By Jim Taggart, FRAIC
To understand the principles of seismic design, one must first understand the nature of seismic forces. Initiated by the movement of the Earth’s tectonic plates, they take the form of waves that travel either in the body of the planet or at its surface. Body waves are further subdivided into primary (P) waves—behaving like the repeated compression and release of a spring—shaking a building in the horizontal plane, and secondary (S) waves—transverse in nature—shaking a building in the vertical plane.
As knowledge of earthquake behavior has evolved, more sophisticated approaches to the seismic design of buildings have been developed. An important consideration is that of ductility. In the case of major earthquake events, the energy-dissipative components are designed to perform ‘plastically’—absorbing energy through deformation and permitting a certain level of damage to the structure, but preventing the catastrophic collapse of the building. In typical wood buildings, the main source of ductility are the connections. (Even after moderate earthquakes, it is prudent practice for building owners to have an inspection carried out by a qualified structural engineer to ensure that the integrity of the structure has not been compromised).
Although seismic events occur all over the world, the areas most susceptible to large earthquakes lie along the boundaries of tectonic plates, including those on the so-called ‘Ring of Fire’ encircling the Pacific Ocean and passing through British Columbia
British Columbia’s seismic upgrade program
With several major earthquakes having struck other countries on the Ring of Fire in the past two decades, there is a heightened awareness of the risk faced in British Columbia. A survey commissioned by the provincial government and conducted by the Association of Professional Engineers and Geoscientists of British Columbia (APEGBC) in 2004 determined a significant number of older B.C. schools did not meet the then-current safety requirements in terms of seismic design.
Of these, 339 were found to include structures in the highest risk category (known as H1)—those most likely to experience widespread and irreparable damage or structural failure in a seismic event. In response to these findings, the province initiated a seismic upgrade program, which has retrofitted or replaced 224 of the highest risk schools in 37 districts to date. A number of these projects used wood as the primary structural material. (The seismic mitigation program and the associated risk categories are described on the Province of British Columbia website. It is important to note classifications are applied not to schools as a whole, but to ‘blocks’ within the building. Blocks represent areas within a school that are of different construction types and have different structural characteristics. For example, gymnasiums will typically have a different structural system than classroom or administration blocks, and as a result may have a different risk rating).
Seismic upgrading of Wellington Secondary School
Located in Nanaimo on Vancouver Island, Wellington Secondary School is a two-storey, 10,750-m2 (115,712-sf) structure with a capacity of 900 students. It was built in several phases from 1969 to 2000, on a radial plan with a circular central block—known as Block F—surrounded by five other blocks (Blocks A, B, C, D, and E) as shown in Figure 1.
Seismic assessment and design approach
In 2012, structural engineers from Nanaimo-based Herold Engineering prepared a Seismic Project Investigation Report identifying four of the blocks (all except C and D) as risk category H1. Two seismic mitigation options were considered:
- comprehensive upgrades to bring all the structures up to current code standard; or
- a partial upgrade, together with the demolition and replacement of the highest risk portions of the building (Blocks A and F).
More detailed consideration of site constraints, the provision of temporary classroom accommodation, parking, site access, and staging confirmed demolition and rebuild was the more economical option.
This option included the seismic upgrades of Blocks B, D, and E, the demolition and rebuilding of Block F, and the demolition and replacement of Block A with a new classroom block (in another location referred to as Block G).
The upgrading of Blocks B and E were carried out at the beginning of the renovation. Both had been constructed in 1987 with a combination of precast concrete panels, unreinforced (or partially reinforced) concrete masonry unit (CMU) walls, and heavy timber roofs with glued-laminated (glulam) beams, tongue-and-groove decking, and plywood sheathing. The seismic upgrade included additional reinforcement of the masonry walls, higher ductility connections between the walls and roof, and the strengthening of the roof diaphragm with an additional layer of plywood. (Cross-laminated wood products, such as plywood and CLT, are particularly well-suited to use as diaphragms as they resist racking when subjected to lateral forces).