limitation of the concrete beam width 2

[h-badawy]

Dear all

is there any limitation in the codes regarding the concrete beam width (minimum or maximum limitation)?

 

[rapt]

Most codes do not really define what a beam is. That is left up to an engineers understanding of structures.

It is normally related to the depth as well. If the width to depth ratio is too high, then the beam is too wide to transfer the loads in it to the supports via shear. There ends up being secondary flexural action in the transverse direction at the support.

 

[KootK]

Some codes proscribe minimum dimensions for fire ratings. Mine says 140mm for width. That's pretty small 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.

 

[h-badawy]

i found only this item (see the attachment below) in the ACI-318 code but this item concerning by special moment resisting frames only

 

[JAE]

ACI 318, section 10.4 states that the maximum distance between lateral supports for a beam shall not exceed 50 times the beam width, b, which is the least width of the compression flange/face of the beam.

So this in effect limits the beam width based on span and lateral bracing.



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[h-badawy]

thank you JAE
but the sentence "which is the least width of the compression flange/face of the beam" not clear for me , would you please to give me more clarification.

 

[JAE]

What they are saying is that if you have a concrete beam with no slab on top (i.e. not a "T" beam but a simple rectangular shape) and that beam is spanning through space without any lateral bracing attached to it, then the maximum span would be 50b.

Your question is what they mean by "least width of the compression flange/face of beam". That is the width of the beam on its compression face (usually top face for a simple span gravity beam).

So if your beam is 12 inches wide and 24 inches deep, the maximum span is 12 x 50 = 600 inches = 50 ft.
If the beam varies in width - say the ends are tapered wider - then the "least width" is the one you use for the maximum span.

So using the L(max) = 50b criteria, ACI is essentially making a minimum width requirement (b(min) = L/50)



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[BUGGAR]

If it's reinforced, reinforcement clearances, stirrup bend radii, cover, etc. will drive beam width.

 

[h-badawy]

Thanks

so now only cases will be critical and governed by the equation cantilever beams and inverted simple or continues otherwise the compression width is very long because the b will be the width of flange.

 

[JAE]

This ACI provision doesn't happen to apply much at all in typical concrete structures since (as BUGGAR stated) the width of the beam needs to be a realistic size for reinforcing and this usually results in a fairly long "allowable" span - not to mention that most concrete structures are monolithic with the slabs.

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[rapt]

Jae's solution gives you a minimum width. The maximum width requires a good knowledge of engineering and a lot more detail than you have supplied to us. There is no simple maximum width.

 

[KootK]

Some codes will limit the transverse distance between stirrup legs in strut and tie designed beams in acknowledgement of the fact that compression diagonals don't stray too far from the ties that restrain then. This is an indirect limit on beam width of sorts. In effect, it turns very wide beams into a series of side by side narrower beams. It's a stretch, I know.

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.

 

[ahmedhegazi]

A side note , have a look on fire rating requirements .from memory it is part of IBC

my rule of thumb , width never less than 200mm (8 inches) for ease of construction placing reinforcement etc...