1) What are the height limitations for five stories of residential use in wood construction over one story of concrete construction?
The total building height is limited to a maximum of 85 ft above grade plane when using type III-A construction in accordance with Chapter 5 of the Oregon Structural Specialty Code (OSSC).
In addition, where lateral resistance is provided by light-framed walls sheathed with wood structural panels rated for shear resistance or steel sheets or flat strap bracing, the maximum height of this lateral system is limited to 65 feet measured from the top of the concrete (type I-A construction), provided the engineer can demonstrate through calculations that the entire structure meets the requirements for a two-stage analysis of section 188.8.131.52 of ASCE 7-10.
See Q2 and Q3 below for further limits on building heights.
2) May non-fire-retardant-treated wood be used in type III construction?
In some instances, the Bureau of Development Services will allow the use of non-fire-retardant treated wood. See Code Guide IBC/6/#4, “Non-Fire-Retardant-Treated Wood Framing Within Exterior Walls of R-2 Occupancy Buildings of Type III Construction.
Other use of non-fire-retardant-treated wood would need to be approved through the building code appeals process.
3) Can the concrete podium, type 1-A construction, be two or more stories in height?
Special provisions in chapter 5 of the OSSC limit the number of stories of concrete (type 1-A) construction to one story above grade plane with or without a basement. Other configurations would need to be approved through the building code appeals process.
Where lateral resistance is provided by light-framed walls sheathed with wood structural panels rated for shear resistance or steel sheets or flat strap bracing, the maximum height of this lateral system is limited to 65 ft measured from top of concrete (Type I-A construction), provided the engineer can demonstrate through calculations that the entire structure meets the requirements for a two-stage analysis of section 184.108.40.206 of ASCE 7-10.
4) What Special Inspections or Structural Observations required for the wood framed structure?
Special inspections are required for key elements of the lateral force resisting systems of the wood framed structure including, but not limited to, the following:
- The grade of structural wood panels used in the sheer walls and horizontal diaphragms;
- Nail size and pattern of the sheer walls and horizontal diaphragms;
- Framing, location and length of all shear walls;
- Hold down installations at all shear wall locations;
- Diaphragm chord, drag strut and related details;
- Blocking to top plate nailing.
Special Inspections may be required for other elements of the structure including but not limited to concrete, steel, anchors installed in concrete etc.
Structural observations are required and shall be provided by the engineer of record. The engineer of record is required to submit copies of the structural observations report to the Bureau of Development Services.
5) Does the podium slab supporting the discontinued wood shear walls and the connections of the wood shear walls to the podium slab be designed for the seismic overstrength factor (Ωo)?
Section 220.127.116.11 of ASCE 7-10 requires that “ …Columns, beams, trusses or slabs supporting discontinuous walls ……… shall be designed to resist the seismic load effects including overstrength factor of section 12.4.3….”This requirement therefore requires that the podium slab be designed for forces due to overturning considering the seismic overstrength factor (Ωo).
Section 12.3.3 further states that “… The connections of such discontinuous elements to the supporting members shall be adequate to transmit the forces for which the discontinuous elements were required to be designed.”
This section requires that the connection between the podium slab and the wood shear walls be designed for the forces for which the discontinuous elements (wood shear walls) are designed for. Since typically wood shear walls are not designed for the overstrength factor (Ωo), unless required by other provisions of the code, connections of the wood walls to the podium slab are not required to be designed for the overstrength factor.