Structures: Efficient Steel Framing Systems

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In the quest for greater structural efficiency, the versatility of steel construction lends itself to a variety of forms and concepts. Two notable framing options that have both economic and practical design advantages — one an older system that has been “revived,” and one a new proprietary system — are described here. Both applications integrate precast concrete slabs into the floor construction.

Staggered Truss System

The Staggered Truss System offers distinct benefits for mid-rise residential buildings requiring a low floor-to-floor height. Storey-high trusses are located at alternate column lines in a vertically staggered pattern (see Figure 1), providing room layouts with large column-free spaces. The low storey height cuts building envelope costs and is achieved partly by using precast slabs to provide a large span-to-depth ratio. Moreover, the simplicity of the truss fabrication reduces construction times, resulting in an effective and economical structural system.

Although not used in Canada for 20 years, staggered truss construction was recently adopted for the Toronto Christian Resource Centre in Toronto. This multi-purpose facility provides 87 units of affordable housing in the social housing community of Regent Park. The project includes five storeys and a basement, with a total area of 6,500 sq.m, and was designed to meet the equivalent of a silver LEED certification. The rectangular shape, floor plan and building height influenced the choice of staggered trusses, which were designed by the firm of Halsall Associates.

Spanning the entire 16.8-m width of the building, each truss supports two levels, one on each chord. Both chords are made of W250 wide-flange sections, while vertical and diagonal members consist of W200 sections. The 2.7-m truss depth is equal to the floor-to-floor height. The member sizes were chosen so that the trusses could be hidden within the walls. Precast slabs supported on the top flange of the truss chords complete the floor framing.

With the W310 columns placed only along the long sides of the rectangular building, and due to the absence of trusses between the ground and second floor, the ground floor is entirely column-free. This result was achieved by supporting the second floor using hangers from the trusses located above. At the same time, the small number of columns reduced the foundation costs.

Structural analysis for wind and earthquake loads is somewhat different from other buildings because the precast slab diaphragms receive lateral loads from the staggered trusses and transmit them from truss to truss at each floor. The diaphragms and connections must be designed for adequate strength, stiffness and ductility to transfer the lateral forces to the lateral-load-resisting system.

A special feature of the staggered truss system is the provision of Vierendeel panels (i.e. without diagonals) near mid-span, allowing for the passage of a corridor along the length of the building. Similar panels were also provided to accommodate windows at the east and west ends.

Girder-Slab System

The Girder-Slab System is a composite structural assembly consisting of precast slabs with an integral steel girder. Originally developed by Girder-Slab Technologies, this innovative system is most cost-effective for mid- to high-rise residential construction.

In Girder-Slab construction an interior girder, called a “D-Beam,” and prestressed hollow-core slabs are connected by cementitious grout. The slabs are supported on the bottom flange of the girder, while the web and top flange are concealed within the slab, thus minimizing the floor-to-floor height (see Figure 2). The assembly is fire-rated for use in high-rise buildings when constructed according to ULC guidelines. The exposed bottom flange can be fire-protected by sprayed-on fireproofing or standard gypsum board.

The special shape of the D-Beam is created by cutting the web in a saw-tooth pattern, producing two identical T-sections and leaving little material waste. A narrow flat bar is then welded to the top of the web, and the resulting section is termed an open-web dissymmetric beam.

The concept was successfully used recently in the Courtyard Marriott Hotel at the Edmonton International Airport. Supreme Group proposed the Girder-Slab system for the 16,500-m2, six-storey tower to meet the aggressive schedule and to minimize costs and construction time in Edmonton’s cold climate.

For the hotel it was decided to apply a 20-mm levelling compound as topping for the hollow-core slabs, although the system can accommodate up to 50 mm of topping. Details of the slab diaphragms require special attention to achieve the proper horizontal load transfer in seismic regions, although this was not a significant factor in Edmonton.

Unlike conventional cast-in-place concrete structures, the Girder-Slab System is assembled in place. The near “dry” construction techniques reduced both the erection period and the heating costs as compared with wet concrete. Thanks to the budget savings, and also due to the minimum structural height, an additional level of hospitality space could be incorporated in the hotel.

As a final note, this method of construction requires a qualified steel fabricator recognized by Girder-Slab Technologies’ licensed distributor network. Various elements of the system are patented, but the application itself is non-proprietary and allows open-market suppliers — both in the steel and precast industries — to compete for projects.cce

Charles Albert, P.Eng. is manager of Technical Publications at the Canadian Institute of Steel Construction (CISC-ICCA), Markham, Ontario. www.cisc-icca.ca