Trussed roof design

[thoko]

Hie Everyone,

I would like to find out on the procedures of correct design of a trussed roof.

Is it possible to go straight into the design without the analysis?

 

[civilperson]

The analysis comes after the design, it is iterative unless the first effort succeeds.

 

[dik]

Wood or steel? If wood, then you may get someone to run the truss through one of the lumber suppliers. Else, as the civilperson suggests, it iterative... For steel, it's a matter of determining the chord forces from ql^2/8 and dividing by centroid distance 0.95*d (as a first guess) and checking the web members based on the end shear. This will give you an approximate first guess..

Dik

 

[thoko]

Do you know any web site where I can down load valuable information as regards to the truss design?

 

[RARWOOD]

In many cases the profile is established first. For example the architect may want a gable truss instead of a scissor truss. There are also decisions to be made regarding the truss depth, whether it is exposed or not,ect.

The important thing from an engineering stand point is to get enough truss depth to work with. There are span to depth ratios that are general guidlines for different types of trusses.

After the profile is determined then I estimate the chord size by determing the moment and dividing by the center to center spacing of the truss chords at midspan.

If you are working on a heavy timber truss many times the architect will already have made msany of the decisions ahead of time. So the engineer's job may just be to analyze the truss to verify the member sizes and detail the connections.

As civilperson indicated it is an iterative process. Part of the answer to your question, is a matter of semantics. It depends on how you deifne design and analysis.

In my work I normally start with an architect's profile. Many times I run an analysis first using nominal size members. What I do after that I consider design.

I generally do not consider what I do before the analysis as design. However some people would consider the intial work as preliminary design.

Based on experience most engineers can work out the profile and member sizes with out running an anaylsis. However at some point you have to run an anlysis to verify your profile works.

I don't think there is any right or wrong approach.