Change in Wood Density and Dimension as the Result of a Change in Moisture Content

[MagicFarmer]

Good afternoon,

I am currently looking at a nail laminated bridge deck with 2x longitudinal running boards as a wearing surface.

I have already calculated the shrinkage/swelling of the deck due to changes in relative humidity. The total deck (~42m) will experience a dimensional change of appox. 744 mm, according to the equations in Structural Wood Design (Aghayere and Vigil). This seems excessive to me... Am I in the neighborhood here? That's ~1.8% over an equilibrium moisture content change of ~10%. If the deck is unrestrained, as a whole, against this expansion, am I correct in the assumption that there will be little/no stress buildup in the deck? There is a 'slip' connection between the deck and steel stringers.

Next, the waterproofing membrane exists between the nail laminated deck and the wood wearing surface. With the possibility of a multi-day rain event (relative humidity of 100% at the wearing surface), I have been trying to determine the load due to the change in equilibrium moisture content of the running boards. Aghayere/Vigil has equations for calculating the specific gravity of wood based on moisture content, but they require the specific gravity of the wood at 0% moisture content... which I have been unable to find for SPF or pressure treated SPF.

Has anyone looked at this before or have any sort of guidance?

Thanks!
MF

 

[dik]

Your 10% moisture change appears to be excessive; I would have thought that 1/2 that amount would have been lots. That said, there is a table with expansion and moisture coefficients attached. Multiply the width of the board x the coef x the change in moisture for a ballpark... this works with the width... dimensional change in length is almost nil. Expansion depends a lot on the species.

QS is quarter sawn and FS is flat sawn

Dik

 

[OldBldgGuy]

I think you're confusing width and length. As dik says, change in length is negligible. Change in width is different tangentially and radially with the growth rings, average it.

 

[MagicFarmer]

@dik, I get the same result using your numbers.

@OldBldgGuy, The deck is laminated in the transverse direction, so the swelling would be perpendicular to the grain. Using your chart for eastern white pine... unless the percentage should be represented by a decimal? If that's the case, 9 mm seems too small for a 42 m deck.

deltaL = 42 000 x 0.00212 x 10 = 890 mm

Using a spruce species results in a larger value.

Also, any insight on weight change?

Thanks again,
MF

 

[Settingsun]

Interesting system. Does the nailing cause all the shrinkage to 'appear' at the ends of the laminated panel as an overall change in dimension, or do the individual boards separate slightly by ~1mm with the overall panel dimension staying almost constant? I'd have thought the former but not sure.

For the weight at zero percent moisture content, you could probably extrapolate if you can get two points on the moisture content vs weight relationship, couldn't you? Eg fibre saturation point and kiln-dried. (I realise this is kind-of throwing your own question back at you but I think these two data points may be more readily available than zero moisture which isn't a real-world situation.)

 

[OldBldgGuy]

My apologies, if the decking is transverse then you are in the right ballpark with your numbers. You have to remember though that the deck lamination joints are unlikely to be tight despite the best intentions and the expansion and contraction will be absorbed incrementally in the full length, just like a hardwood floor in a gymnasium, so the 744 mm is spread across 42m of bridge. I can't imagine that the weight of the difference in moisture content of the decking is even within rounding errors of the bridge design. Fibre saturation point is never reached without complete submersion for an extended period, and there's no such thing as 0% MC other than in a lab. 10% is probably more than the actual moisture content change season to season, but even 6 or 7% (12% to 18% is well within reasonable values) results in large absolute numbers but negligible differences for a properly constructed deck for weight or dimensional change. You also have to remember that the wearing surface is nailed to the bearing deck and nails will easily compress the wood grain laterally or separate it longitudinally for the minuscule amount that is required in a given lamination. Nail laminated floors and roof decks were common 75 years ago, and in my area I know of and have worked on hockey arenas with NLT decks 40 or 50m wide (over an 85' ice surface plus seating areas) that display no issues at all even though they are mounted on steel trusses with masonry walls at each side (end). They don't get direct precipitation but they get interior atmospheric changes as wide as can be imagined. Keep the big picture in mind.