Browse Forums Renovation + Home Improvement 1 Feb 19, 2014 12:06 pm Hi all, I am part way through a house remodel and the engineer I was using has disappeared with the number now disconnected and web site gone ...... oh the joy! Luckily he gave me he structural drawings I paid for but I now have questions with no one to ask Essentially I am looking to knock out a house stump and put in a PFC beam. Span will be 5.3m and the engineer has specified a single PFC beam 230mm by 75mm at 25kg/m bolted to the existing wooden floor bearer and house stumps. Unfortunately this reduces the head height under the house 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? Re: PFC Beam Question 2Feb 21, 2014 8:14 am While on paper ( or on screen in this case ) that sounds fine to me, the only person who can give you an informed answer is another engineer. Maybe shoot a PM to some of the structural guys who lurk on these forums from time to time. Stewie Re: PFC Beam Question 3Feb 21, 2014 11:39 am Thanks Stewie, I have done a bit more research and there was a problem with my assumption. I actually need to look at the moment of inertia of the 230PFC. Essentially the moment of inertia for the two 150PFCs added together is around 40% under what the moment of inertia is for a single 230PFC. However the moment of inertia for two 180PFCs added together is about 10% more than a single 230PFC. The cost and weight is more for two 180PFCs but I save 50mm in head height which is a more important consideration so that is the direction I will go in. Cheers and thanks for the reply. Re: PFC Beam Question 4Feb 21, 2014 12:45 pm Hi Beechworth, yes you are correct in your calculations. 2 x 180PFC's back to back will have greater capacity than a single 230PFC or 2 x 150PFCs you could possibly look at using the following as substitutes: 1 X 200UC46 (I = 46 x10 mm4) 1 X 200UB30 (I = 29 x10 mm4) Re: PFC Beam Question 5Feb 21, 2014 1:10 pm Hi there - thanks for these ideas. I had thought about just replacing the whole existing bearer with a UC or UB but thought it would end up being too deep. I hadn't considered leaving the wooden beam in an using a single 200UC or 200UB next to it. As it is I would still go with the two 180PFCs in this instance as I still get an additional 20mm head height but I will take a look to see if I could use two smaller UC/UB either side of the bearer ..... will report back what I find. Re: PFC Beam Question 6Feb 21, 2014 1:31 pm Ok - 150UB18's are out - the moment of inertia is almost the same as the 150PFCs. However two 150UC30's would have a moment of inertia 24% more than a single 230PFC but they are 150mm wide which just wouldnt work without adding in additional house stumps to take the load. I could use a single 150UC30 and a single 150UB18 together. Their combined moment of inertia is almost identical to a single 230PFC but I think I will still have the issue of not having a house sump to support them on...... Re: PFC Beam Question 8Feb 21, 2014 2:14 pm hi beechworth, I am an engineer and its not just the moment of inertia (MI) that matters. What actually matters is the Z (section modulus), which is MI/y. 2x 150 PFC would give you almost the same Z as that of 1 x 230PFC 230PFC, Z = 232.7E3 mm3 150PFC, Z = 111.2E3 mm3 I am not sure what factors of safety (FOS) was assumed when your engineer did the calculations. But my gut feel tells me that with the usual FOS values 2x150PFC should do the job. My Building Thread: viewtopic.php?f=31&t=62776 Re: PFC Beam Question 9Feb 21, 2014 3:39 pm wrexter what are your size constraints? Mainly around the 2 house stumps the beams will need to rest on and which currently support the wooden beam. It is a wooden stump at one end and concrete at the other. Both of which I need to cut in order to allow the PFC/UB/UC to sit on. The width of UC is such that over half the beam will be in mid air Re: PFC Beam Question 10Feb 21, 2014 3:40 pm navarch hi beechworth, I am an engineer and its not just the moment of inertia (MI) that matters. What actually matters is the Z (section modulus), which is MI/y. 2x 150 PFC would give you almost the same Z as that of 1 x 230PFC 230PFC, Z = 232.7E3 mm3 150PFC, Z = 111.2E3 mm3 I am not sure what factors of safety (FOS) was assumed when your engineer did the calculations. But my gut feel tells me that with the usual FOS values 2x150PFC should do the job. Thanks for that. Will do some further calculations - what would the Z of a 180PFC be? Re: PFC Beam Question 11Feb 21, 2014 4:22 pm 180 PFC ; Z=157e3 mm3. Two of these connected appropriately should give you more Z than a single 230PFC and should be fine for your job. My Building Thread: viewtopic.php?f=31&t=62776 Hello everyone, After some suggestions and ideas about how to put a concrete path around the drop edge beam area on our new build. We are required to have a concrete path… 0 12860 The distance between my DEBs varies from 4.1m at the narrowest to 8.1m at the widest. 5 27281 |