Improving Architectural Flexibility of Wood Frame Buildings Through Innovative Structural DesignApr 16 2014 · 0 comments · NISHKIAN DEAN, Wood Frame ·0
Basics of Lateral Load Resisting Systems in Wood Frame Buildings
Buildings resist wind and seismic forces through a combination of horizontal and vertical lateral force resisting systems. The lateral forces are first transferred through the horizontal elements at each floor which then act as deep beams to distribute the loads to the vertical elements. In wood frame construction, the horizontal elements are typically floor and roof diaphragms consisting of plywood sheathing nailed to wood framing members, such as joists, beams, and blocking. The vertical elements are typically shear walls consisting of plywood sheathing nailed to studs and blocking. Shear walls are anchored to the building foundation or an elevated concrete podium slab, both of which are designed to resist lateral loads and uplift forces.
“Traditional” and “Open-Front” Design Methods
In traditional multi-unit wood frame construction, interior corridor walls, unit separation walls, and exterior walls are used as shear walls. In both the longitudinal and transverse directions, the diaphragm is assumed to act like a deep beam that is supported by the shear walls. Shear walls must meet a certain aspect ratio as defined in the current code in order to be effective at resisting lateral loads; very short lengths of wall are generally not permitted. Locating efficient and code-compliant shear walls in multi-unit housing buildings can be a challenge since exterior walls are often perforated by numerous windows, doors, and louver openings, leaving few solid sections of wall available for use as shear walls. In addition, architecturally popular “pop outs” along the face of the building can result in exterior walls that often don’t align vertically over the height of the building. Where exterior wall segments are available for use as shear walls, the demand on these walls is often high due to their short length. This results in large anchorage forces and the need for more hardware connectors. One design approach to address these challenges is to eliminate exterior shear walls, using only the interior corridor and unit separation walls to resist lateral forces. In this method, the diaphragm is designed to cantilever from the interior corridor walls. Eliminating exterior shear walls allows for much more architectural flexibility and mitigates the constructability concerns associated with short lengths of shear walls.
The building code classifies this type of design as an “Open-Front Structure,” consisting of a diaphragm supported by plywood shear walls on only three sides, as shown in the diagram. The code restricts the use of this method to conditions where the diaphragm aspect ratio (length, L, to width, W) does not exceed 0.67 and the length does not exceed 25 feet. However, the code provides an exception to this rule, allowing the overall length and aspect ratio to be increased as long as the diaphragm deflections “can be tolerated.” While the code does not provide specific instructions on how to prove that deflections can be tolerated, it implies that the code requirements for building drift limits must not be exceeded.
Designing the building as an open-front structure increases the aesthetic flexibility of the building. By accommodating larger door and window openings and avoiding costly, short exterior shear walls, owners, architects, contractors, and future residents can all benefit from this structural design methodology.