Structural calculation advise needed

[lesnick]

Hi All,


I am after some advice on the structural calculation I have for first floor beam support which replaces internal brick wall.


The calculations have been done a while ago and I can’t get in touch with the architect and the structural engineer who did it.


At the time it was done I didn’t look in the details, but now when it came to build it I noticed that the load assumptions are not correct for few reasons:


1) The beam supports floor joist to both sides of the house and the total floor space is around 18m2

2) As the project started it has been decided to make the beam slightly longer – 3.45m span

3) Due to above it now also picks up 8x3 joist (0.3m from the post) which supports block partition wall on the first floor.


With some assumptions I found online the load may look like:


1)Floor load (0.5kN * 1.4 + 1.5kN * 1.6 = 3.15kN/m2 – first floor factored dead+ factored live load)

3.15kN *18 / 2 = 28.35kN

2) Bathroom wall straight above the beam – 2.50kN x 3m x 2.2m = 16.5kN * 1.4 = 23.1kN

3)1st floor partition wall 2.5kN x 2.5m x 2.2m / 2 =6.8kN * 1.4 = 9.6kN


Total load – 61kN, or per meter of the beam – 18kN/m


I am not adding 1st floor celling load as celling joist are connected to the loft floor joist (loft conversion).


Without understanding full details of the below calculations I can see that provisioned 152UC37 and supporting 90x90x5 SHS post are well over dimensioned?

Like

Fv = 128.3 > 11

Mcx = 110 > 8

Mb = 77 > 8


Am I right assuming that extra load which the beam/post going to get well within the safe limits?


Any advice/comments much appreciated!

 

[^woody^]

I dont know what you have done there, just changed some figures?

You need it recalculating anyway as you now have a point load which alters things

 

[lesnick]

Woody, I have not changed the calculations. What you see attached are the original calcs I have, however I listed the load I believe is correct which is higher than originally calculation, but still several times less they are compared within the calcs.
I agree, load from the 8x3 is higher than from other joists (8x2) and it makes load distribution a bit uneven, however my question is with the scale of the over dimension can it be ignored?

 

[^woody^]

You can't really ignore a point load and assume that it will be catered for in some over design - and you can't just look at the shear and bending capacity values either and say that they are over dimensioned without working out the actual forces involved from the loading.

 

[tony1851]

I've just come in from the garden after spending a couple of hours on the sun lounger reading a book and with copious ice-cold drinks, so may have missed something on your figures, but - correct me if i'm wrong - you don't seem to have factored your loads up? 1.4 for dead load and 1.6 for live load?

 

[lesnick]

Tony, not sure what was in mind of the architect/structural engineer, but I can't see correct floor load in the original calcs.
For my assumptions I took 1.4kN (dead) +1.6kN (live) = 3kN/m2 for the floor load. It looks to me as overkill, but that's what I came across in a couple of docs I found.

 

[tony1851]

Not quite; floor live load (domestic) is 1.5kN/m², which has to be multiplied by 1.6; floor dead load varies - i usually take 0.5kN/m² for timber floors, which has to be multiplied by 1.4. Have you factored up (ie multiplied by 1.4) the dead load of the partition / wall as well?
This might be why the loads you see in the figures appear larger than you might assume.

 

[lesnick]

Have you factored up (ie multiplied by 1.4) the dead load of the partition / wall as well?

I haven't originally, but now updated the original post.

 

[tony1851]

To be fair, I'm not really sure of your loading situation so have only commented in broad terms. But looking at the beam and post they have specified, I can't see what issue you have with them and they look broadly OK.

Sure, some of the figures seem well in excess of the capacity of the beam but it is not that simple. Beams have to be checked for several criteria and for your 152-deep UC section, deflection is probably the critical issue. If you look at the figure for maximum deflection, it has come out at 16mm, while the maximum permissible deflection for that span is 20mm. A lighter beam, such as a 152 x 152 x 30, may well exceed the allowable limit for deflection and so would fail that test, even though it might be OK in bending stress/shear capacity/buckling etc.

Similarly for the column; an 8 or 9ft long 90 x 90 SHS would fail by buckling well before it reached its theoretical crushing strength. There has to be a reduction in allowable stress to allow for slenderness.

Unlike some nonsense from SEs you see posted here by perplexed DIYers, I would say the sections you have been given are not unreasonable, even allowing for the slightly longer span.

 

[^woody^]

But the calculation is now wrong. You can't get a beam designed for one situation with one set of loadings, and then just increase the span and alter the loads, and then hope for the best.

 

[lesnick]

Thanks Tony and Woody. Any technical input is appreciated.

 

[lesnick]

But the calculation is now wrong. You can't get a beam designed for one situation with one set of loadings, and then just increase the span and alter the loads, and then hope for the best.

You can if you understand the input/function/output. That's why I post here for an advise. For someone who knows the subject it's probably 5 min job to look at the calcs, changed input and judge the output. Furthermore, there will be many people who installed tens of beams in exactly same application in a standard UK semi - 6m wide with internal load bearing wall.

 

[tony1851]

@woody; I agree that the calculation is now invalid because of the altered span and loading condition, but it doesn't necessarily mean that the beam/post specified won't work - just a case of proving it.