Seismic design with wood

Seismic analysis of Block F
The original 1969 library structure (Block F) presented a considerable challenge to upgrade, and the analysis merited a more detailed discussion. The roof structure consisted of radially arranged concrete T-beams, resting on inner and outer concrete ring beams supported on concrete columns. In turn, these columns were supported on concrete walls at the basement level. The entire structure was heavy and, having been designed to a much less demanding seismic standard, had neither the required ductile connections between the elements nor the adequate lateral restraint in the radial direction.

Generally, when comparing two buildings of equal height, in the same geographic location, and with the same soil conditions, a heavier building will attract larger seismic forces than a lighter building. The heavy weight of the Block F structure would have required a large number of custom steel brackets, substantial cross-bracing, and enlarged foundations to transfer the required loads to the ground. The more desirable, and cost-neutral, alternative was to dismantle the existing structure (Figure 2—see image carousel below) and replace it with a new lightweight building.

Wood was chosen for this replacement structure due to its economy, speed of construction, and esthetics. The wood solution met the constraints of a fast-track schedule and a tight budget, while introducing a warm look to the core of the school. Demolition of Block A opened up an area adjacent to this core that is now a new glazed entrance (Figure 3—see image carousel below).

Detailed design
The structural engineers developed a retrofit system for Block F that used the existing foundations and replicated the geometry of the original structure (Figure 4—see image carousel below). Block F is divided into two distinct but connected components. The ‘main street’ surrounding the central courtyard forms a circle with an inner and outer ring of columns connected by beams that support a sloping roof. The inner ring of columns delineates the exterior glazed wall that encircles the courtyard, while the outer ring forms a colonnade separating the main street from the rest of the school.

The main street is circular in plan, and the surrounding school is in the form of a pentagon, leaving an irregularly shaped zone between them. This zone was covered by an existing flat roof originally framed with solid timbers. The longest of these members were reinforced with laminated veneer lumber (LVL) beams, so the roof could perform as an effective diaphragm between Block F and the surrounding blocks. The engineers also added a second layer of plywood sheathing to meet load transfer and drift requirements.

This upgraded roof connects to the outer ring of posts below the eave line of the central sloping roof. This results in a discontinuous section where the roof diaphragms are not in the same plane, requiring lateral loads be transferred into the vertical structure by a pair of drag rings consisting of continuous steel cross-bracing (Figure 5—see image carousel below).

In the vertical plane, lateral resistance is provided by a series of 16 steel cross-braced frames that tie into adjacent pairs of glulam columns in the outer ring. Full-height cross-bracing is also used between pairs of columns in the inner ring (Figure 6—see image carousel below). Throughout the timber structure of Block F, connections are designed to be relatively simple and economical—the majority being exposed steel plates and brackets (Figure 7—see image carousel below).

The light weight, versatility, and economy of wood have combined to bring this project to a successful resolution, on time and on budget. Wood has also contributed additional value, creating a warm and welcoming atmosphere, one that has transformed the identity of this aging school (Figures 8—see image carousel below).

Other applications of wood in seismic design
Two other recently completed B.C. school projects illustrate alternative approaches to seismic design using wood. Cordova Bay Elementary in Victoria employs a combination of cross-laminated timber (CLT) and nail-laminated timber (NLT) panels, while Surrey Christian School combines a glulam post-and-beam system and light-wood frame shear walls with NLT roof panels.

CLT and NLT panel solutions
Cordova Bay Elementary dates from 1945, and like Wellington Secondary, has undergone multiple renovations and expansions since. Of these, the 1965 addition, constructed with unreinforced concrete masonry exterior walls and a glulam and heavy timber roof, was designated as risk category H1.

As with Wellington, a detailed assessment of site logistics temporary accommodation resulted in the decision to replace the existing structure. It was proposed the new classroom block be built with CLT walls and roof panels, and light-wood frame construction for interior non-loadbearing partitions (Figure 9—see image carousel below).

Despite the replacement option having been chosen for its low cost, the project came in a little over budget. The low bidder for the supply and installation of the CLT components, offered a savings to change the roof panels to NLT. The final result is all the loadbearing and shear walls are constructed from five-ply CLT, and the roof is NLT panels made from 38 x 190-mm (2 x 8-in.) material nailed together face to face, creating a solid deck.

The CLT panels are set vertically, extending from the ground floor slab to the underside of the roof. Base connections are steel plates set into the concrete and recessed into rebbates factory-milled into the panels. The plates are secured using long, high-strength, self-tapping screws, then covered with a wood plug so the connections are hidden and the CLT can be left exposed (Figure 10—see image carousel below).

The vertical edges of the panels are milled with a profile so they form a lap joint when brought together. This joint is then stitched together using pairs of similar self-tapping screws set at opposing angles. Where an internal wall meets an external wall, the butt joint is secured in the same manner. The use of a large number of small connections (rather than a smaller number of large connections) is the most efficient way to dissipate seismic forces. This is because it spreads the load more evenly through the structural section.

