L_Bey
Structural
- Aug 8, 2017
- 18
We have been having a discussion in my office lately on the requirements for designing townhouses per the IRC. The main question is do we need to comply with the braced wall requirements per unit, or is the entire group of townhouses to be designed as a single structure. Often these structures are 4 or more units, maybe a garage door on one end and entry/windows on the other, leading to pretty limited available wall in one direction and plenty in the other (with the demising walls between the units).
The IRC defines a townhouse as "A single-family dwelling unit constructed in a group of three or more attached units in which each unit extends from foundation to roof and with a yard or public way on not less than two sides."
The section on wall bracing reads as follows: "R602.10 Wall bracing. Buildings shall be braced in accordance with this section or, when applicable, Section R602.12. Where a building, or portion thereof, does not comply with one or more of the bracing requirements in this section, those portions shall be designed and constructed in accordance with Section R301.1." (Section R301.1 allows an engineered solution in accordance with the IBC where the prescriptive requirements are not met)
Table R602.10.1.3 lists "Townhouse" as a building type for the maximum braced wall line spacing requirements for wind and seismic. Section R602.10.3 item 3 notes that townhouses in seismic design category C se the greater of the wind or seismic braced wall line lengths.
Both of these requirements lead me to believe that the intent of the code is to design the townhouse (a single unit in a group of 3 or more, with exterior walls on 2 sides) per the braced wall requirements, meaning that each unit in the group will have the full braced wall requirement met, and if one unit burns down (or isn't built at all) the individual is still acceptable. The alternative would be to design the entire structure per the IBC and not use the prescriptive requirements at all. My boss has argued that if the entire group of townhouses (since R602.10 states that a Building shall be braced) can meet the braced wall panel length requirements. This would mean that each individual townhouse unit would not need to meet the full braced wall line requirements by itself.
What say you?
The IRC defines a townhouse as "A single-family dwelling unit constructed in a group of three or more attached units in which each unit extends from foundation to roof and with a yard or public way on not less than two sides."
The section on wall bracing reads as follows: "R602.10 Wall bracing. Buildings shall be braced in accordance with this section or, when applicable, Section R602.12. Where a building, or portion thereof, does not comply with one or more of the bracing requirements in this section, those portions shall be designed and constructed in accordance with Section R301.1." (Section R301.1 allows an engineered solution in accordance with the IBC where the prescriptive requirements are not met)
Table R602.10.1.3 lists "Townhouse" as a building type for the maximum braced wall line spacing requirements for wind and seismic. Section R602.10.3 item 3 notes that townhouses in seismic design category C se the greater of the wind or seismic braced wall line lengths.
Both of these requirements lead me to believe that the intent of the code is to design the townhouse (a single unit in a group of 3 or more, with exterior walls on 2 sides) per the braced wall requirements, meaning that each unit in the group will have the full braced wall requirement met, and if one unit burns down (or isn't built at all) the individual is still acceptable. The alternative would be to design the entire structure per the IBC and not use the prescriptive requirements at all. My boss has argued that if the entire group of townhouses (since R602.10 states that a Building shall be braced) can meet the braced wall panel length requirements. This would mean that each individual townhouse unit would not need to meet the full braced wall line requirements by itself.
What say you?