Steel beam size

[sami69]

Can someone confirm

Which of these beams is stonger and can take a heavier load
305X305X118UC or 254X254X167UC

My understanding is that the latter is heavier, 167kg/m and thus can take a heavier load

I have two seperate structural engineers who have spec each of the above beams for an extension. I want the one that can take the most load and is the stronger.

Why would I want 12 inch beam if the smaller 10 inch can take a heavier load assuming the supporting piers can hold either...

 

[noseall]

I have two seperate structural engineers who have spec each of the above beams for an extension. I want the one that can take the most load and is the stronger.

Why would I want 12 inch beam if the smaller 10 inch can take a heavier load assuming the supporting piers can hold either...

Why would you want TWO structural engineers?

 

[sami69]

long story - but I had to get two independent structural enginners to give me calculations for insurance purposes to replace a beam that was advised by another SE but turned out to be inadequate and had to be taken out.

 

[Aqua]

Not really understanding this, is the second beam an RSJ with the first being a UB?

On the face of it a 12 inch will take a greater load than a 10" but what on earth are you trying to hold up with 10 and 12 inch beams...? Full width of the house?

 

[tony1851]

Not really understanding this, is the second beam an RSJ with the first being a UB?

The two beam sections the OP is referring to are neither RSJs (they are hardly ever used now anyway) nor universal beams; they are universal column sections (UCs) which are 'H' shaped rather than 'I' shaped.

In terms of comparison of load-bearing capacity, both are pretty much the same, though the 305 has the edge. But 'strength' is not always the main criterion in domestic situations; deflection is often the governing factor. The 254 will deflect about 20% more than the 305, under equal loading, though that may not be significant in your situation.

IMO both SEs are 'correct' and there is little to choose. As a matter of interest, what section did the first SE suggest that was so hopelessly wrong?

 

[Aqua]

I see said the blindman

My interest in steel beams has always been t'other way round

 

[^woody^]

There is no such thing as "stronger" with steel beam design. Its all about being "suitable" to support the loads placed upon it.

 

[sami69]

I have an large 10m opening for sliding doors on a single strorey extension.
The original beam spec for the project was a single steel 203x203x60 UC grade 275

The structural engineer has obviously made a big mistake as when a five course two sided brick parapet was built on top of the beam it started to deflect. The beam was only holding up the parapet and 9x3 roof joists at the time.
Still to go was the rest of the roof - 3X98kg roof lights, asphalt, insulation etc. Deflection was about 15mm before the rest of the roof materials. The sliding door company have a tolerence of 7mm deflection on the completed roof.

After a short battle, the original structural engineer is going through his PI insurance to do the remedial work(9K). He has given a new beam spec of 254 x 254 x 132 UC grade S275.

I don't 100% trust him now and I appointed two other structural engineers to give me their beam specs for comfort and use in possible litigation if the SE carried on refusing to use his PI insurance. They have inturn given me two different beam specs. All 3 SE are telling me that deflection will be less than 7mm with their beam specified.

I just want to make sure that I have the right beam in place this time so want the one that will deflect the least..

So I have three beams that have been advised

254 x 254 x 132 UC
254 x 254 x 167 UC
305 x 305 x 118 UC

Assuming all three can be supported by the pillars at each end, I just want to know which one will deflect the least.
My simple logic was - the beam with the higher mass per kg (heavier)would be the one that would deflect the least... Is this not a correct assumption?

 

[noseall]

The two beam sections the OP is referring to are neither RSJs (they are hardly ever used now anyway)

so ubs are not much used then.... so do please say what has taken their place

Read the post again, he is saying RSJ's are hardly ever used.

Regards the OP. How have you calculated the heat loss in such a mahoosive opening? I bet it's like being in a goldfish bowl!

 

[^woody^]

.. Is this not a correct assumption?

No. The self weight of beam will influence it's deflection.

 

[sami69]

The extension is 6m deep and 12m wide and we are using top range glass etc to manage heat issues.

3 structural engineers, 3 answers.. what did I expect

What would you guys do in my position - which one for least deflection?

 

[^woody^]

I'd detail it, so that any deflection does not affect the frame.

 

[^woody^]

Or redesign the roof to remove some of the load

 

[^woody^]

Or specify a different type of beam

 

[tony1851]

I just want to make sure that I have the right beam in place this time so want the one that will deflect the least..

So I have three beams that have been advised

254 x 254 x 132 UC
254 x 254 x 167 UC
305 x 305 x 118 UC

For a given load, the deflection is proportional to the moment of inertia (I) of the beam (aka 'second moment of area').

Figures for I are given in the standard steel tables and are as follows;

254 x 254 x 132 I = 22500 cm^4
254 x 254 x 167 I = 29900 cm^4
305 x 305 x 118 I - 27600 cm^4

In this case the heaviest beam will deflect the least.

(On a 33ft span, it's hardly surprising that an 8" deep beam as originally spec'd would deflect that much).