metal web joists

Can these type of joists be used structurally instead of metal beams for internal work, where only timber is involved (notwithstanding their size) ? i.e. Could I use these instead of a steel in a loft conversion, in order to support floor joists and a purlin wall ? And to support the ridge if required (aka a ridge beam).

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Reply to
sm_jamieson
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I've seen them used structurally (in fact I can't see another purpose for them) - more than one company was promoting them at Interbuild this year - that you get structural strength, without blocking pipe and cable routing.

The downside may be the design effort - I'll read the docs on that website when I have time.

Reply to
dom

deflection considerations. Stiffness is proportional to breadth x depth cubed. So taking the smallest size, 35mm edge sections separated by 125mm, the stiffness is proportional to

72x(195^3-125^3)=393246016, which is the same stiffness as a piece of solid timber 47mm wide x 203mm. The manufactured joist does the same job as a solid joist using nearly twice the timber (with added advantages re straightness, service installation etc).

For this size the solid timber is 20% stronger, and has a significantly higher shear capacity. Meanwhile (for a 3m span) a little 127x76 steel will take more than three times the load, a similarly sized 203x102 more than nine times the load.

So whilst these joists make a good replacement for traditional solid joists, they are not dissimilar structurally to a solid piece of timber, so won't be an adequate replacement for steel beams (assuming a steel beam is needed in the first place.

Reply to
Tony Bryer

OK. However, these are basically trussed structures. Presumably you could make a trussed structure to replace a steel joist if it was tall enough. For example, say you had 500mm height available under a roof ridge (say 6000mm length, clay tiled, cut roof) that needed supporting, and the usual solution was a ridge steel. What type of timber structure would be needed to replace the ridge steel ? Thanks, Simon.

Reply to
sm_jamieson

Possibly several cut sheets of wbp glued together, BUT you need to do the maths and be damned sure the glue is done properly. I wouldn't want to do it on site, I would use steel.

Reply to
dennis

yes, with caveats that you still need a structural engineer to specify them.

Reply to
The Natural Philosopher

the issue being of curse that although you might need to go much larger for the stiffness, it might still be lighter and cheaper than the steel.

Reply to
The Natural Philosopher

Yes, of course, the point is it may be cheaper and easier for a DIYer, as you mention in your other post.

Simon.

Reply to
sm_jamieson

I need to support the ends of some joists, where the joists change direction in the middle of a room. They are currently supported by a transverse lintel which runs below ceiling level, but I want to get rid of that and support them from within the ceiling space, so as to have a flat ceiling.

An RSJ would do, with the ends of the joists cut into the webspace *except* that I need to get some pipes (up to 22mm)and cables through it.

Anyone know whether you can get something which looks like an RSJ, but with the web in the form of a lattice rather than solid?

Reply to
Roger Mills

In larger sizes, "Universal Beams" rather than RSJs, it is possible to have them "castellated". This involves cutting a regular zigzag through the web, and re-welding the two halves together peak to peak rather than peak to trough, making the beam taller. But this would be quite expensive.

For your purpose, it would be cheaper and easier to cut some holes in the web of an RSJ to allow pipes and cables to pass through. A structural engineer will specify the beam to suit your loadings and the number/size of holes you need.

Reply to
Bruce

Its quite common to cut an I beam and weld it back together so it is full of holes. It makes it stiffer while only using the same amount of steel. A steel fabricator will do it if they don't have a stock of them.

You could DIY it but you would need something like a plasma cutter and a welder and be good at welding.

Reply to
dennis

I don't think enough emphasis is being given in this discussion to these trusses' comparative lack of shear capacity. While they would appear to be potentially highly efficient in bending, and offer strong resistance to torsion, shear forces are normally taken by the web of a steel or timber beam, and no web means little or no shear capacity.

I find it difficult to imagine a situation where these trusses would not be subjected to significant shear forces. I do wonder whether some of the end details shown in the Technical Guide would offer sufficient shear capacity for larger spans.

This is a product that is best suited to new build (where close adherence to the original design and to the required standards of workmanship may be more easily checked) rather than conversions, where engineered I-joists may be a more logical choice, given their intrinsically greater shear capacity.

Reply to
Bruce

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may not be economic for a one-off.

