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Staically Indeterminate Trusses

Staically Indeterminate Trusses

Staically Indeterminate Trusses

(OP)
Hello:

I am a BSME in need of some Structural Engineering help. I am looking to engineer an awning truss system, accounting for snow load and possibly wind load (required?). The truss has a basic right triangle perimeter, and is supported along the wall side (short side) by three fasteners (all equally spaced). The end of the truss (furthest point away from the wall) on the bottom side (long side) is supported by a column. I wanted to add a snow load to the top side (hypotenuse).

With external forces being generated from a snow load (known), a possible wind load (known), a dead load (known), a reaction at the column (unknown), and 6 (3 fasteners x 2 directions each = 6) reactions at the fasteners, I end up with statically indeterminate equations after summing moments and forces.

Can anyone tell me how to proceed? Can I derive statically determinate equations from this setup? If not, how would this truss be analyzed? Thanks all in advance for your help.

RE: Staically Indeterminate Trusses

There are ways to analyze this by hand.... however I have long given up on this.... AFTER my last exam in college.

Get some software to help you out.  Much faster and probably more accurate - if you model it correctly..

You could possibly simplify and assume only two reations - the post and the connections.  Then split the connection load up over the three fasteners.  Be conservative with the fasteners and you should be OK.  I have often used this method and it seems work just fine - and actaully is not far off what the software says.

RE: Staically Indeterminate Trusses

You can model it with RISA2D or RISA3D... these can be downloaded from their website and the demo program will be adequate.

You might want to consider moving one of the attachments against the wall so that you only have 2 reactions and the frame may not be statically indeterminate.

Dik

RE: Staically Indeterminate Trusses

For a continuous prismatic beam with two equal spans and uniform load, the central support takes 62.5% of the total. In this case due to the stiifnes over the support I would imagine it would take more like 70% of the total load.

for uniform load on one span only it will be roughly 50% to each of the 2 supports under the load, a little more to the central one.

There may be other issues such as bracing of the bottom chord that only a structural engineer would pick up so I would advise you to spend a bit of money on one.  

RE: Staically Indeterminate Trusses

I would use roller at the column, and a pin at the wall (just one at the bottom).  You should be able to solve this.  

Once you are done with this, then you can figure out the vertical force where it is attached to the wall.  Then you just just design the 3 clips to hold that load.  

My theory is, if you get rid constaints to make it "determinate" structure, you are designing it more conservatively. (Because you brace it more when you build it).  So just use one fastener when you design it.

RE: Staically Indeterminate Trusses

csd72... I would think the opposite since the top and bottom chord would be more axially stiff than the vertical in flexure.

Dik

RE: Staically Indeterminate Trusses

Contact a local truss manufacturer and ask them to run this for you. We call it a Mansard Truss. Most truss companies are willing to run a truss here or there to help engineers. This will make their job easier in the future as the engineer knows what he is doing, and gets their name out there as you could mention to the customer the ABC Truss helped you out.

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