Calculating the deflection of a wood truss
Calculating the deflection of a wood truss
(OP)
I don't have truss or 2D frame software - if I assume the Moment Area of Inertia based on the cross section of the top and bottom chords, as if they were connected with solid web that had no deflection resistance by itself, i.e. solving it as a simple beam, is that ballpark or will give a deflection result too much, or too little?






RE: Calculating the deflection of a wood truss
RE: Calculating the deflection of a wood truss
Quando Omni Flunkus Moritati
RE: Calculating the deflection of a wood truss
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Calculating the deflection of a wood truss
When I did a hand calculation as described above, I got the exact same value (by coincidence) as their truss software.
Therefore I suspect their software is a bit unconservative, especially because this girder truss is so shallow (span to depth) and is very heavily loaded.
RE: Calculating the deflection of a wood truss
RE: Calculating the deflection of a wood truss
Thanks.
RE: Calculating the deflection of a wood truss
deflection based on MoI of chords > deflection based on full MoI, no?
Quando Omni Flunkus Moritati
RE: Calculating the deflection of a wood truss
RE: Calculating the deflection of a wood truss
Yes
Yes
RE: Calculating the deflection of a wood truss
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Calculating the deflection of a wood truss
RE: Calculating the deflection of a wood truss
Yes that is worrying me a lot about this girder repair that the truss mfr came up with - I recommended they make it deeper but they couldn't, claiming it was impossible because of existing electrical conduit - grrrr.
It is only 28" deep, has a span of 33', and a dead load of about 480 plf
Then to add another layer of anxiety, they stupidly did not originally check how the supported trusses load transfer to each of the 5-plies, so they stupidly had to (in a hurry) used 1/2" dia bolts that further reduced the MoI, and they stupidly made the statement, well...the truss software can't take that into consideration.
Of course, worrying about the huge torsion on that girder would be something they wouldn't be capable of either - they had to use top flange case Simpson THA222-3 hangers, which IMO exacerbate the torsion problem.
All this had to be resolved Friday because they had to fab the girder Friday night and ship it to the jobsite Saturday (yesterday)
RE: Calculating the deflection of a wood truss
RE: Calculating the deflection of a wood truss
RE: Calculating the deflection of a wood truss
RE: Calculating the deflection of a wood truss
I asked them how they came up with that number of nails but got no response - see attached
RE: Calculating the deflection of a wood truss
Twice the deflection calculated for a beam, with the beam moment of inertia calculated only from the chord areas (excluding the bh^3/12 of the individual chord elements)
as AELLC referred to in his original post.
See the attached reference from Andrew Orton's, "The Way We Build Now: Form, Scale and Technique" that seems to agree.
RE: Calculating the deflection of a wood truss
However I am doing a study in my spare time, saying that is moderately inadequate.
Where did he get that? Sounds over-conservative.
RE: Calculating the deflection of a wood truss
It doesn't say twice the delta, it just says delta is higher. See above posts by others, they are estimating 15-20% higher, not 100% higher.
RE: Calculating the deflection of a wood truss
The posts above and elsewhere are saying additional, hard-to calculate delta from:
1) Axial deformation of web members under their axial loads
2) Elastic (spring) deformation of the splice plates, especially at chords
3) Long-term creep of truss plates (not the plate itself, its grip into the wood)
RE: Calculating the deflection of a wood truss
The reference says the equation for delta is 10WL^3/384EI. That is twice the classical beam theory equation 5WL^3/384EI.
It all depends on span/depth ratio and the fixity of the web connections. With a SJI steel joist you have very high span/depth ratio, and the web members have high axial and rotational fixity to the chord members. It is approaching the behavior of a solid beam. So, SJI has empirically found their factor. The last long span, parallel chord timber truss I designed had a span/depth ratio of 7, and approaching truly pinned web member connections. The deflection using GTStrudl was ~1.8 times the beam analogy I referenced, and actual measurements showed ~1.9 (for dead load).
I spent three minutes running a quick truss through GTStrudl and got the following:
Span = 70'
Depth (c-c of chords) = 10'
Chords: W8x21
Load: 3 kips @ each of 7 top chord joints ==> .30 k/ft = .025 k/in
Beam-analogy moment of inertia = 2 x (6.16 x (60^2)) = 44352 in4
Deflection using beam-analogy = (5wL^4)/(384EI) = .126"
Deflection using GTStrudl = .2164"
Ratio of Theory/Beam-Analogy = .2164/.126 = 1.71
I've used this technique for years and have gotten a "feel" for what factor to use depending on joint type and span/depth ratio. Again, I only use this as a starting point with a spreadsheet to begin the design of a truss. That usually gives a good estimate for selecting your initial chord size.
RE: Calculating the deflection of a wood truss
OK I missed that.
What you are saying, is all those expensive truss softwares such as Alpine Truss and Mitek are completely wrong for the deflection calculation.
RE: Calculating the deflection of a wood truss
I have no clue with respect to truss-specific software. Never used them. However, those packages should be using virtual work or another "true" analysis approach rather than a beam-analogy approximation. I just didn't want someone in the future coming to this thread and using 1.2 x beam-analogy deflection numbers when its not appropriate.
RE: Calculating the deflection of a wood truss
I don't have access to the truss mfr software. My only ax to grind is the resolution of a serious mistake made by the truss designer.
Also in the future I would like to have a wood truss "rough estimate" design capability in my design Excels so I can put more stringent notes on my framing plans, directing the truss mfr to take extra care.
Right now they totally ignore what I indicate anyway, ugh.