roof beam rafters vs trusses

A truss for the same span and loading is normally lighter than an equivalent wide flange beam. But that doesn't necessarily make the truss more economical, as the fabrication cost may override the material savings. Another factor is that the truss will probably be deeper.

Rafters are site constructed items; whereas, trusses are "manufactured" items. For site built items, you are at the mercy of the expertise of the individual doing the work. For manufactured products such as trusses, the manufacturer is often required to adhere to industry standards in the manufacturing and quality control functions....thus there is a greater likelihood of a more consistent, potentially inspected product than a site built item. Note that your likelihood of the manufacturer following a quality assurance/control program is greater, but not guaranteed.

I don't see the distinction, Ron. I think the OP is just trying to compare two different types of members, i.e. wide flange beams or trusses.

Ron... all our trusses are site constructed... we haven't got any ready made manufactured trusses.. first time to hear of this..

So between both site constructed trusses and site constructed wide flange rafters.. isn't it that the members of the trusses can bend in response to wind, whereas in wide flange.. it is often stiffer than the stress are transferred to the connections? Which is better.. stress in the members of trusses or stress in the connections of stiffer wide flange rafters?

You fabricate steel trusses on site?

BA

You fabricate steel trusses on site?

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It's just cutting and welding angle bars. It could be done at site or at workplace but these are connected manually.. not done in a manufacturing plan welded by machine.

Some folks only make trusses.. so I wonder if these are really advantageous for roofing.. maybe the members in the trusses flexing and stretching in response to wind pressure is the main advantage of it?

hocho said:

isn't it that the members of the trusses can bend in response to wind, whereas in wide flange.. it is often stiffer than the stress are transferred to the connections? Which is better.. stress in the members of trusses or stress in the connections of stiffer wide flange rafters?

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Comparing these three systems:

1) A pitched roof truss with a flat bottom chord.
2) A pitched roof with rafters supported on a central ridge beam.
3) A pitched roof with rafters not supported on a central ridge beam.

In terms of material efficiency, which is not necessarily overall economy, I would say that #1 is the most efficient and #3 is the least efficient. That said, all of these systems are workable if engineered properly. And, for small spans considering economy only, the order may actually be reversed with #3 being the cheapest and #1 being the most expensive.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

Comparing these three systems:

1) A pitched roof truss with a flat bottom chord.
2) A pitched roof with rafters supported on a central ridge beam.
3) A pitched roof with rafters not supported on a central ridge beam.

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It's difficult to connect 2 wide flange at a pitch without any central ridge beam.. how do you detail the connections.. by simply cutting and welding the wide flange at an angle? Would this even be stable?

hocho said:

how do you detail the connections.. by simply cutting and welding the wide flange at an angle?

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That's one way. I would normally include a full depth vertical stiffener at the joint. For lighter loads, this could be conveniently done using a bolted, end plate moment connection.

hocho said:

Would this even be stable?

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The beams would need to be restrained laterally by connection to a roof diaphragm of some sort, either using competent decking or discrete bracing members. The same would be true of trusses of course.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

Can anyone see any problems with using rafter that is stronger than needed? For example.. using w8x18 or 8 inches x 5 inches by 18 weight for light roofing? This is just for additional safety and considering the prices don't vary much compare to lower sizes.. any complications of using stiffer rafter?

No problem using a beam stronger than required.

BA