Help with calculating RSJ sizes

[nbritton]

Hi all

I need some help from someone who can assist with doing the appropriate calculations for the installation of an RSJ for a kitchen renovation in my house. basically there is an existing steel girder in at the moment quite low (about 1.9M from the floor) and I want to raise this to about 2.5M to give a level ceiling. It will be supported at either end on concerete pad stones with an overlap of 150mm at each end. The total span is 3.4m (inc the overhang) and there is 1 floor above.

Anyone able to assist

many thanks

 

[^woody^]

Just raise the beam then

 

[nbritton]

I have been advised that the existing beam is undersized and I need to show proof of suitability to building control via the calculations

 

[^woody^]

But if the existing beam has been there for many years with no problems, and you are not increasing any loading, then it can't be under size

 

[tony1851]

I have been advised that the existing beam is undersized

As a matter of interest (i) what is the actual cross-section size of the beam (ie total depth, and width across the flange) and (ii) who advised it is undersized ?

 

[nbritton]

current beam(s) are 157mm high and 157mm wide by 3.3m long - only a 100mm support on each end and it was a surveyor who said as far as he was concerned they looked inadequate, building control wont help all they say is they need to see calculations before they will confirm anything.

 

[tony1851]

That beam size is produced in 3 different weights; assuming it is the lightest beam (ie the 'weakest') and also assuming that the inner beam carries approx. 1.8m of floor + 2.7m 1/2 brick wall+1.8m of roof and ceiling, then it would be OK.
Unfortunately it is easy for the surveyor to dump a problem in your lap by saying he doesn't 'think' it will be OK, without actually knowing one way or the other - very frustrating.
Buiding Control will also not give an opinion - that's not their job. Ultimately the proof of the pudding is in the eating and the last poster hit the nail on the head. However, if you HAVE to prove it's OK for whatever reason (and I can't see why), all you can do is bite the bullet and pay a local SE to prove it.

 

[chapeau]

current beam(s) are 157mm high and 157mm wide by 3.3m long - only a 100mm support on each end

... and I want to raise this to about 2.5M to give a level ceiling. It will be supported at either end on concerete pad stones with an overlap of 150mm at each end. The total span is 3.4m (inc the overhang)

Surely that means the existing beam isn't long enough anyway? My experience with structural engineers in this situation is they will tell you verbally it's not going to fall down but will not necessarily put that in writing.

 

[nbritton]

Thanks - I am glad you see my predicament - the existing beam sits in the pad 100mm either side - I have been told this is not enough and should be 150 hence I have to replace the beam however once I take the old one out and replace it building compliance insist on seeing calculations confirming the new one is suitable.

I know I can pay and get an SE to do this for me but as the post suggests I am looking for help from anyone on here who can do these calculations with / for me.

 

[^woody^]

- the existing beam sits in the pad 100mm either side - I have been told this is not enough and should be 1500 .

Do you really mean 1.5m or 150mm

A bearing does not have to be 150mm and 100mm is normal in most situations. Who told you that it must be 150mm

 

[tony1851]

As I see it now, your problem seems to be not so much the size of the beam itself but the bearings. What is the size of the padstone (length x breadth x height)?
The critical area is the contact face between the underside of the padstone and the brickwork immediately below. The aim is to ensure that the padstone is big enough to spread the load over a large-enough area of brickwork so that the brickwork itself is not overstressed.
If you can work out the total load on the beam - both the weight of the structure (wall, floor roof etc)- and add an allowance for live load on the floor and roof, you can work out the load on the padstone (normally one-half the total load).
Knowing this, by simple division, you can work out the stress on the brickwork immediately below the padstone. ( =total load/area of padstone).
In the older text books there are published tables of the maximum permissible stresses allowed at beam bearings on brickwork of various types in different mortars.

 

[^woody^]

Maybe so, but the bearing stress and shear at the bearing of the beam will be exactly the same as it is now

The beam is only being moved vertically, so there is no way it can be inadequate if it has performed OK since installation

There could even be an argument that this is not a structural alteration, so nothing to do with building control

 

[nbritton]

The pad stone is 400mm wide x 225mm high and 10mm deep (approx)

 

[tony1851]

the bearing stress and shear at the bearing of the beam will be exactly the same as it is now

In fact, if he's moving the beam up by 600, he will be taaking out a considerable weight of brickwork, and the loading would be less than that now.

With a 400 long pad, 225 high (and presumably 100 wide, not 10), the spread of the load from the underside of the beam downwards at 45 deg through the depth of the pad would distribute the load over a good area of brickwork. If its OK now it would be more than OK when raised up (this is assuming, of course, that there are no lateral stability issues such as a too-slender pier)