Calculating the deflection of a wood truss 3

[AELLC]

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?

 

[AELLC]

Shoot me and put me out of my misery, now I am getting emails saying the truss repair wasn't done Saturday as planned, they are still tweaking their method.

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)

 

[theonlynamenottaken]

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.

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.

 

[AELLC]

theonly,

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.

 

[theonlynamenottaken]

AELLC,

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.

 

[AELLC]

theonly,

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.