Widths for Continuous Beams

[AggieYank]

Thanks a lot ahead of time for your responses.

My question is this: Do continuous beams have to be the same widths, and if they don’t, is there a limit on the difference in widths. It seems obvious to me that there is a limit on this type of situation, but I am not familiar with any codes referencing it.

Example: I’m designing a concrete building. I have a joist that needs to be widened to around 20 inches. I’m using 8 inch wide joists at 6’2” o.c., 20” pans, with a 4.75” slab. So basically I have a run of 4 joists, the 1st is 8”, the 2nd is 20”, the 3rd is 8”, and the 4th is 8”. My spans are around 35’ for each. I would design the 20” joist as a simple span, but its carrying CMU, and so deflection just doesn’t work unless it’s a continuous span.

Possible solutions:
1) Make the joists continuous, with steel detailed as such.
2) Make the 20” joist single span and make it deeper as necessary.
3) Something else creative, such as widening the joists on each side, or one side of the 20” joist…..

Thanks guys/girls.

 

[AggieYank]

I reread my post and I left out a very important part. When I say a continuous beam/joist, I mean the top steel is continuous, carrying negative moment over the support. When I say simple span instead of continuous, I mean the beam/joist is designed as being a simple span, with minimum steel at the top at each end (not carrying negative moment), and enough steel at the bottom of the joist to resist wL^2/8, even though the joist is still in line with the others. Thanks again.

 

[JAE]

OK - this should probably be in the concrete forum - but here goes:

The idea of designing a simple span, when it is in fact a continuous system is a bit wrong - Letting the concrete crack at the negative moment area to have it behave as a simple span creates cracking right where the max. shear is. A little disconcerting.

The difference in widths simply makes for a difference in stiffnesses across the continuous spans. The 20" wide joist will be approximately 20/8 times as stiff as the 8" joist. You need to include this stiffness difference in your model to correctly get the shears and moments in each span.

 

[rapt]

AggieYank,

You can change the width, however there will be a transition zone at the start of the 20" wide beam where the full width does not operate.With the width differences you are talking about, assuming the centrelines of the beam are lined up, this transition length would only be about 12" or so.

It is normal in this situation to try to extend the wider beam into the spans either side by about .2 of the span as the negative moment area of the beam will be critical and will need that width in the joining span.

 

[LdsUtd]

how about running the top steel from the 20" joist into the slab on either side of your 8" joist.

thoughts / opinions on this?