You can cut holes in the web of any I-beam as long as the effect of doing so is checked. Our ProSteel software includes this option, but I'm not suggesting DIY calculations. Background info (which may make your eyes glaze over) at

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decide where you want the openings, get an engineer to design accordingly, and fabricator to cut openings before beam is delivered.

Reply to
Tony Bryer

What shear forces? In what plane?

I beams are designed to resist bending loads. Period. The only shear forces as such, that I can envisage, are at the beam ends where its supported, and, provided these are less at its maximum loading than the strength of the structure, which it usually is by a huge margin, there is no problem.

The other shear forces encountered by the web are to resist parallelogram type movement: but that is exactly what the composite warren truss type structure does efficiently anyway. Neiher they, bo the copmpresive/stretching type fores in teh wen to resist the Euler bucklng modes are in anyway large. It is veryty easy to design a lattce braced structire such that hwat keeps the webs part ois of much lighter constrctio than the top and bottom sections: in essence they are where teh strength resides: the function of the web is purely to prevent them moving relative to one another. In this the steel I beam is relatvely wasteful.

Reply to
The Natural Philosopher

Before answering the questions I posed, it might have been a good idea for you to study the theory of structures.

Your use of terms such as "bending loads" and your lack of understanding both of the term "shear force" and of the relationship between bending and shear, means that you really don't have the faintest idea what you are talking about.

Reply to
Bruce

See

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is a difference between the external shear forces and the internal shear forces generated to resist bending

Reply to
Mark Dumbrill

Some shear stupidity being shown here. Although I'm not going to be bending anyone's ear about it.

I've already got my coat!

Reply to
1501

Nice of you to display your profound ignorance of structural engineering again. God help us if you are ever in a position to design a structure to withstand shear forces, because you clearly have not the faintest idea what they are, or where they come from!

Thank you - it's a long time since I laughed as much as this! ;-)

Reply to
Bruce

Good thing I actually have a degree in it, and indeed do design structures routinely.

Though not for buildings

If anyone else is reading this, don't take my word for it, look up beams on wiki for full and reasonable explanations.

Laugh away, but then ask yourself why if these beams are so unsuitable as beams, why they are allowed to be used at all.

Then look at the innumerable Warren trusses in use round the world, also used as low deflection load carrying beams. and ask yourself why they don't flex and bow.

The only reason to use a solid steel I beam is that it is simple to make. It uses far more material in the web than it needs to. The web forces are extremely low compared with the top and bottom elements.

A truss design is far lighter and more efficient use of material. It just is more complex to fabricate.

You will note that in a warren truss structure, there can be no 'shear stresses' within the elements. since here are no elements big enough to have substantial shear applied across them. That analysis is reserved for beams with solid webs. The truss elements go into tension and compression alone.

I.e. the tendency of the beam under load to have lateral shifting between the top and bottom element is not constrained by a solid web resisting 'shear' but by a series of struts connecting the upper and lower elements with diagonal braces. If you HAD done proper beam analysis, you would know that these lateral forces are relatively low compared with the actual tensile and compressive forces in the beam upper an lower elements, and in the limit the failure mode is usually - with wood anyway - the failure of the top element under compression. Or due to Euler buckling if its not constrained in the lateral plane. Or if the trussing is TOO sparse. However that is, whilst not trivial, a fairly simple calculation to do, if the elasticity of the member under compression is known, there being a critical length for a given size and elasticity below which failure by buckling simply will not occur before the material itself undergoes plastic deformation.

So I suugest you actually instead of trying to apply solid beam or solid web ideas to a truss, go back and get the NEXT chapter of the book you haven't understood, and try and understand that.

TNP. MA, Eng.

Reply to
The Natural Philosopher

Yes, long span structures carrying relatively light loads, where the sizing of a solid member would be determined by deflection considerations.

It doesn't perhaps occur to you that the proportions of a steel UB are not an accident. Not only are there considerations of shear, web bucking and web crushing, but if the web is considered slender according to BS5950 the section is unlikely to be a good choice as a column (

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- not for the faint of heart) and you don't then have a lot of metal for the normal angle cleat, end plate and fin plate connections.

Reply to
Tony Bryer

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