The NLT panels bear directly on the CLT walls and are connected to them in a similar way. They arrive onsite with a plywood diaphragm factory installed over most of the panel, but held back from the edges. The panels are lifted into place by a crane (Figure 11—see image carousel below), and the diaphragm is completed by installing a final row of plywood sheets that cover the joint between panels. This approach results in a continuous diaphragm across the entire roof, requiring only a single layer of plywood.

This project demonstrates that factory-produced CLT and NLT panels can be successfully combined to create economical and esthetically pleasing buildings (Figure 12—see image carousel below). It also confirms simply detailed CLT panel systems can provide a cost-competitive, code-compliant solution for lateral design in high seismic zones.

Glulam post-and-beam and light-wood frame shear walls
Located in the Lower Mainland, the new two-storey Surrey Christian School building includes a total of 15 daycare, kindergarten, and primary classrooms, along with their support spaces. The classrooms are organized along a linear two-storey atrium extending the full length of the building.

The need to connect at main floor level to the adjacent middle school meant the new building, which is on a sloping site, was constructed atop a new single-level parking garage that is partially tucked into the hillside.

To address the client’s concerns for economy and speed, and deliver an attractive, high-quality building simultaneously, the design team proposed a simple engineered wood post-beam-panel structure that could be prefabricated.

The vertical structure consists of glulam posts at 2.7 m (8.8 ft) centres along the length of the building. Each bay consists of four posts—two at the exterior walls and two at the atrium walls (Figure 13—see image carousel below). The glulam posts were factory-fitted with custom steel base plates (Figure 14—see image carousel below) that were attached using long, high-strength, self-tapping screws installed at opposing angles.

The posts on the main floor were bolted to the concrete slab of the parking structure, and braced longitudinally using light-wood frame infill panels. There are no longitudinal beams in the building. The posts were then ready to receive prefabricated floor and roof panels, 2.7 m in width and spanning the full 8.5 m (27.8 ft) depth of the classrooms. Each panel has two glulam edge beams, connected with light-wood frame header panels at both ends and bridged by a deck made up of nail-laminated 38 x 89-mm (2 x 4-in.) material (Figure 15—see image carousel below).

These panels were installed in alternate bays along the length of the building, leaving the spaces between them to be filled with a second panel type that consisted only of 38 x 89-mm nail-laminated timbers. The edge beams of the main panels rest directly on top of the posts, and are connected to them with a similar detail to that used at the base.

Once all the main floor panels were installed, plywood sheathing was laid by the general contractor in order to create a horizontal diaphragm. (Carpenters used plywood reclaimed from the formwork for the concrete parking garage. It was field-installed one sheet at a time). For the vertical plane, lateral stability is achieved by plywood-sheathed light-wood frame walls running north-south at either end of the building, and east-west along the length of the corridor between door openings. These shear walls were also prefabricated. The lateral system was designed to resist all the required seismic loads, enabling the exterior walls of the classrooms to be fully glazed (Figure 16—see image carousel below).

For this project, the use of factory prefabrication compensated in part for the additional time required to construct the parking garage. It was possible for the wood components to be prefabricated at the same time as the concrete was being poured. Installation of all the prefabricated wood components took approximately one week. Prefabrication in wood was also compatible with the use of site-built light-wood frame construction for the interior partitions. The result is a building with a warm and welcoming atmosphere that greatly exceeded the client’s expectations (Figure 17—see image carousel below).

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A final note
The projects described in this article were all designed to meet the requirements of the 2012 edition of the British Columbia Building Code (BCBC), which was based on the 2010 edition of the National Building Code of Canada (NBC). It is worth noting the seismic design values used in the new 2015 edition of the NBC (upon which subsequent editions of the BCBC will be based) are significantly higher than those in the 2010 edition of the code. Therefore, the solutions described here may not comply with those more stringent requirements—although new solutions using the same basic principles will of course be developed. (Of course, designing structural wood buildings goes far beyond solely seismic requirements. For more information, see this author’s previous Construction Canada case studies and articles, including “Mid-rise Makeovers,” by visiting www.constructioncanada.net. Other features of interest would include “Specifying Combustible Construction in Canada” by Jack Keays, MSc., P.Eng., and “Specifying Modern Timber Connections” by Maik Gehloff, Dipl.-Ing. (FH), M.A.Sc. For further reading, also visit www.constructionspecifier.com for the article, “Solid Timber, Solid Construction Performance,” by Ryan E. Smith).

Jim Taggart, FRAIC, teaches architecture at the British Columbia Institute of Technology (BCIT) in Vancouver. He is also the editor of Sustainable Architecture and Building Magazine (SABMag) and the author or editor of more than a dozen books, including the award-winning Toward a Culture of Wood Architecture (2011). Taggart has also lectured extensively on this subject throughout North America, Scandinavia, and Australasia. He is a Fellow of the Royal Architectural Institute of Canada (RAIC) and the recipient of the 2012 Premier of British Columbia’s Wood Champion Award. He can be reached at architext@telus.net.