Eccentric moment 4

[struggle66]

Hi Good day Everyone,

Question again As you can see, my beam B2 isn't supported on column concentrically. There is an eccentric moment. But if I design my slab with 9m & 7 m span as shown, do I still to include that eccentric moment in slab design. Because the accredited checker asked me to do so.

Thanks

 

[Trenno]

Are you able to provide some proportions here? Slab depth? Beam depth?

What is the torsion moment magnitude with respects to what minimum torsion ligs will give you?

 

[Trenno]

Are you taking account of when your thin slab reaches the edge of your beam, the stiffness and effective depth increases? Your code should be able to provide guidance on what span length you should take when designing one way slab systems.

 

[hokie66]

I believe you only need to design the one way slabs to span between the band beams as normal, then also make sure the slab can cope with the compatibility torsion.

 

[BAretired]

The eccentric moment P.e is shared by the column above, the column below and two haunched slabs according to their relative stiffness.

BA

 

[Agent666]

The only way to not design the slab for the moment that I would consider would be to prove that the beams and columns could deal with the 'equilibrium' torsion required for this scenario, i.e in simple terms slabs apply loads to the beams edges, this results in both bending and torsion in the beams, this is transferred to the supporting columns. If the beams and columns can be detailed for these actions then it works, if not then moment capacity of the slab is required for stability/equilibrium. I would imagine this might be quite hard to achieve in practice as designing for larger equilibrium torsions can result in a lot of stirrups being required at close centers depending on the size of the beams.

In general though, in this sort of scenario I would just put another beam in between the columns and design this in simple terms to take the moment generated. This beam will deal with the moment a lot better than a 'thin' slab. Don't know the exact proportions but this sounds like a much more direct load path all round, especially if your structural system of slab and beams needs to carry any significant lateral loads due to earthquakes or wind.

 

[KootK]

But if I design my slab with 9m & 7 m span as shown, do I still to include that eccentric moment in slab design.

I vote no. In general, I think that would be fine for the design of the slab. The span adjustment essentially is how you've dealt with the compatibility torsion. I would, however, consider the eccentricity in the design of both the supporting column(s) and the joint between the columns and slab band beams (one-way/punching). Those elements are more brittle and not able to redistribute load as well.

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.

 

[struggle66]

Thanks everyone

I am using 2D equivalent frame software. The software will consider the column as support and I can do exact model as per my sketch (model in the beam eccentrically).

This is not a band beam so no effective depth and stiffness increased.

My uncertainty is that if I design the 9 m span slab with eccentric moment which tributary area is part of the 9m slab panel, am I kind of double taking the loadings?

What if it is a band beam? What would be the considerations since band beam is part of the slab in the slab direction?

Thanks

 

[BAretired]

Adding a beam seems to be a sensible plan.

BA

 

[KootK]

My uncertainty is that if I design the 9 m span slab with eccentric moment which tributary area is part of the 9m slab panel, am I kind of double taking the loadings?

I agree, that would be redundant. Maybe it would be easiest to go back to the original spans and design for the eccentricity induced moment. The answer should be similar and then you can move on.

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.

 

[hokie66]

So are you now saying those are not band beams, but just strip lines of a flat plate?

 

[BAretired]

This is not a band beam so no effective depth and stiffness increased.

Why is it not a band beam? I thought that it was. What is a band beam? What is a slab band? Are they the same thing? I don't believe so, but perhaps others do.

BA

 

[KootK]

Curious. I've been assuming the following terms to be interchangeable:

1) Slab band (western Canada among other places).
2) Band beam
3) Continuous drop panel (Toronto weirdness).

I've been thinking that all three refer to the fundamentally two-way slab system in which drop panels are made continuous in one direction yet some aspects of design are one-way-ish.



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.

 

[BAretired]

KootK,
I define band beam in the same way you do, i.e. a wide flat beam thicker than the slab. Slab bands (according to my understanding) are not dropped; they are simply heavily reinforced areas within the slab. Post-tensioned flat plates are often reinforced with slab bands in one direction and uniformly spaced tendons in the other but the slab is of uniform thickness throughout.

If we are talking here about slab bands, then it makes no sense to talk about eccentricities because the band can simply be considered to be centered on every column, although not necessarily straight from end to end.

If we are talking about dropped band beams, then the eccentricity must be taken into account in the analysis because without some rather unusual formwork, the beams cannot be centered on every column.

BA

 

[struggle66]

Hi

BA
Thanks & sorry for my sketch. It looks like band.

What I meant is it is just beam (Not wide enough to be considered as band).I have this eccentric scenario in a few locations. And I just found out that at other locations they are wide enough to be considered as band beam (neither slab band nor extended drop pane).

Here is my understanding of band beam and beam. Rapt told me that before which I believe is logical.