words, the joists may have deflected under the weight of the wet concrete during the pour and the slab just increased in thickness to make up the difference.