Lumber sheathed diaphragm
Lumber sheathed diaphragm
(OP)
The project involves the renovation of an 100 year old building. Clay brick exterior bearing walls, steel frame interior supporting timber joists. Floor diaphragm is lumber sheathed (perpendicular to joists) with hardwood over top of planks (perpendicular to lumber sheathing).
All interior gypsum block/lath and plaster walls are being removed. We are providing new LMF shear walls to replace existing walls, although not in the same locations. IBC 2006 does not allow an increase in seismic force in any existing structural element of more than 10% without the entire lateral system meeting the requirements of ASCE 7 for new structures. The new locations of the shear walls will increase the force in the horizontal diaphragm by more than 10%. NDS and FEMA/NEHRP have values for lumber sheathed diaphragms, but I could not find any values or discussion of this type of diaphragm in IBC. Am I stuck having to add plywood to every floor, or locate shear walls to maintain the same sub-diaphragm aspect ratios?
All interior gypsum block/lath and plaster walls are being removed. We are providing new LMF shear walls to replace existing walls, although not in the same locations. IBC 2006 does not allow an increase in seismic force in any existing structural element of more than 10% without the entire lateral system meeting the requirements of ASCE 7 for new structures. The new locations of the shear walls will increase the force in the horizontal diaphragm by more than 10%. NDS and FEMA/NEHRP have values for lumber sheathed diaphragms, but I could not find any values or discussion of this type of diaphragm in IBC. Am I stuck having to add plywood to every floor, or locate shear walls to maintain the same sub-diaphragm aspect ratios?






RE: Lumber sheathed diaphragm
Don Phillips
http://worthingtonengineering.com
RE: Lumber sheathed diaphragm
RE: Lumber sheathed diaphragm
Don, under the alternative methods section, 104.11, it would appear that the "alternative method" is based on the building officials opinion. So I guess, depending on the building official this may or may not be an accepted method.
RE: Lumber sheathed diaphragm
Tranverse lumber sheathed diaphragms have very low shear values and usually large deflections. Another advantage of adding the plywood, is that you have well established shear values so when you are done you know you have a very reliable diaphragm.
RE: Lumber sheathed diaphragm
If the only sheathing were horizontal planks, the answer would be easy (NDS allowable shear = 100 plf) due to the very limited allowable shear value. However there is hardwood flooring on top of the planks spanning perpendicular to the planks. FEMA 356 gives values for this type of diaphragm (400 plf +/-) that I could live with, probably just using some additional nailing and chord reinforcing.
RE: Lumber sheathed diaphragm
From a design professional liability prespective, if you are following the code, I think you would be on safe ground regarding neglience claims.
Don Phillips
http://worthingtonengineering.com