Wednesday, January 2, 2013

things with decks ...

Couldn’t resist.  I snuck this snap of a friend’s deck to illustrate what we have been talking about in class ... and to illustrate some new stuff.


1.  Here the general design provision of the end of the beam (at bearing), being held in place to resist rotation, has not been satisfied ... and the end of the beam has rotated.  (Duhhh.)  It is the practice of some to assume that the connector (or strapping) prevents rotation.  Well, here it didn’t.  (Well, maybe it prevented more rotation, and catastrophe, at least to date, but rotation has still occurred.)  Here the end(s) of the beam could be held in place from rotation by cross or angled bracing members to the joists.


2.  The sag in the closest span beam appears to be excessive, particularly considering the beam being subject to Dead load only at the time of the photograph.   Possible cause may be a prior sustained heavy load as perhaps by a hot tub(?).


3.  Note that the framing is accomplished using FOUR simple span (series) beams.  In my opinion, a SINGLE four-span continuous beam would have tied everything together better visually, structurally, and would have knocked any deflections way down (including the excessive deflection in the nearest span). 


Note that the minimum `size’ beam, whether four simple spans or one continuous, may indeed have been the same (based on bending stress), a continuous beam is a better `solution’.


4.  The end supports are brought in a bit, which, in my opinion, and excluding the stuff that looks bad, `looks better’.  It also allows the use of a CC or PC type connector, same as for the interior supports.  Otherwise the ECC or EPC (End-Column-Cap or End-Post-Cap) connector must be used.


5.  Notice the end split that appears to coincide with the top row of connector bolts.  Remember, wood shrinks and swells across the grain.  Where shrinkage is prevented, say, due to direct connection to something that won’t shrink, like steel side plates, the wood is forced to split.  Ways to prevent this condition include making sure the moisture content of the wood during time of installation (drilling the bolt holes) is LOW (so that the wood only swells in service) ... or ... providing oversize or slotted bolt holes for the upper bolts.


6.  Gosh, please be consistent with bolt heads vs. nuts.  Probably would have looked nicer to have all the bolt heads to the outside, and nuts to the inside.


7.  Lateral support, FOR THE DECK AS A WHOLE, does not appear to be present.  Resistance to lateral loads (wind, ground motion, incidental inertial loads) must be prevented.  Some decks have lateral support provided by knee braces at the outside ends or corners.  Alternately, the deck may be `tied’ laterally to the main superstructure (lateral ties or deck ties, as we have discussed in class).  In reality, many decks are not adequately supported laterally and have collapsed (or  soon will). 


8.  Finally, the railing!  (I don’t deal with railings.)



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