## modeling RC beam - hand calculation

## modeling RC beam - hand calculation

(OP)

I have to design RC beams that are partly integrated into RC slab. I dont know how to model/calculate this since I dont trust FEM softwares for this kind of things since I almost made a huge mistake with something similar once upon a time.

RC beams are marked yellow and red line is a bearing masonry wall above beams. Design load on beams are approx. 50 kN/m.

Id model beam 1 and beam 4 as cantilever beams but Im wondering how much does beam 4 actually helps since load is transfered in direction of stiffnes (bearing wall above beams - red line)? Is it problematic that beam 1 and beam 2 are not in the same line? If I move beam 1 in the same line as beam 2 I cant design it as cantilever since it would be a simple supported beam. How would you model this?

RC beams are marked yellow and red line is a bearing masonry wall above beams. Design load on beams are approx. 50 kN/m.

Id model beam 1 and beam 4 as cantilever beams but Im wondering how much does beam 4 actually helps since load is transfered in direction of stiffnes (bearing wall above beams - red line)? Is it problematic that beam 1 and beam 2 are not in the same line? If I move beam 1 in the same line as beam 2 I cant design it as cantilever since it would be a simple supported beam. How would you model this?

## RE: modeling RC beam - hand calculation

Dik

## RE: modeling RC beam - hand calculation

## RE: modeling RC beam - hand calculation

Dik

## RE: modeling RC beam - hand calculation

Im waiting for Kootk to post some interesting thoughts about this one

## RE: modeling RC beam - hand calculation

It might.....then again it might not. Since you are not doing a FEA model you won't have those answers. You'd be stuck designing for a worst case torsion (via paper calcs)....or sizing the stirrups for the compatibility torque required by ACI 318 for a indeterminate system.

## RE: modeling RC beam - hand calculation

I don't agree with this assessment. If anything, any arching in the wall system is likely to draw load

disproportionatelytowards the cantilevered end of beam four. I see no reason to discount beam four's contribution other than, perhaps, conservative simplicity.It's only a problem if you're planning to make beams one and two continuous across beam four. It all three are to be cantilevers then it's not a big deal.

Wouldn't that be an improvement? That's exactly how I'd do it. Simpler to design, simpler to construct, and more reliable. Cantilevers have no redundancy. Although I suppose that multiple, cojoined cantilevers sort of do.

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.

## RE: modeling RC beam - hand calculation

## RE: modeling RC beam - hand calculation

## RE: modeling RC beam - hand calculation

Dik

## RE: modeling RC beam - hand calculation

## RE: modeling RC beam - hand calculation

Your sketch at the bottom right showing the intersection of beams 1, 2, and 4 is a really bad condition to reinforce... it should be avoided if possible. You will have a mess of stirrups due to the near fixed end moment torsion condition (the support at the intersection will not behave as a simple span, but, more likely as a continuous beam) as well as the shear from the reaction of Beam 1.

Dik

## RE: modeling RC beam - hand calculation

Yeah, that's a factor. I doubt you'll be flying any forms out past that beam so the easiest is probably just to make the cantilevered slab as thick as it needs to be, bearing or no. Another option is to just fatten the beam over the B1 section. Whatever produces the most economical formwork scenario in your market. Or maybe you go back to your original proposal. It's a game of trade offs of course...

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.

## RE: modeling RC beam - hand calculation

## RE: modeling RC beam - hand calculation

You make a fair point about end fixity. Moments in beams 1 and 2 cannot redistribute until something cracks. I was trying to convey the idea that they should be designed so that redistribution is possible and the torsional capacity of beam 4 is not required for equilibrium of the structure.

## RE: modeling RC beam - hand calculation

Kootk - i was thinking about this - to fatten the beam over the B1 section. That way beam 2 in 300 mm wide and beam 1 is 550 mm wide. it looks kinda wierd to me, but i think this is still better than my original system. In this case is beam 4 needed at all? maybe the best thing is to leave beam 4 but design beam 1 and 2 as there was no beam 4. So beam 1, 2 and 3 form continuous beam that is simply supported at both ends.

## RE: modeling RC beam - hand calculation

## RE: modeling RC beam - hand calculation

Designer's choice. If design the fattened beam as a continuation of the slab, then you don't have a torsion problem and you could live without B4 if you don't need it for deflection control etc. On the other hand, if you designed the fattened beam as beam in torsion, as I likely would, then B4 is pretty handle to have around for rectifying that torsion.

[quote greznik][/In this case is beam 4 needed at all? maybe the best thing is to leave beam 4 but design beam 1 and 2 as there was no beam 4. So beam 1, 2 and 3 form continuous beam that is simply supported at both ends.quote]

I like it.

Some questions for you:

1) without fattening the beam, how close would the interior side of your wall be from the exterior edge of the beam?

2) How thick would you have to make the slab to be able to handle 50 kN/m shear? I don't have a good feel in metric and haven't bothered to run any numbers.

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.

## RE: modeling RC beam - hand calculation

1) without fattening the beam, how close would the interior side of your wall be from the exterior edge of the beam?

150 mm - aprox. 6 inches2) How thick would you have to make the slab to be able to handle 50 kN/m shear? I don't have a good feel in metric and haven't bothered to run any numbers.

Well 160 mm thick slab has aprox. 60 kN/m of shear resistance which is fine, but I want to avoid that at all cost since there are openings in a bearing wall (windows) so at some parts of the wall the load on a slab edge actually exceeds shear strenght of a slab - concetration of loads between openings. To cover this slab should be 200 mm thick but i dont like that either since there will be too many levels of formwork (160 mm slab (inside), 200 mm (outside) + beams taht are partly integrated into slab. I think its easier to just fatten the beam.## RE: modeling RC beam - hand calculation

Im asking what is a proper reinforcement detail when width of the beam change in the middle of the span - like in this case from 'fattened' beam B1 to beam B2?

Is my sketch alright?

## RE: modeling RC beam - hand calculation

## RE: modeling RC beam - hand calculation

## RE: modeling RC beam - hand calculation