actual design loads

huangyhg

actual design loads.
i attended a seminar last week presented by lawrence a. kloiber of lejeune steel company minneapolis, mn on practical connections using the 2005 aisc.  i work for a glulam fabricate and design glulam connections.  however i could relate very well to his discussion on the use of actual loads.
he showed several examples of cases where the eor used a standard table for specifying connection load requirements  and where the eor uped the load for cya reasons.  in his examples he should the connections designed for the loads on the drawings and what the connection would look like if designed for the actual load.
i haven't done any major steel design for almost 10 years so i could not relate his examples to any steel connections.  however i could relate them to numerous cases where the drawings required the design of glulam connections for many times the actual load.
one example i thought of was where the engineer added 15 psf to the roof dead load just in case the jowner added a ballasted roof in the future.  in another example the engineer required designing for a drift load which would have been 2' above the roof parapet causing the drift.
the problem with not specifying the actual loads for wood connections is that it goes against the general principle of using the smallest number of fastners in the wood to reduce potential problems in the connections.  
as you increase the number of connectors you increase the stiffness of the joint and you also increase the potential for the connectors to cause splitting as the member dries in place.
so as a wood designer i strongly endorse mr. kloiber advice to specify the actual loads.  
   
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wood and steel are two different materials.  i don't have much experience with timber construction for buildings.
however, i would submit that deisgning connections for some additional load is engineering judgement.  several things need to be considered by the eor.  type of building, owner of the building, use of the building are just a few.  on certain types of buildings that i do, the use changes, the occupancy changes, or the owner adds equipment after construction.  if the connections had been designed for the actual loads, modifications to the connection would have been required.
sperlingpe
you are right, the key in your statment is some additional load and engineering judgement.  in mr kloiber examples he was talking about such things as using standard plan notes from previous jobs with out really thinking about what the results are on the current job. or deciding to add in another 25 psf just to be safe with out really thinking things through.
a problem i run into all the time is engineers who will list  beam reactions based on a combined dead and live load that exceeds the capacity of the 2" decking specified.  on a timber job there is a huge cost increase in going from 2" decking to 3" decking.  i often will end up calculating the actual dead load ajnd check the decking out using that dead load.

you should detail the wood joint so that restraint will not cause splitting due to shrinkage... nearly all failures occur at connections hence the concern and over design...
dik