Comparison of GluLam Vs RSJ Vs other?

[tony1851]

No rule of thumb or guesstimates?

I said above that one company reckoned a 90x405mm glulam was required based on 6.4m span, 6.0m unsupported. I'd happy entertain educated guesses from people with some experience what approximate RSJ might be equivalent.

If I can get my SE to verify, you could win bragging rights

You told the company who gave you that approximate size the span of the beam, but did you tell them the width of roof the beam was supporting? - obviously that has a direct bearing on the load the beam is to carry. Most SEs will not sit down and work out a size of glulam beam because it's not cost-effective time-wise - you generally go off manufacturer's tables for commonly-produced sizes, but they need to know the load it is carrying. (If you want to see the figures, Google 'Structural Timber Design' by Abdy Kermani and you'll see why most SEs won't be bothered).
Design of steel beams is an 'iterative' process, in that you choose a section you think will work, and then check it for stress, buckling and deflection until you get the smallest-size section which satisfies all three. I'll stick my neck out and, on the sparse info you have given, would say one of the 254 x 146 beams might work? or possibly a slightly bigger one, or smaller one - who knows?

 

[d000hg]

@tony1851 I gave them the plans and the SE's report.

That's interesting so even in the world of modern computer modelled design, you can't reverse engineer the beam you need - you put something in with given properties and test it? I have seen in calculations where it seems they've run a test at a particular point on a beam/joist to check everything is in range - that's the sort of thing you mean?

I'm not sure I follow why this is a different process for steel Vs GluLam though? If standard sizes have advertised values then couldn't a SE iteratively try a few GL options in the same way as they would RSJ, or use the manufacturer's tables?

You say but they need to know the load it is carrying. How would that information be provided, given it's not a point load but varying force along the length of the beam - are there standard parameters I should be requesting? Should my SE know what information to give or should the GL supplier be asking him for specific things? The SE is a bit hard to get hold of and I'm not sure what I should reasonably be asking him to do, given he took on this job to do calculations for the whole building and to make this design change to introduce a beam.

 

[d000hg]

BTW someone said an RSJ would be cheaper. I have one quote for a 90x405 GL for £468 (the beam is £250 the shipping is nearly as much!) and when I looked up an 254x146x31 RSJ the price I was quoted was £570. The steel would weigh 200kg and I think the GL 170ish. Only one quote for each but I was quite surprised.

Update: GlulamBeams.co.uk say they will charge over £500 just for delivery!

 

[DIYhard]

I have a prefabricated timber garage with 6m span and 15 degree pitch roof. There is one truss mid-way along the 5.4 building, so the bay length is effectively 2.7m. The dimensions are roughly equivalent to those of the OP's building.

The truss is a very simple design, using 150x70 timber for the top chord (sloping rafter) and 70x70 timber for the bottom chord (horizontal tie) and kingpost. The top and bottom chords are connected by M12 coach bolts. There are steel eaves gussets made from 5mm thick plate, with flanges for bolting to the top chords and the wall frame. The bottom chord is raised above eaves height, therefore headroom is excellent over most of the building width (obviously some restriction at the eaves gussets).

This must have been very cheap to produce, in fact it would have been an easy DIY job. I have no idea who specified and/or approved the truss design, and all I can say is that is hasn't shown any signs of failure to date.

 

[Notch7]

BTW someone said an RSJ would be cheaper. I have one quote for a 90x405 GL for £468 (the beam is £250 the shipping is nearly as much!) and when I looked up an 254x146x31 RSJ the price I was quoted was £570. The steel would weigh 200kg and I think the GL 170ish. Only one quote for each but I was quite surprised.

Update: GlulamBeams.co.uk say they will charge over £500 just for delivery!

I used to pay £700 per metre cube for glulam beams.
A 7metre 90 x 405 would be about £200+vat.......mind you I bought about 10 beams in one order.

The costs for an orangery were pretty similar for timber or steel, but timber worked well for construction methods.

Steel is generally cheaper because there are local stockists all over the UK -so delivery will be cheaper for a one off.

7metre 90mm x 405mm about 135kg so easily moveable.

 

[d000hg]

@DIYhard that's probably the difference between what will work and what the regs say!

@Notch7 So a 254x146x31 RSJ gets quoted about £600 Inc delivery and will weigh 200kg.

Since everyone says steel is cheaper, does that imply that steel is more than needed?

 

[tony1851]

@DIYhard


Since everyone says steel is cheaper, does that imply that steel is more than needed?

No, it implies that steel and timber have different strengths.

 

[d000hg]

No, it implies that steel and timber have different strengths.

Everyone says for equivalent load, Steel should be cheaper. Maybe much cheaper?
So I mean if a specific steel costs more than a specific GL wouldn't that mean it's probably stronger. Or have GL come down in price relative to steels recently and GL could genuinely be the cheaper option for the required load?

 

[cdbe]

You should try a few more steel places - that steel is about £200 at my local place and they'll drop it off.

 

[d000hg]

You should try a few more steel places - that steel is about £200 at my local place and they'll drop it off.

Wow. OK, clearly buying online is not a great option for this sort of thing- thanks for letting me know. I'll ask the builders who they'd use locally once we get a spec agreed.

 

[tony1851]

Everyone says for equivalent load, Steel should be cheaper. Maybe much cheaper?
So I mean if a specific steel costs more than a specific GL wouldn't that mean it's probably stronger. Or have GL come down in price relative to steels recently and GL could genuinely be the cheaper option for the required load?

You don't pay 'per unit of strength' for beams! It's not that simple.
In compressive strength, steel might be nearly 20x as strong as timber, but that does not mean that a 450mm deep glulam beam can be replaced by a 22.5mm deep steel beam (not that they make them).
As well as strength, stiffness is also a critical factor, and this depends partly on the material, and partly on the geometric properties of the section.
If you are interested to this extent, and are not simply content to let your SE advise, you really need to refer to a book on structures;
Understanding Structures by D.Seward is a good, readable starter.

 

[d000hg]

I've chatted to the SE, he reckons we might get that steel a little smaller. I will try to remember to report back what is specced and what we go with - I often find these threads when I search and then it never says what the result ended up being!