Service Holes in Composite Beams

[ianrawson]

In the UK, when designing holes in the web of composite beams in accordance with the SCI publication, the moment capacity is obtained by the couple developed between the compression in the concrete and the tension in the steel.
Now, that tension capacity of the steel is limited to the area of steel below the hole. Which means that, giving an extreme example, if you have a 10mm hole 10mm up from the bottom flange then the tension capacity is based on the area of the bottom flange.
This seems rather unfair when there is potentially a lot of material above the hole which can carry tension.
Does anyone know of any other design methods which account for this additioal area - or does anyone have any comments?

 

[JAE]

I'm not sure how applicable it would be in the UK but there is a Steel Design Guide Series (Number 2) by the American Institute of Steel Construction that covers web openings in composite beams. (its not metric so you'll have to struggle with inches and pounds)

http://www.aisc.org

 

[JAE]

Actually, here is a more precise page:

https://www.aisc.org/publications/shop.asp?action=displayitem&show=one&id=221

 

[ishvaaag]

The

Guide to stability design criteria for metal structures 5th ed
Galambos

also gives guide for webs with both rectangular and circular holes.

 

[section]

I assume that you are referring to the SCI publication 068.

Naturally the example that you quote is not covered in this booklet.

However, what do you think of this method?

Calculate the moment capacity of the composite beam at the hole position assuming only the beam flanges and the concrete slab are acting.

Add the effect of the local bending moment (equal to shear force at opening x length of opening) across the opening assuming the moment is carried by the upper T-section only Steel and concrete slab).

Regards

 

Reply to 'section':
If you mean carry the tension, due to the moment, in the bottom flange only, then the moment capacity is limited by the area of the bottom flange, and hence the force the bottom flange can carry.
Ok, you get a large compression flange due to combining the beam top flange and the concrete slab, but it is limited by that beam bottom flange capacity.
What I am trying to recognise is, if the neutral axis is within the depth of the concrete slab, then how can we fully utilise the area of steel below the n/a to carry tension.

 

[ishvaaag]

A somewhat debatable appraisal ot the true moment capacity could be gained through compatibility of deformations sectional analysis. Of course this wouldn't apply to partial composite action, and it is debatable if even to fully composite action, due to some slippage being present. In any case, a so minor hole wouldn't make a difference for the applicability of such method if you dare.