# Beam Calculation basics

Discussion in 'Building' started by chapeau, 16 Oct 2010.

1. ### chapeau

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Hi all,

Just had some calcs done by an engineer for knocking through a cavity wall and I have a suspicion the resulting chunk of steel is for the benefit of his insurance premiums and not our interior design and ceiling height

So am I correct in assuming, for a straighforward hole-in-the-wall where the beam is only holding up what's vertically placed on it...

1) What really matters for the beam itself is the Second Moment of Area (I) calculation for the length of the beam, and that this number simply has to be less than the published value in the corus blue book for any beam.

( where I = 2.3 x total loading on beam x span ^2 )

2) If parallel flange channels are specified, two of them as it's a cavity wall, each beam will only have to support the weight on that leaf. Therefore the maximum beam depth will, in effect, be that beam which is supporting the outer leaf which is denser than the inner leaf. i.e. specifying each beam using weight of both inner and outer leaf is incorrect - or should I say overengineered ?

Thanks for any assistance.

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3. ### noseall

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WTF are you going on about?

If you have employed a pro to spec' up a beam and he has given you a steel size then you neither use it, or if you think you can do better go ahead and deal with the liability yourself.

Either way talking gibberish does not wash with me.

4. ### geraint

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so why did you ask for advice.. it seems that you know it all...

5. ### RonnyRaygun

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You know what they say about a little knowledge...

I'm not quite following your 'I' calculation, maybe you could explain it to me in more detail...

Second moment of area (I) is used in the calculation of deflections, and deflection doesn't matter as long as the beam isn't overstressed. For serviceability reasons BS 5950 gives span / 360 live load deflection as a maximum, although sometimes less may be desirable.

The plastic section modulus is the important one for a steel section. This will determine the maximum moment capacity of the section. If the maximum moment capacity is exceeded, the beam is overstressed and it fails.
However, if a beam is unrestrained - as it almost certainly will be stuck in your wall, it will never be able to develop that maximum moment capacity, as the top flange will buckle long before due to the compressive forces placed on it. So the engineer will design to a maximum buckling resistance moment (Mb).
Unrestrained tall, thin sections such as PFCs will buckle far sooner than squat sections such as UCs.

Also, don't forget the engineer has to design to a factor of safety of 1.4 for all dead loads, and 1.6 for all imposed loads, while also allowing 1.5kN/m^2 imposed for floors (if carrying any), and some more for the attic and snow loads on the roof.

If you still think he's overdesigned the sections, ask him to explain his calcs to you...

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6. ### ^woody^

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Put a Catnic in

7. ### roughcaster

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a Catnic out,
in out,
in out,
and shake it all about.

8. ### ^woody^

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# Pay the Engineer
And do not moan
That's what it's all about. HEY! #

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10. ### roughcaster

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Ohh, well that's a good one Woody,
Ohh, well that's a good one Woody,
Ohh, well that's a good one Woody,
That's wot it's all about. Night Night.

11. ### chapeau

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Quite agree which is why I asked - civilly - and thankyou for taking the time to answer. I have a reasonably good understanding of how forces 'work' but obviously no idea how these are using in practice by structural engineers.

The rest of you wags can go stand on the naughty step

This came from his calculations (and a bit of googling from me to try to understand)

Ah, I had missed that. Those figures are also available for beams.

Yes I am happy to do that, just want to understand the terminology/basics first.

Basically the engineer spec'd a 200mm PFC. If that can be made a 150mm beam it will not require the ceiling to be lowered any more than it is.

12. ### RonnyRaygun

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Couldn't see initially where the 2.3 had come from in the 'I' calc...but have worked it out now and I agree with his calcs. For a span of 3.5m carrying a UDL of 11.5kN/m you need the stated 'I' value of 1134 cm^4. (I make it 1128 but makes little difference...)

The maximum moment is correct as well. I make it 26.5 kNm.

So...I agree with you. A 150x90 PFC will do the job. It has an 'I' value of 1160cm^4 and, at 3.5m, Mb is 34.2 kNm. Depending on the end restraints, I often take a longer effective length, but even with an effective length of 5m a 150x90 section would still work.

It may be that the engineer wants to keep deflections low in order to prevent cracking of the masonry above and so has specified a larger section...a 200x90 gives a deflection of around 4.3mm compared to 9.5mm for a 150x90.

There could be other reasons that the engineer has specified the larger sections that you or I don't know about.

Talk to him...The Handbook of Structural Steelwork is a handy companion to the Blue Book...

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13. ### IJWS15

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Remember the safety margin.

14. ### RonnyRaygun

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Already accounted for if the calcs on Chapeau's earlier post are correct...

15. ### chapeau

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Good morning Ronny,

Again thanks for the assistance which was very helpful.

I spoke with the engineer, a very nice chap, and as expected he was very amenable to explaining his reasoning.

1) The connection detail between the two leafs is easier to form;
2) The 200 section will course better than a 150mm deep section if a gap is left for packing as per the sketch (200+25=one block depth + mortar)

He also provided an alternative solution using the 150 deep PFC.

We also plan to knock out a 8m wall holding up the back of the house and he will be coming round to look at the structure exposed when the builders are in for the 3.5 m beam upstairs. I suspect there will be less room there to argue for a beam that the mrs cannot see in the ceiling.

Thanks again Ronny.

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