rsj help

[happyman1010]

hi had some calculations done for a job
ie a beam over a 6mt opening this was done 12mth ago
SE calculated i needed two 254/146/37 rsj welded together on the flanges
looking at the job wanted to know can i use box section ie 250/250/8
would ask SE but he pass a way
thanks rob

 

[noseall]

You can only use the steel beam that has been specified for your job unless you can prove (via new calc's) that a different beam will be sufficient.

It may well be that your desired box section will be ok, but you need calc's and a qualified bod to underwrite it, otherwise B.C. will not pass it.

 

[tony1851]

Typical!; Why specify two beams to be welded together at the edges of the flanges? Did the SE not realize the cost of this??

203x203x52 would have been fine over that span.

 

[happyman1010]

well i was surprised on the size and beam configurations
its for a conservatory with a bifold door so think he was worried about deflection

 

[tony1851]

What is it supporting?

 

[happyman1010]

a lean to roof 3.89 wide by 7.3 mts long ay 18 degs
has a loading of
LL 0.6
DL 1.27

 

[tony1851]

If the clear span is 6m, then presume approx. 12 sq m of roof is supported by beam, a 203x203x52 will deflect about 0.006m.

Span/400 is regarded as a good standard to aim for, and that would be 0.015m, so the 203 beam would be well within the limit.

The beams the SE specified would deflect around 0.003m, which is very small. Depends what tolerance you need.

 

[happyman1010]

thanks for that looking at the calculations i have he say deflection is 4mm
and need to stay under 8mm, a pair of 203/133/30 would give me 7.5mm deflection which is too close for a 6mt span
what deflection would i expect with a 203/203/52 ?

 

[happyman1010]

sorry being thick yes see its 6mm

 

[happyman1010]

would i get near the 4mm if i went to a 154/254/73 uc?

 

[happyman1010]

sorry 254/254/73 uc

 

[tony1851]

what deflection would i expect with a 203/203/52 ?

about 0.0065, which might be too near your limit.

His dead load of 1.27 seems high for a tiled roof - how has he arrived at that?

 

[happyman1010]

well tile/slate is 9,47 kg sq mt
then batton 0.01 kn/sm
felt 0.01 kn/sm
rafters 0.2 kn/sm
ceiling 0.35 kn/sm

 

[tony1851]

Two of those loads are too high;

1. tiles/slates: at 18 degree pitch, you'll be using one of the interlocking concrete tiles or manufactured slates, right?
These rarely weigh in at more than 0,5kN/m^2.

2. plasterboard + Kingspan is usually 0.15kN/m^2.

I doubt your dead load is more than 1.0kN/m^2

 

[happyman1010]

yes your right on the plasterboard 0.35 is high but the tiles are 9,47kg sq mt his calculation is 0.7
how much does that improve the deflection?