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ctcray (Structural) (OP)
13 Sep 07 9:08
I use Alpine's View software (version 7.34) and the self-weight of the truss does not seem to be included in any of the load cases.

For instance, I run a 50' span truss with 2plf on the bottom chord and I get reactions of 50lbs at each support.  The software can tell me that the truss (with 2x12 bottom chord) weighs 354 lbs, so it knows the weight, just doesn't use it in design.

Can anyone confirm that the other programs DO include self-weight in the design?
CJJS (Structural)
13 Sep 07 10:08
Robbins has the option to include or exclude the self weight.  We use 10 psf bottom chord dead load which is very conservative and ignore the self weight.  
Trussme68 (Structural)
14 Sep 07 9:34
The 10psf is probably more than the self weight and drywall. We also increase the pitch based on the slope of the truss. For instance, if we use a design deal load of 7, the truss is really ran with 7.83 to account for the slope.
ctcray (Structural) (OP)
14 Sep 07 9:59
I asked because I had a 4-ply girder with LVL bottom chord that  weighed around 1400 lbs, 36' long -- about 39plf.  I was trying to shave out as much extra load on the girder as possible. I was looking at deleting the  standard 2' oc uniform load on the girder by adding an extra common directly next to the girder. Then the girder would effectively only be carrying it's girder load.  It just barely worked at 4-ply, but then I realized the selfweight wasn't included and adding it bumped it to 5-ply.


It would be nice to have the option.

Does Mitek have the option for selfweight?

Helpful Member!  woodengineer (Structural)
15 Sep 07 7:51
Mitek does not include the self-weight in the analysis.  It does provide you the weight of the truss leaving the designer responsible for including enough DL for self weight.  I have tackled this problem because we design and manufacture a number of long span agriculture trusses.  We want to account for enough self-weight, but no overload the truss either.  Here is how I did it:

Write a spreadsheet to calculate how much TC DL and BC DL is required given a particular truss spacing, my spreadsheet looks something like this:

Truss weight:
--Span:
--Pitch:
--Maximum TC Pitch:
--Weight of truss: (you get this from Mitek, etc)
(Truss weight / (Spacing)(Span) / 2 gives you PSF DL due to truss alone equally distributed to top and bottom chords

Additional TC DL:
--Felt:
--Roofing:
--Sheathing Decking:
--Purlins:
--Sprinkler:
--Other:
(These boxes have pull down automated menus with cooresponding PSF values.  These values are summed and multiplied by the pitch adjustment and added to the portion of DL from the truss self-weight)

Additional BC DL:
--Ceiling Finish:
--Purlins:
--Sprinkler:
--Other:
(These boxes have pull down automated menus with cooresponding PSF values.  These values are summed added to the portion of DL from the truss self-weight.  If you have scissor trusses you may use a DL pitch adjustment on BC)

For your example (4 PLY GIRDER 36', (5:12), 24" O.C. Trib, 1400# Self weight, Assuming 15# felt, medium asphalt shingles, 7/16" OSB or Com-Ply for top chord loading...1/2" gypsum on bottom chord) The required DL TC = 14.68 PSF and DL BC = 11.92 PSF, again given that spacing.  

You can download a dead load calculator from WTCA called LoadGuide, but, I caution you that the purlin weight at a given spacing is incorrect.  They are too heavy.  I called them and they agreed.  The spreadsheet should take you about 2 hours to put together but it will save you a lot of time.

Good Luck.

woodengineer

ctcray (Structural) (OP)
17 Sep 07 8:58
Thanks for the responses.

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