While many structural forms have been adopted for construction of low-rise timber building superstructures, the types illustrated in Fig. 5.1 can be considered as common. Notably, while the good safety performance of low-rise ancient to modern timber buildings might be thought to have little direct relevance to tall timber building performance expectations when overloaded, this is not the case. The exploitation of the inherent high strength-to-mass ratio of timber and modern timber-based composites, the ability to develop alternative load paths and arrest the propagation of damage, and the capability to absorb energy associated with inertial forces when close to collapse via mechanisms other than material damping are all characteristics for which superstructures of any height should be designed. Also, need for tight control of component manufacturing and site construction practices is apparent from post-mortem field observation of collapsed low-rise buildings. This implies a preference for prefabricated systems for the con-struction of tall timber-framed buildings. Other lessons learned relate to the fact that solutions for providing adequate protection against disproportionate structural damage can be integrated with strategies for fire containment [20]. In addition, designing low-rise superstructure systems against infringing upon limiting states related to dynamic motions and static deflections is often the crucial factor. Furthermore, in tall buildings both vertical and horizontal motions can be problematic from a serviceability perspective [86].
In general, static deformations in well-designed and constructed timber building superstructures are modest in magnitude. Experience suggests that it is very rare for such buildings that are
Traditional buildings
• Many types of occupancies
• Composite walls
• Walls resist all types of loads
• Timber upper floors
• Timber roof framing
• Two or three storeys most typical
• Five or six storeys examples exist
• Constructed based on experience
Modern heavy-frame buildings
• Mainly non-residential occupancies
• Heavy timber columns and beams
• Diagonal bracing added to resist lateral forces
• Connections designed for axial and shear forces
• Timber joisted or composite upper floors
• Timber or composite roof, typically joisted
• More than two storeys is unusual
• Engineering design is always mandatory
Modern platform buildings
• Mainly residential occupancies
• All timber or composite walls
• Walls resist all types of loads
• Timber joisted, timber plate or composite floors
• Storeys are like stacked shoeboxes tied together
• More than four storeys is unusual
• Six storey examples exist in several countries
• Engineering design is not always mandatory
Fig. 5.1: Common structural forms for low-rise timber buildings
5.2 USEFUL LESSONS FROM LOW-RISE TIMBER CONSTRUCTION (CIRCA LESS THAN 20 M TALL) 59
serviceable with respect to vibration response to be unserviceable with respect to deformations under static loadings. Part of this assertion is the presumption that ensuring acceptable vibra-tion serviceability is based on a realistic three-dimensional vibravibra-tion analysis. Such an analysis combines the effects of the structural form and the construction detailing to avoid vibration-induced serviceability problems and sound transmission problems. These need to be considered by structural designers, even if they fall outside the traditional scope of responsibility. The best solutions are those that combine isolation of propagation sites from receptor sites with a concen-tration of the mass at selected locations. In low-rise construction, this is often not done, but in taller construction it is frequently seen. For tall timber buildings, the potential for vibration and sound transmission must be designed against based on the recognition that isolation, damping provisions, and placement of relatively massive elements are key components for good solutions.
Traditional timber framed buildings are relatively flexible with respect to global response. Also flexibility exists at subsystem levels. Furthermore, such systems have the ability to adjust the geometry of their substructures during seismic events [14,87]. Relative to other structural sys-tems there can be high levels of embodied structural damping, reflecting that the building fabric is often not monolithic and can consume energy via frictional and other contacts. However, the ability to achieve this is not always realized, because it requires careful and deliberate attention to construction detailing. Lateral bracing of low-rise frameworks is normally achievable without the need to make frameworks act compositely with shear walls. This is in contrast to tall timber structures. However, various factors related to the intensities of design loads usually combine so that structural systems can normally be very simple, with well-defined flow paths for resisting gravity and lateral forces (Fig. 5.2).
In modern times, glued laminated timber (glulam) elements have been the most common type of framing element, reflecting the lack of reliable supplies of new, large, sawn timbers in most parts of the world. Length restrictions for glulam members depend mostly on transportation methods and on-site lifting capabilities, which means that limits on possible dimensions match restrictions for prefabricated steel and RC elements. As illustrated in Fig. 5.3, employing con-tinuous girders and concon-tinuous columns in various configurations is technically possible and often done. Importantly, continuity of primary framing members is employed to reduce moment peaks and deflections within the girders and column members, but for reasons explained below not to transfer moment forces between beams and columns.
Floors and roof plans Exterior walls
Column Girder Wall bracing Horizontal diaphragm
Fig. 5.2: Essential elements of low-rise multi-storey timber frameworks
In the design and construction of timber heavy-frame systems, special attention must be given to the connections between framework elements and for those between the superstructure frame-work and the foundations. Fundamentally, choices for interconnecting system parts are car-pentry joints, connections made using metal parts and metal fasteners (with and without local reinforcement of timber members), hybrid carpentry-metal part connections, glued joints, and hybrid glue-metal part connections. Figure 5.4 shows preferred types for timber frameworks.
Decisions about connection methods are always intrinsically linked to the overall structural design strategy. Three-dimensional timber frameworks are not usually intended to work in an analogous manner to most three-dimensional RC and structural steel frameworks. As shown in Figs. 5.2 and 5.3, arrangements of timber columns and girders are not usually self-stabilizing (i.e. they need to be supported by temporary bracing during erection and then permanently braced). An arrangement of floor and roof diaphragms in conjunction with diagonal bracing in wall planes is the normal approach to stabilizing systems. Wall bracing is normally widely dis-persed across bay locations in timber superstructure frameworks. Bays that are not directly sta-bilized must be anchored to those that are. Modern multi-storey buildings of any height nearly always have elevators and stair shaft walls and/or fire walls with significant shear wall capabili-ties. In contrast, timber low-rise superstructures are normally not designed to work compos-itely with shear walls (i.e. the two types of substructures are not structurally interconnected).
Grider to column pin connection Column to foundation pin connection Wall bracing
Fig. 5.3: Illustration of the use on continuous members in low-rise frameworks
Metal bearing element
(a) (b)
Laminate/grain direction Element internal force
Metal link element Laminate/grain direction Element internal force
Fig. 5.4: Logic of how timber/glulam framework elements should interface with connections:
(a) bearing transfers of gravity forces; (b) axial transfers of tensile forces resulting from wind and seismic forces
5.3 MODERN RENAISSANCE TALL TIMBER FRAME SYSTEMS (CIRCA 20–80 M TALL) 61 Low-rise timber buildings that are large in plan are often designed so t hat each fire compartment is an independent structure, with each fire compartment deriving its three-dimensional stabil-ity from geometry and an independent bracing system. Such practices yield buildings that are inherently robust and not prone to disproportionate spread of damage whatever the cause of the damage. However, when large building volumes derive from them having significant height, the same strategy cannot be followed.
Present timber design codes are written predicated on the design of traditional low-rise ings. Therefore as usage of timber transitions to also include construction of high-rise build-ings, it becomes prudent to consider whether codes still adequately reflect needs of designers.
When this Structural Engineering Document (SED) was written Canadian code committees, for example, had commenced the task of reviewing relevance of provisions in their national build-ing and timber design codes.