1996 LRFD Manual for Engineered Wood Construction

GUIDELINELRFD MANUALTO LRFDFORFORENGINEEREDSTRUCTURALWOODCOMPOSITECONSTRUCTIONLUMBER 1919CommercialReservoir Cover2PROJECT PROFILES: CASE STUDIESProject DescriptionDuring 1992 the City of Los Angeles, Department ofWater and Power erected a reservoir cover over the VanNorman Bypass Reservoir in San Fernando, California,part of the Los Angeles drinking water supply system. Acovered reservoir offers several advantages. The mostobvious reasons include the minimization of both waterevaporation and contaminants (e.g., airborne pollutants,bird feces, etc.). The secondary benefits of a reservoircover include: reduced chemical usage (i.e., primarilychlorine) and improved safety and security. An enclosedreservoir assists in keeping people and animals from enteringthe area.The reservoir cover is a dome-like structure. Thecenter of the roof is approximately eight feet higher thanthe perimeter. Total roof area exceeds 600,000 squarefeet, or approximately 14 acres. The roof stands 44 feetabove the reservoir floor, and is supported by glulambeams and metal web wood trusses on a 60 x 60-foot concretecolumn grid. On top of the wood structural systemare 3 x 20-foot aluminum roof panels, installed to minimizetotal dead load and provide a continuous, water-tightbarrier.Design ConsiderationsPrimary engineering design considerations for thecover included both seismic activity and wind uplift wherethe Uniform Building Code was used. However, with theexception of seismic and wind consideration, the primarydesign issues were not governed by the UBC, but ratherby the end-use specification and defined by the Departmentof Water and Power. Because this water was actualdrinking water, the primary issues were: strict maintenanceof water quality and roof structure longevity/durability.Materials SpecificationsIt was essential that the roof protect the water fromcontamination for the specified design lifetime. But itwas equally important that the building materials themselvesdid not contribute any contamination. To this end,Alaskan yellow cedar was selected as the wood species tobe used in both the primary and secondary structural members.This species was used because of its balance betweennatural durability and strength characteristics.Alaskan yellow cedar is not commonly used in glulambeams, therefore, a series of tests were conducted by theAPA-EWS to verify their performance capability.AMERICAN FOREST & PAPER ASSOCIATION