RSJ - Two or One?

[Boyts81]

Out of interest can anyone briefly sum up the difference in properties between two smaller RSJs bolted together and one bigger RSJ? E.g two ub203x133x25 bolted together versus one UB 533x210x82

Is it a simple case of the bigger one being stronger in all areas or is it more nuanced?

(I Appreciate this is novice question but please humour me )

 

[Nakajo]

The clue is in the question. One weighs 50kg/m. The other weighs 82kg/m.

 

[Boyts81]

Thanks Nakajo. Are there any nuanced difference in their structural properties or is one simply stronger than the other?

 

[Nakajo]

I scored 40 out of 100 in my structural exam, so I'm not sure I'm qualified to answer that (not, admittedly, that that usually stops me)

 

[tony1851]

The question is too abstract because steel beams have to satisfy several criteria, including; checking maximum bending stress, maximum deflection, maximum shear, lateral buckling etc.

It's not just a case of one being 'stronger' than the other.

If this was an actual practical problem in a building project, rather than a theoretical problem, it would be easier to suggest an answer.

 

[bernardgreen]

Bolted side by side two would almost always be "stronger" ( support more weight shared between them )

Two bolted end to end with the correct joining pieces would almost always be weaker than one continuous RSJ if the load was evenly spread along the length of the RSJ.

But as siad the answere depends on what load the RSJ(s) are supporting and how it is distributed along the RSJ(s)

 

[tony1851]

Two bolted end to end with the correct joining pieces would almost always be weaker than one continuous RSJ if the load was evenly spread along the length of the RSJ.

By that you mean a beam splice. The resulting beam would not be 'weaker' than one long beam, because the splice would be designed to accomodate the
bending stress and shear force at the point where the join is.

 

[^woody^]

The whole concept of designing is to design economically, at the least cost, using the minimum section size, and with the practicalities (and again costs) of installation.

That determines whether it's one, two or a splice.

 

[Boyts81]

Thanks for all the input all, here's the reason I ask:

We're having a rear extension built. Calcs done and approved by BC. Steel is now in place and again approved by BC.

I happened to be reading through our engineers design package yesterday and noticed he had recommended two ub203x133x25s bolted together side by side. Now I'm 90% confident that only one steel was installed, around 200 in width.

I'm seeing the builder tomorrow so will talk it through with him and will also chat to BC who have approved everything thus far.

 

[tony1851]

Thanks for all the input all, here's the reason I ask:

We're having a rear extension built. Calcs done and approved by BC. Steel is now in place and again approved by BC.

I happened to be reading through our engineers design package yesterday and noticed he had recommended two ub203x133x25s bolted together side by side. Now I'm 90% confident that only one steel was installed, around 200 in width.

I'm seeing the builder tomorrow so will talk it through with him and will also chat to BC who have approved everything thus far.

Has he put in a 203 x 203 x 46, by chance?; if so, it should be reasonably equivalent to the 2 specified beams.

 

[Boyts81]

Thanks tony. It's certainly approx 200 in width but would've guessed at it being higher than that so I'm not sure. Will find out tomorrow.

Can I expect some form of certification as to the spec of the beam in place?

 

[tony1851]

Out of interest, what is the span? It's highly unlikely the beam is a 533 x 210 - that would hold up the whole street, not just your back wall.
I suspect that you may have looked up the section-size tables for Universal Beams, and assumed those beams that are 210 wide are all 533 high?

Try looking in the tables of Universal Columns - there you will see several sections that are serial size 203 x 203. That's more likely the size your builder has used.
Universal columns are frequently used as beams (confusing isn't it!) - they are 'H' -shaped rather than 'I' shaped.

 

[Boyts81]

Span is just shy of 4.5m. Yeah it was more H than I. I'm assuming that since BC has inspected and approved that it is fit for purpose but somewhat frustrated they wouldn't consult with me before changing the design. Thanks Tony

 

[Boyts81]

PS you're absolutely right, I did look up UB tables!