Universal beam question

[geraldthehamster]

I have a question about universal beam sizes. I appreciate this is something a structural engineer would need to calculate but just wanted to get a feel, for design purposes, as depth is an issue.

The beam would support joist ends part way under a flat roof, where a wall would be removed. From one side are 7x3 joists at 600mm centres, 3500 span. On the other, same joists, same centres, 1500 span. Roof is a warm flat roof (OSB, Celotex, OSB, EPDM).

Question – does anyone have a feel for the maximum span that could safely be supported by a 152 x 89 x 16, or a 178 x 102 x 19 universal beam, before a column was needed?

Or 152 x 89 x 16 doubled up?

Or any idea how I could calculate it?


Thanks

Richard

 

[geraldthehamster]

Put it another way. I've now calculated a load of 30kg/m (0.3KN/m). The total distance the beam needs to span is 5 metres. In principle what size do I need? I'm still playing with online calculators but I am not an engineer.

Cheers
Richard

 

[geraldthehamster]

According to this, a 150 UB should do it, which surprises me, so I've probably got it wrong.

http://engs-comp.com/unibeams/index.shtml

 

[jeds]

Beams normally fail in deflection before anything else but these small beams tend to fail in buckling when they get long. If you said a live load of 1.00kN/m² and dead load of 0.85 you'll have about 4.6kN per metre of beam. A 152x89x16 will span about 4m at that rate - a 178x102x19 will do about four and half.

 

[geraldthehamster]

Beams normally fail in deflection before anything else but these small beams tend to fail in buckling when they get long. If you said a live load of 1.00kN/m² and dead load of 0.85 you'll have about 4.6kN per metre of beam. A 152x89x16 will span about 4m at that rate - a 178x102x19 will do about four and half.

Thanks Jeds, much appreciated. I assume the 0.85 dead load per m² includes the two layers of OSB, PB and skim as well as the 7x3 joists? Is the live load allowing for snow, pigeons and so forth?

What if I were to double up 152x89x16 side by side? Would that increase the available span on the steels? I appreciate width doesn't add strength in the way that depth does. I need just shy of 5 metres.

Cheers
Richard

 

[^woody^]

You need to factor in that the load is not uniformly distributed

 

[jeds]

The live load allows for snow etc. Do you live below an altitude of 100m or in the South West? If so you could reduce the live load to 0.75, which will give you another 300-400mm of span. A dead load of 0.85 is an off the top of the head coverall for 99% of situations. Looking more closely yours is probably more like 0.65-0.75, in which case a 178 beam would comfortably span 5m. A 152 is still on the borderline though.

 

[tomfe]

That calculator presumes full lateral restraint, which is more like the beam cast into concrete.

 

[geraldthehamster]

That calculator presumes full lateral restraint, which is more like the beam cast into concrete.

There wouldn't be a lot of lateral restraint. The UB would have timber joists sat on it and be encased in PB and timber.

 

[geraldthehamster]

This is how I calculate the roof dead load, as a non-engineer.

When complete the roof would be 5m x 5m. The UB would be 1.5m from one end, 3.5m from the other. Joists overlap on it.

Roof contains:
50m2 OSB @ 5.76 kg/m2 = 288 kg
0.32 m3 C16 @ 500 kg/m3 = 160 kg
25 m2 PB @ 15 kg/m2 = 375 kg
25m2 EPDM @ 1.3 kg/m2 = 32.5 kg

(weights per m2 and m3 sourced from random places on Internet. I'm taking Celotex to weigh nothing)

Total roof weight = 855.5 kg

As the roof is supported at either side do I halve this for the dead load on the UB?

If so, I have a dead load of 427.75 kg, or 4.16 KN, distributed to the UB at eight points where the joists sit. That's 0.83 KN/m. To which I need to add a figure for live load, and for the self-weight of the beam.

Would anyone have time to tell me of those figures look plausible, and if not, where I've gone wrong?

I do have the option of putting a post in, 1m from one end, though I'd prefer not to.

Many thanks

Richard

 

[^woody^]

I do have the option of putting a post in, 1m from one end, though I'd prefer not to

Your engineer will advise on that

 

[jeds]

You've got 2.5m of roof loading on the beam, the beam is 5m span. That's 2.5x5.0=12.5m². Live load is 1.00kN/m². (or 0.75) Dead load is, say, 0.75. Therefore, 12.5m²x1.75=22kN. Multiply by a factor of 1.5 for safety and add self weight of beam and you are around 34kN. (3400kg)

 

[geraldthehamster]

You've got 2.5m of roof loading on the beam, the beam is 5m span. That's 2.5x5.0=12.5m². Live load is 1.00kN/m². (or 0.75) Dead load is, say, 0.75. Therefore, 12.5m²x1.75=22kN. Multiply by a factor of 1.5 for safety and add self weight of beam and you are around 34kN. (3400kg)

Thanks John. Indicating what size beam (with or without a column 1m from the end)?

Could you tell me how you arrive at the live load and dead load figures?

Cheers
Richard

 

[geraldthehamster]

Sorry, I've re-read your earlier post so I assume we're still talking about

A 152x89x16 will span about 4m at that rate - a 178x102x19 will do about four and half

are we? If so, it seems I'm where I need to be with a 152x89x16 and a post to reduce the span to 4m. Also, I'm not in the south west but I am 70m above sea level so I'll take that extra foot (in which case I'm nearly there with a 178x102x19 and no post).

Obviously I'll get this calculated before doing anything but it confirms the feasibility. Jeds, please tell me if I've misunderstood anything you've said.

Cheers
Richard

 

[tony1851]

Have you considered a universal column section? Something like a 152 x 152 x 23 might do.
These sections are less-prone to lateral buckling because they are wide-flange.
That section would deflect less than a 152 x 89, and only a tiny bit more than a 178 x 102.