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02-18-2014, 06:03 PM   #1
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## PFC Beam Question

Hi all,

I am part way through a house remodel and the engineer I was using has disappeared ...... oh the joy! Luckily he gave me he structural drawings I paid for but I now have questions with no one to ask - hence this forum

Essentially I am looking to knock out a structural wall and but in a PFC beam. Span is 6m and the engineer has specified a single PFC beam 230mm by 75mm at 25kg/m bolted to the existing wooden floor bearer. Unfortunately this reduces the head height in the room below so I was thinking of using two 150mm by 75mm at 17.7kg/m PFCs with the existing wooden bearer sandwiched in between. That way I get an extra 80mm head height.

Does anyone know if this will have any problems? I am assuming there is a linear relationship in PFC strength but am unsure is this assumption is correct.

That is I assume 1 * 230 PFC with a webbing of 12mm and a flange of 6.5mm will be equivalent of 2 * 150 PFC which each have a webbing of 9.5mm and a flange of 6mm.

Any views?

02-19-2014, 04:06 AM   #2
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Beechworth - it doesn't work like that.
In your case, the critical factor will probably be deflection of the beam, and this has to be kept within certain limits.
All other things being equal, the deflection of a beam is directly proportional to a property of the cross-sction of the beam known as the 'moment of inertia'.
Put simply, the moment of inertia of a 230x75 is 2748 units; the moment of inertia of a 150x75 is 861 units (so the moment of inertia of two 150s will be around 1700 units).
This means that the two smaller beams together will deflect about 1.6 times as much as the deeper beam.
You need to check this with another SE.

 02-20-2014, 05:19 AM #3 Newbie   Join Date: Feb 2014 Posts: 2 Rewards Points: 10 Thanks for the quick reply! Great to have the required information. I have done as you said and need to go with two 180PFCs to get a moment of inertia just above what I can achieve with a 230PFC. Cost will be 150% over the 230PFC but the additional 50mm head room will be worth the costs. Thanks again.
 02-20-2014, 09:43 AM #4 Member   Join Date: Apr 2012 Location: UK Posts: 714 Rewards Points: 502 Intended to say that deflection was 'inversely proportional' to 'I', but you appear to have sussed that!

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