Wood Beam Girder
Wood Beam Girder
(OP)
I have a (7) 2x12 joist girder supporting ground level floor that is overstressed (1400 psi actual - 1060 psi allowable). The joist girder spans 4 bays 13'-2" each long supported on brick piers. The ground level will be converted to a restaurant and the basement will be used for storage. Therefore, the owner opposes the use of additional posts in the basement to reduce the joist girder spans. The other option that could be used is using 1/8 inch steel plates on each side of the beam girder and bolt them together through the (7)2x12 members. Any other ideas?






RE: Wood Beam Girder
I assume the floor joists go OVER the supporting girder. If they are face mounted to the side of the girder then that might require another solution as the LVL's won't be as wide as the (7) 2x12 assembly.
RE: Wood Beam Girder
A way that may allow use of 1/8" thick plates would be to add a 2x12 on each side of the existing 7 members and sandwich the plates on each side between the existing members and the added 2x12s. This sandwich should keep the plates vertical without an excess number of bolts.
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RE: Wood Beam Girder
I'm assuming the 7 joists have staggered splices though out the length? At 52' seems unluckily they would be continuous. Are you assuming any continuity over supports or are you just treating it as "pin-pin"
I'm not a huge fan of plates on the outside for the buckling reasons mentioned above. We do use flitch beams somewhat often, but those have wood on the outside to stabilize. What about channels bolted to the outside of the built up joists? We do this often for residential structures when adding basements below existing floor girders.
RE: Wood Beam Girder
I'd be reluctant to drill bolt holes through really good lumber, unless of course, one must.
But then again, what is the origin on the 1400 number. Is there a potential for an excessive load in this restaurant from a default exit corridor or a dance floor or..?
Just some thoughts, happy 4th!
RE: Wood Beam Girder
RE: Wood Beam Girder
RE: Wood Beam Girder
RE: Wood Beam Girder
To make any new sistered members effective in sharing the loads and stresses, you have to jack the existing members up; in affect de-stressing (unloading) them. Then apply the new side members, and then let the girder back down. This allows the new fasteners to do some bending and crushing in bearing, so that they do start to transfer some of the loads to the new side members. Otherwise, the existing members start out with their stress, whatever that is, and that stress (and deflection) must get even larger before the new fasteners and side members are brought into play. 7-2x12's is already a lot of plys and you still didn’t say that the floor jsts. are bearing on top of the girder, but I assume they are. From that era, those 2x12's are probably 1.625" wide, so your new side members will not be bearing on a 12" pier. What are you going to do about that? Also, look at the existing splices and consider that at any given location you probably only have 5 or fewer 2x12 making up the actual member, at that splice location. Stretch a string (piano wire) on that girder and study its deflection, maybe load one span and compare its deflection to a calced. deflection. This, and a lack of much permanent set in the girder should give you come confidence in the quality of the lumber used. How has the girder done under the type of loading it has seen over the years? Look for any grade stamps. In that era 1400-1500psi was a pretty common extreme fiber stress for both Doug. Fir and Sou. Pine. Look at some of the new structural screws for fixing the new side members. They are quite effective in shear and in pulling members together, both important in this application. They or bolts or nails shouldn’t appreciably weaken the girder since you shouldn’t be applying them that close to the extreme fiber, of course, they should be staggered.
RE: Wood Beam Girder
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RE: Wood Beam Girder
destressing the existing member is key and engaging the new channels. the steel at each face works well to force a nice clamping action to engage the VQ/I of your composite shape. i am assuming shear is a non issue for the span. the channel has a much better Ix than flat plate, and with the steel Member EI versus your wood beams, channels will suck the load.