Wet Use / Service Factor and Deck Design

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So, think this through with me ...

(Yes, there is a deck under here.  The framing may or may not be wet - but it is certainly not warm.)

This deck (the one we have been looking at in class), intermountain Idaho, cold, high elevation, absolute humidity really low (even when snowing) ... for the design snow event I am going to judge the joists (and ledger) relatively dry (as long as they are allowed to breath). So, C_M = 1.00 for the gravity load design condition.

Now let’s think of the spring of the year, when there is less snow ... it has been melting, and everything is wet, and has been for weeks. Then the earthquake hits. I will do the design checks for the seismic event using the Wet Service Factor for wet service. For the anchor bolts ... C_M = 0.7. I could (and maybe should) check the joists ... the load will be less (and the Code will provide for `just how much’ less) ... the Load Duration factor will go up (C_D = 1.6) ... and the Wet Service Factor will drive the wood strength down. Probably we’ll be okay. (But it would be `fun’ to check.)

Now let’s think of preservative treatment and decay. The deck joists will be wet for short periods of time during (and after) rains ... when it’s (relatively) warm. But after the rain the humidity falls again and if the joists are allowed to reach equilibrium moisture content ... they will get pretty dry pretty fast. In fact, in any particular rain ... my guess is that the surface gets moist without the moisture content (MC) of the interior wood necessarily changing much. That’s for rain.

For the snow melt ... which may be occurring over months ... the joists are wet – but everything is still cool – and I am thinking there is little decay activity. So, all this to say, then, that I will probably not spec. P.T. joists unless there is some condition that will cause them to be wet or moist for prolonged periods when it is also warm.

Now let’s direct our attention to the ledger. The ledger is in contact with concrete. Typically in this type of construction the concrete will be of a quality that can transport moisture ... so, even if the air is dry ... the concrete can be bringing moisture up from the ground/foundation ... any time of year. Plus, the interface (for lack of better word) between the concrete wall and the ledger is one place that might indeed trap (again for lack of better) water or moisture. Thus! ... I will probably spec. a P.T. ledger.


Note: as said in class, if this deck were in the Coastal or Cascade Range, where the snow is heavy, wet, warm, and the air humid, ... I would carry Wet Use/Service Factor into all my calcs.

JRF