Steel Truss Preliminary Design 1

[atakancengiz]

Hello Everyone,

I have a question about truss systems. I have been designing steel truss systems since last year. I am not the one making decisions of type of truss, height of truss or other parameters. My supervisors tell me these parameters and I am making the rest of the job. But I am not happy to fullfil what I am told. Because I want to learn how people making decisions according to the projects.

I am quite sure that my supervisor making decisions according to the their former job experience. I, of course, understand and respect their experience. But any advice will be very helpful.

I have attached a photo to show what I am thinking.

 

[robyengIT]

google : "roof trusses" or "roof trusses calculator"

 

[Ron]

atakancengiz....Such decisions are often dictated by the use of the building. For instance, the clear height from the floor to the bottom of the trusses would be different for different uses of the building. If used for offices, that distance might be 3 or 4 meters, but if used for a commercial or industrial application it could be as much as 10m.

The height of the truss at the center is dictated by the desired slope of the roof and the span of the truss. It can vary from almost flat to a pitch of 6/12 or some other pitch of the roof. This can be influenced by wind loading and snow loading. In areas of high snowfall, the pitch of the roof could be high to prevent snow drifts. If you lower the pitch of the roof in such areas, the truss has to be stronger because of the higher loads. For wind loads, the pitch is desired to be lower so that the lateral wind load is lower.

These are generalizations of these decisions.....there are many variables that can go into the decisions that you are questioning. The client often dictates many of these variables and some are dictated by engineering constraints.

 

[atakancengiz]

Ron, thank you for your answer, it will be very helpful for anyone that look this topic in web. One of your sentences [pre]"In areas of high snowfall, the pitch of the roof could be high to prevent snow drifts. If you lower the pitch of the roof in such areas, the truss has to be stronger because of the higher loads. For wind loads, the pitch is desired to be lower so that the lateral wind load is lower."[/pre]

The real thing that making me uncomfortable is limits of this system. For instance, after 10m spans we have to think a truss system rather than a Beam-Column frame system. Because of a frame system becomes uneconomical and sections of beams become unnecessarily bold. These differences are key for a design

Thank you so much.

 

[jayrod12]

The majority of the questions you have are based on client requirments for the building more often than not. Do they require a large clear floor area, or can they deal with beams, columns and bracing? How high of a ceiling do they want? What is the outside look they are going for?

After the building takes a basic shape, i.e. floor area, height, roof line, then the sizes of the structure can start to be determined preliminarily (using rough calculations or rules of thumb). This preliminary sizing allows the client to get a feel for the proposed cost and final structure shape, and more often than not a client realizes what they want may exceed what they can afford so this is the stage to make modifications to size, height, acceptability of interior columns etc.