Beam and Trusses: Substitution 2

[dbelov72]

Hello everyone!

I recently looked at the Burrard bridge here in Vancouver and noticed that some sections the road are supported by beams and some sections by trusses (I should have taken pictures myself, since none of the pictures on the Internet show the section supported by beams). As a future structural engineer, I became curious on structural purposes of beams and trusses. As I know so far, the only purpose of beams and trusses is to carry vertical loads by means of using shear and axial strength of steel or some other material.
So, by knowing that, is it any reasonable to assume that beams can always be substituted by trusses as long as constructibility requirements are met?
If yes, why don't we substitute beams by trusses all the time?
Wouldn't truss substitution make structures lighter?

Plus, how does a structural engineer decide whether to use a truss or beam in a given problem? Or it is the architect who decides that?

Please, let me know if I am thinking in a wrong direction. I am surprised I haven't asked that question to professors during school term....


Kind regards,
Denis

 

[BAretired]

Nothing wrong with your thinking. In general, trusses and beams can be interchangeable but when spans are short, beams tend to be more economical than trusses because they require much less fabrication even if the truss is lighter in weight. Labor is a very significant part of the cost of any product and structural steel is no exception. Castellated beams are truss-like and are deemed economical in some parts of the world. In my area, they are frowned upon because the local fabrication shops are not set up to do the web cutting economically. Open web steel joists are truss-like and are used extensively for framing floors and roofs. They are produced very economically, particularly when there is plenty of repetition.

Long heavily loaded spans may suit trusses more than beams because the weight saving is considerable and long span beams may not be readily available.

The structural engineer has a pretty good idea of whether to use a beam or a truss by doing a few quick calculations and determining which he thinks is more economical. When the decision is close, he may wish to check with local fabricators to get their input. Alternatively, if he doesn't care which of the two is used, he can design beams and allow trusses to be substituted at the option of the contractor subject to his approval of the design. Sometimes, the architect may wish to create a special effect and detail a special type of truss which the structural engineer has to ensure carries the loads safely.

When the project is far away from structural steel fabricators, transportation costs can become a deciding factor.

BA

 

[JAE]

Architects don't think...they only feel.



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[TLHS]

The approach on the Burrard Bridge is over reasonably solid, clear ground. So they have a series of vertical supports and short spans where it's economical to use girders. When it approaches the water there's a longer span or two, where it becomes significantly more efficient to use a truss. Then, over the water, they flip the truss above the roadway to give more clearance for boats.

The decision to switch to trusses is partially preference, partially math and partially "feel" or experience. It depends on a combination of span, load, clear depth, labour costs, material costs and erection constraints.

 

[SlideRuleEra]

dbelov72 - On industrial projects there can be a couple of other concerns. Point loads on structural members from heavy plant equipment are common. Beams lend themselves more than trusses to point loads. This is especially true when modifications to equipment / structure are made.

Another issue is maintenance. Beams withstand physical damage (e.g. impact from mobile equipment) better than trusses.

Also, in industry you probably will never have to work with an architect.

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[dbelov72]

Thanks for your answers!

BAretired - Absolutely great post. I am now actually curious how to analyse castellated beams... thanks for throwing newer things into my vision!

TLHS - thanks for clarifying their reasoning!

SlideRuleEra - I always thought that beams are more suitable for distributed loads...And for trusses, are they ever loaded with distributed weights? I always thought that only point loads should be applied to trusses. Also why is it so that beams are better at withstanding impact loads? Is it because trusses have lots of connections?

Kind regards,
Denis

 

[SlideRuleEra]

dbelov72 - Both beams and trusses are fine for distributed loads, like floors, roofs, bridge decks, etc.

To analyze a truss with the "method of joints" you assume that the loads are applied at the truss nodes, but in reality the load are often distributed. A common example is a typical residential roof truss - the dead load of the roof deck / shingles are distributed loads. So are many live loads, wind load and snow load.

If you want to place heavy point loads on a truss, you have to not only consider where it is on the overall truss, but where it is in relation to the truss nodes. On a beam, the point load can go wherever necessary as long as the beam in not overloaded in shear, moment, or deflection (for moment, be sure to take into account the beam's unbraced length).

I agree with your conclusion about beams being better at resisting impact than trusses. A typical rolled beam is, of course, one piece. A reasonable impact is going to be distributed, in some fashion, throughout the member. If it is dented or bent, it will still work... just not quite as well. A truss depends on every member contributing to load carrying (otherwise, the member would not have been included in the truss). If one member gets significant damage it can jeopardize the entire truss.

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