Deflection in Truss Design 1

[GetOnTrack]

Hello - New to the forum and looking for help! Here's the problem:

I'm helping my brother build a permanent dock (for boats) and want to span as far as possible; shooting for 65' at this point. I have a roof truss design that I believe will be a good starting point except I cannot determine "I" for the geometry. This particular roof truss is made of steel angles on top and bottom (4 total) with ~ 1" diameter wire rod connecting the top two steel angles to the bottom two steel angles (up and down at some angle). Hopefully the design sounds familiar... I can calculate "I" for the steel angles in this particular geometry, but can't get any meaningful information regarding overall stiffness because I don't know how to handle the rod (straight members) in the calculations...

Any help would be appreciated.

 

[civilperson]

Ignore the contribution of the rods for flexural strength, when you check shear at support location or concentrated loads then the web is important. Composite I of four equal angles is the area of the angles times the distance from the original centroid of the angle to the centroid of the combined areas squared plus the original I of each angle.
Example: four each L-2x2x3/8, two top and two bottom with total dpth of 30". Composite I of joist equal 4 times ((1.36 in<2)x(14.4 in x 14.4 in) + 0.479 in<4), or 1130 in<4.

 

[GetOnTrack]

thanks civilperson - I'm coming up with the same "I" as you when I use the 2x2x3/8. This was the only part I could actually calculate. I'll use this information when calculating flexural strength.

With regard to the web, my main concern is buckling - right? I suspect buckling before failing in tension? Is there a preferred method for evaluation of the web? I have an old statics book from college and found the truss section with "method of joints." I must admit - it has been a while since I've had to do anything like this, is there anything online to help? Recommended reading?

 

[civilperson]

Self taught engineers need to get the results checked by an expert, (structural engineer with an license), prior to building or purchasing materials. The vocabulary is confusing. Method of joints assumes no flexural strenth across a connection to form a truss, (only axial loads in truss components). I think you are describing a steel joist with continuous top and bottom chords. Bracing or bridging is important in joist systems. Read a manufacturer's catalogue, i.e. Vulcraft.

 

[GetOnTrack]

Thanks for the help; will check the Vulcraft as well.

 

[billhorton7]

You can also check with the Steel Joist Institute at

http://www.steeljoist.org

for publications.