water and wood

[diggingdeeper]

i have a 70' bowstring truss with the 6x6 bottom chord split through the bolts from the heel past the first panel point. the truss supports a mech load above the 1st top chord panel point (1st in from the heel), and there is evidence of water on the underside of the sheathing in this area. the top chord consists of 4-2x6 flat, apparently nail laminated together.
with the break, the top chord has essentially 'lost it's curvature' over that 1st top chord panel point, ie. 'flattening out'.
can you help me understand and quantify the impact that water may have had on this scenario?

 

[civilperson]

Wet lumber has 30% less load carrying capacity.

 

[msquared48]

Is this the same 70 foot Bowstring truss that is in the Structural Engineering Other Topics section of the forum?

Mike McCann
MMC Engineering

 

[hokie66]

Water is not good for the wood, but it is even a worse enemy of steel nails and bolts. My first thought is that the nails have corroded, allowing the plies to straighten, and the bolts have corroded, causing the bottom chord splitting.

This sounds a lot like a DIY job. Carpenters building something without engineering assistance.

 

[diggingdeeper]

Mike: yes it is the same 70' truss project. there are actually 4 broken trusses. 2 have similar conditions loading and water wise and are what this thread is asking about. the other two have breaks at bottom chord 2nd panel point from heel, no sign of water, and vertical break essentially at the vertical bolt through the bottom chord.

 

[diggingdeeper]

sharp(hokie)66: excellent sleuthing. very interesting idea. thanks

 

[diggingdeeper]

civilperson:
can you provide me with reference(s) for 30% number?
thanks

 

[Ron]

The mechanical properties of wood change with the moisture content. Generally, the properties increase when the moisture content is below 24% during the initial drying process. When wood is re-wet to various moisture contents, the properties again change. Generally there is about a 25% reduction in most properties (bending, compression, etc.) when the moisture content reaches the Fiber Saturation Point.

Attached is Chapter 4 from the Wood Handbook (public domain)that outlines such changes.

 

[diggingdeeper]

Ron: perfect, thanks.

 

[dik]

The biggest problem, I've encountered, with moisture is the deterioration from Brown Rot (aka Dryrot).