Steel beam size

[sami69]

Tony,

Having looked at the Steel tables after your post there is a 305x305x137UC with I value of 32814 cm^4

From what you are saying the above would deflect even less than the
254 x 254 x 167 I = 29900 cm^4 and would also be lighter.

Maybe after considering all 3 recs given by the SE's, the 305x305x137UC would be the best option?

What do you think?

 

[gissle]

The structural dimensions of the beam give it it's strength in different directions, it's not just a case of the mass of the beam. It is how the mass is distributed and what it is being used for.

The beams look fairly similar in size, and I would guess that the two engineers have run through the calculations and have selected similar but not identical beams. i.e. you could probably use either.

You can easily find the charts showing the beam properties online.

 

[tony1851]

Tony,

Having looked at the Steel tables after your post there is a 305x305x137UC with I value of 32814 cm^4

Having a higher 'I', that beam would deflect lsss than the others.

However, as gissle, woody, et al have pointed out, there are other factors to consider; any alternative would have to be checked for such things as bending stress and buckling as well.

Don't take on the responsibility of choosing a beam yourself - hand that to the SE - it's what he's paid for.

 

[sami69]

Unfortunately I have to trust someone and go with it. Having 3 SE and 3 different opinions makes it even more difficult to do. It's been cocked up once at great expense and I have to get it right this time round. The glass and tracks have all been manufacutured and delivered so I can't mess about with changing pier widths etc

Apart from trying to find some middle ground between all the beams specified I don't know what else to do.

Thanks everyone for your input.

 

[tony1851]

Your structural opening is 10m wide, but what length of roof spans on to it?
(ie what is one-half the projection out of the extension).

When the SEs said that deflection would be <7mm, I was just curious to know what live loading they have allowed for, and this depnds on the area of roof supported by the beam.

 

[sami69]

Tony,

The opening is 10m
The piers are .75m each - cavity wall brick & breeze block construction. These are now going to be strengthed with concrete down the cavity.

The roof span is 6m
9x3 roof joists
0.75kn/msq loading
5 courses of two sided bricks on top of the beam + coping stone

asphalt roof + 3 roof lights of 98kg each
usual warm deck insulation etc
below is picture of the opening - beam and parapet have now been taken off while we decide what steel to put in.

Back extension

• sami69
• 6 Oct 2012
• 1

[/img]

 

[tony1851]

Assuming 30 sq m of roof supported on beam; total live load will be 22.5kN. Using the standard equation for deflection, this will be <7mm for the beams noted.

DEAD LOAD (ie weight of structure)
roof dead load @ 1.0 kN/m2 = 30kN.
brickwork + coping, say 10m x 0.45m = 4.5m2 @ 4.5kn/m2 = 20kN approx.
beam self-weight 10 x 120 x 9.81 = 12kN approx.

total dead load 62kN; deflection approx. 12-15mm - this will be'built-in' (ie indpendant of deflection caused by live load) and needs to be allowed for when installing doors.

Get SEs to confirm this.

 

[noseall]

That's the most bonkers opening i've ever seen.

The question i would have to ask is WHY?

 

[sami69]

Why is it bonkers?

Glass across will give an amazing view of 150ft garden. I've seen many similar or even bigger on many archictectural websites..

 

[noseall]

I can look out over my entire 60ft garden via my own window. I don't need a ridiculously big door to do it either.

Is it for people to see in or for you to see out?

 

[sami69]

Noseall,

It's more of a glass wall than a big door, with 2 fixed panel of glass at each end and 2 in the middle that slide open. Sorry you don't like it...

Tony,

Are you saying that you think the total deflection on the steel will be more than 12-15mm + live load deflection say 3mm with a completed roof?

These SE are all telling me total deflection will be 7mm or less on the steel when roof is completed with the beam specified...

 

[noseall]

Will there be any 'diaphragming' of the roof on windy days causing additional deflection of the beam?

Its not that i don't like it it seems so impractical in a domestic situation. for example, you have lost useable wall space on that elevation otherwise you may as well have had a small wall.

Now if your house was a Ferrari showroom........

 

[tony1851]

These SE are all telling me total deflection will be 7mm or less on the steel when roof is completed with the beam specified...

This is incorrect. The code for steelwork (BS 5950) sets limits for beam deflection under LIVE load only (eg snow on the roof). This is intended to prevent, among other things, cracking of brittle finishes such as plaster casing of the beam.

In your case and with the beams suggested, the deflection uner live load will be below the maximum stated by the code (5-7mm seems about right for the beams suggested).

However, the code does not give a limit for the deflection of the beam under DEAD load (ie in your case the weight of the roof, parapet and beam self-weight). This is left to the discretion of the designer

In your case, the dead load will be considerably more than the live load, so the deflection caused by the structure above will be proportionately greater.

In short, once the structure is up, you will have an initial deflection of 12 - 15mm depending on beam chosen, and then with snow on the roof, a possible further deflection of anything up to 7mm. Your SEs are effectively saying that the beam will not deflect under the dead load, which is nonsense.

 

[sami69]

Tony,

I will ask these SE about the points that you have talked about.
I have been clear with them in saying that I want total deflection to be 7mm or less with the roof completed ie parapet,roof lights,asphalt etc. I have even seen one guys workings - he has live loads & dead loads identified.

Thanks for all your help today.

 

[RonnyRaygun]

In short, once the structure is up, you will have an initial deflection of 12 - 15mm depending on beam chosen, and then with snow on the roof, a possible further deflection of anything up to 7mm. Your SEs are effectively saying that the beam will not deflect under the dead load, which is nonsense.

Just to add something to this, you are going to have a set of multifold doors that run across the span of your opening. if you have a total deflection of around 22mm then the beam will need to be 22mm higher at the ends to prevent the doors binding under maximum load.
A 10 metre span is a particularly long one for a domestic situation, and while it is something that is not usually done in domestic situations, you could consider getting the beam precambered to 15mm (or whatever the dead load deflection is).
This will mean that when the doors are hung the beam is flat and level, and the only deflection that has to be allowed for is the 7mm from the imposed load.
Another thing to note is that sometimes multifold doors are top hung, in which case there is an extra load to the beam which needs to be considered, and which will increase the deflection still further.