Moment Frames In Combinations with Shear Walls. Large Brace forces.
Moment Frames In Combinations with Shear Walls. Large Brace forces.
(OP)
I live in SF CA and I'm currently involved in a number of seismic retrofit jobs. These jobs are all wood framed residential units 3 to 4 stories over mostly open garage spaces. Basically your classic soft story type structure. Almost all of them are rectangular in shape aprox 25'X75'
Often these lower levels (in the short direction) have one or two available interior wall lines that can be strengthened to act as a shear wall, a full wall at the rear that is easily strengthened, and an open garage front that begs for a frame.
Per ASCE7 12.2.3.3, if I use a combination of moment frame and shear walls in the short direction, I must use the lowest R value of the two systems. This means that if I use OMF at the front, the forces to my shear walls get factored up by (6.5/3.5)=1.86. This (more often than not) seems to either render the shear wall ineffective (such as the rear wall where we only want to add ply to the inside face to avoid exterior work) or the diaphragm becomes over loaded (straight board sheathing) without adding more wall lines.
Using a SMF would circumvent this issue because I can use the lower R=6.5 to design both the frame and the shear walls, but I've run through a couple designs and the out of plain bracing requirements for the SMF seem impossible to reasonably develop into the wood diaphragm. This seems to be a selling point for proprietary products such as the Simpson Strong Frame SMF which requires no additional bracing Link
Parking is a premium in these units, so adding walls is not really an option. Are ppl out on here using SMF in wood retrofits and if so how are you getting these brace forces to work out? Any tips on getting a OMF in there without the shear walls loads blowing up??
thanks so much.
Often these lower levels (in the short direction) have one or two available interior wall lines that can be strengthened to act as a shear wall, a full wall at the rear that is easily strengthened, and an open garage front that begs for a frame.
Per ASCE7 12.2.3.3, if I use a combination of moment frame and shear walls in the short direction, I must use the lowest R value of the two systems. This means that if I use OMF at the front, the forces to my shear walls get factored up by (6.5/3.5)=1.86. This (more often than not) seems to either render the shear wall ineffective (such as the rear wall where we only want to add ply to the inside face to avoid exterior work) or the diaphragm becomes over loaded (straight board sheathing) without adding more wall lines.
Using a SMF would circumvent this issue because I can use the lower R=6.5 to design both the frame and the shear walls, but I've run through a couple designs and the out of plain bracing requirements for the SMF seem impossible to reasonably develop into the wood diaphragm. This seems to be a selling point for proprietary products such as the Simpson Strong Frame SMF which requires no additional bracing Link
Parking is a premium in these units, so adding walls is not really an option. Are ppl out on here using SMF in wood retrofits and if so how are you getting these brace forces to work out? Any tips on getting a OMF in there without the shear walls loads blowing up??
thanks so much.






RE: Moment Frames In Combinations with Shear Walls. Large Brace forces.
RE: Moment Frames In Combinations with Shear Walls. Large Brace forces.
Would that get you there?
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RE: Moment Frames In Combinations with Shear Walls. Large Brace forces.
JAE, thank you. None of these buildings are 2 stories or less (exception 2) so that wouldn't apply unfortunately.
RE: Moment Frames In Combinations with Shear Walls. Large Brace forces.
DaveAtkins
RE: Moment Frames In Combinations with Shear Walls. Large Brace forces.
Check out section A6.3 of AB-107. This might sound crazy, but the section modifies the R factor for a cantilevered column system if you meet the requirements. Therefore, per this section you can use an R = 8 for cantilevered columns.
RE: Moment Frames In Combinations with Shear Walls. Large Brace forces.
We use A$ in this office as we typically don't know (or don't get) 2nd floor wall layouts required for the FEMA analysis.
What the the deal with the wording in section A6.3 regarding a cantilevered column system meeting the detailing requirements of special, intermediate, or ordinary. Is this just referring to what column sections are permitted to be used based on 341-10?
RE: Moment Frames In Combinations with Shear Walls. Large Brace forces.
A6.3. Use of Cantilevered Column Systems. Cantilevered column systems conforming to the following provisions may be considered as moment frame systems (Special, Intermediate, or Ordinary, as applicable, based on detailing) with regard to determination of the R, Ω0, and Cd factors.
1. Columns shall not carry gravity load.
2. Columns shall be configured in pairs (or larger groups) connected by a continuous foundation or grade beam.
3. The continuous foundation or grade beam shall be designed to resist the expected plastic moment at the base of each column, computed as RyFyZ, as defined in AISC 341-10.
4. The flexibility of the continuous foundation or grade beam, considering cracked section properties of reinforced concrete, shall be included in computing the deformation of the cantilevered column system.
5. Cantilevered columns shall be considered as twice their actual height when checking lateral torsional buckling.
RE: Moment Frames In Combinations with Shear Walls. Large Brace forces.
ASCE7 Table 12.2-1 kicks me to Section 14.1 which then kicks me to AISC 341, but I cant seem to find anything there. Am I overlooking it or in the wrong place?!
RE: Moment Frames In Combinations with Shear Walls. Large Brace forces.
RE: Moment Frames In Combinations with Shear Walls. Large Brace forces.