Designing for Earthquakes
Terminology adapted from ASCE 7-10
EFFECTIVE SEISMIC WEIGHT, W: The effective seismic weight, W, of a structure includes the dead load above the base and other loads above the base as follows:
▶ In areas used for storage, a minimum of 25% of the floor live load. Exceptions: a) Where the inclusion of storage loads adds no more than 5% to the effective seismic weight at that level, it need not be included in the effective seismic weight, and b) floor live load in public garages and opening parking structures need not be included in the effective seismic weight.
▶ Where provision for partitions is required in the floor load design, the actual partition weight or a minimum weight of 10 pounds per square foot (psf) of floor area, whichever is greater
▶ Total operating weight of permanent equipment
▶ Where the flat roof snow load, Pf, exceeds 30 psf, 20% of the uniform design snow load, regardless of actual roof slope
▶ Weight of landscaping and other materials at roof gardens and similar area
STRUCTURAL HEIGHT, hn: The vertical distance from the base to the highest level of the seismic force-resisting system of the structure; for pitched or sloped roofs, measured from the base to the average height of the roof
SEISMIC DESIGN CATEGORY: A classification assigned to a structure based on its risk category and the severity of the design earthquake ground motion at the site
SHEAR PANEL:
A floor, roof, or wall element sheathed
to act as a shear
wall or diaphragm
LIGHT-FRAME WOOD SHEAR WALL: A wall constructed with wood studs and sheathed with material rated for shear resistance
BEARING WALL: Any wood stud wall that supports more than 100 pounds per linear foot (plf) of vertical load in addition to its own weight
BUILDING FRAME SYSTEM: A structural system with an essentially complete space frame providing support for vertical loads; seismic force resistance is provided by shear walls or braced frames
INVERTED PENDULUM-TYPE STRUCTURES: Structures in which more than 50% of the structure’s mass is concentrated at the top of a slender, cantilevered structure and in which stability of the mass at the top of the structure relies on rotational restraint to the top of the cantilevered element
For proper design, it is critical to identify the risk category of the building or structure. Detailed descriptions of buildings and structures associated with Risk Category I, II, III and IV are described in IBC Table 1604.5. (For a summary, see Table 1.)
The value of design seismic base shear increases with increasing values of the importance factor, which range from 1.0 for Risk Category I and II structures to a maximum value of 1.5 for Risk Category IV structures. (See Table 1.) The importance factor is equal to 1.25 for Risk Category III structures. Requirements for drift control are also linked to building risk category. Reduced values of permissible drift are associated with higher risk category structures. For example, allowable story drifts range from a maximum of 2.5% of the story height for Risk Category I or II structures to a minimum of 1.0% of the story height for Risk Category IV structures. More stringent drift requirements for higher risk category structures are expected to limit structural and non-structural damage associated with building deformation relative to lower risk category structures.
All except the tallest wood-frame shear wall buildings will be classified as short-period buildings due to the stiffness inherent in wood-frame shear wall structures coupled with the ASCE 7 maximum structural height of 65 ft for wood-frame construction. Within ASCE 7, the applicable equation for determining the approximate fundamental period, Ta, for a wood-frame building is shown in Equation 2.
From Equation 2, values of approximate fundamental period are observed to vary by structural height. The relationship between height and approximate fundamental period is shown in Table 2.
Alternative Simplified Procedure
An alternative simplified version of the ELF procedure is provided in ASCE 7-10 for Risk Category I or II buildings three stories or less in height, including building configuration limits to avoid a significant torsional response. The simplified procedure is not applicable in Site Class E or F (e.g., soft clay soils, peats and/or highly organic clays, very high plasticity clays, and very thick soft/medium stiff clays). The design base shear is roughly equivalent to that derived from the full ELF procedure for one-story structures, but results in a slightly more conservative first-story design base shear in multi-story structures. Features of the simplified procedure are: 1) a simplified story shear distribution assumption—e.g., story shear is taken as proportional to the effective seismic weight at each level, 2) omission of requirements to check story drift, and 3) use of seismic R-factors associated with the full ELF procedure.
A SEISMICALLY SOUND STRUCTURE FOR SCHOOLS
Cordova Bay Elementary School | Photographer: Krista Jahnke | Architect: Iredale Architecture
Surrey Christian School | Photographer: Ed White | Architect: KMBR Architects Planners Inc.
Because a heavier building will generally attract larger seismic forces, wood’s lighter weight provides an effective option for new construction and seismic upgrades of existing buildings, as was the case for two schools in British Columbia, Canada.
Cordova Bay Elementary in Victoria, B.C. uses a combination of cross-laminated timber (CLT) and nail laminated timber (NLT) as part of its overall renovation and seismic upgrade. The project demonstrates how a CLT panel systems can provide a cost-competitive solution for lateral design in a high seismic zone.
Surrey Christian School in Surrey, B.C. combines a glulam post-and-beam system and light-wood frame shear walls with NLT roof panels. Its lateral system was designed to resist all the required seismic load, achieved by plywood-sheathed light-wood frame walls.
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