How to avoid structural timber design failure

Watch it! Timber does not behave in the same way as inorganic materials such as steel and concrete. Here, Ishan Abeysekera highlights aspects of timber design that engineers used to designing in more traditional materials should be aware of.

Timber: the material

Timber is a natural material harvested from trees. The microstructure of timber consists of tubes that transport nutrients and water through the tree, so it is helpful to think of timber as a bundle of straws. This structure makes timber much stronger parallel to the grain, for forces acting in the direction of straws, than perpendicular to the grain. Imagine trying to squash a bunch of tightly packed straws top down (axial loading), compared with how easy it would be to put a hand around the bundle and squeeze, bending them through their middles.

Since trees need branches, timber incorporates knots. Compared with ‘perfect’ straight-grained, knot-free timber, the tension strength of structural timber is more affected by knots in an off-axis grain direction than the compression strength. During axial (top-down) loading, a knot will be at the point of maximum stress wherever it is in the section. So, while the presence of a knot will always reduce axial capacity, its effect on the bending capacity will depend on where in the section it is. Knots at the edges of a timber beam will reduce bending strength significantly, while a knot at its centre will have little effect.

Duration of load also affects timber strength. At higher loads, creep behaviour of timber will not be stable (logarithmic with time) and will instead be unstable (exponential with time), creeping towards failure. So, engineers must apply load duration factors to keep the applied stress in real, long-term construction projects below documented strength levels that are measured in relatively short-term tests. When assessing design strength, the load case with the shortest load duration will govern the load duration factor for any given load combination.

Double-check sections and elements

Curved and tapered timber sections are readily available from several manufacturers. With curved elements, in addition to standard checks, engineers should carry out additional checks on tension perpendicular to grain, locked-in stresses due to glueing of curved laminates and non-linear section stress.

Tapered sections also require additional checks of shear forces and tension perpendicular to grain. >>

Calculate connection strength at concept

Connections are generally the weak points of timber structures. There is limited space to fit in screws, and slots for steel flitch plates can weaken the member. Design engineers who are used to adding reinforcement into a concrete connection, or welding on extra steel to make a connection work later in the design process, must learn to account for the nature of timber in an earlier design phase. If not, they risk having to increase member sizes to carry the connection forces.

Timber connections should be checked very early in the design and span to depth tables should be used with caution. As a rough rule, at concept design stage, beams should be sized to assume that connections have only 60% of the strength of the full timber cross-section.

Understand slip at timber connections

Timber connections are relatively flexible due to slip on the bolts, screws etc. This is due to oversizing of the hole and local crushing of the timber.

The stiffness of timber connections is highly variable and impossible to predict with any accuracy. Figure 1 provides just one example of just how variable a single lateral dowel type connection can be. So, the values in design codes should be used with extreme caution.

This has several implications:

• Connections significantly increase the deflection of trusses and portals – the increase is most easily calculated by hand using a virtual work method.

• When working with cross laminated timber (CLT) panels, connection stiffness dominates the flexibility of shear walls because the panels themselves are very stiff compared to the connections. Concrete cores make more sense for taller buildings.

• The uncertainty over connection stiffness means that, in indeterminate systems, it can be difficult to determine which path the loads will actually follow.

Minimise moment connections

Joints that transfer bending moment forces between a column and beam, or two or more beams, are known as moment connections. While not impossible, timber column to timber beam moment connections are expensive, difficult to achieve and have relatively low stiffness. Where possible it is often easier to avoid using this type of joint. Where two or more members cross in the same plane, moment connections are extremely difficult to achieve without a complex steel node. Most timber grillages – where one or more tiers of beams are superimposed at right angles to each other to disperse load over an extensive area – are in fact built as one-way spanning structures with short infill pieces in the secondary direction.

Avoid glueing on site

Reliable glueing of timber requires stringent conditions of temperature, moisture content, pressure and cleanliness. This is practically impossible to achieve under site conditions. There is also no non-destructive testing (NDT) that can be undertaken to confirm whether the glueing is adequate. Therefore, structures which rely on glueing on site should be avoided. In addition, even glueing in the factory should only be carried out by experienced fabricators. Where glueing of elements is carried out in the factory the engineer should check whether the manufacturer has adequate quality control measures in place.

Consider the cost of connections

In contrast to steel, where minimising the weight of steel will benefit the bottom line, an engineer must find alternative solutions to balance the influence of connections on the budget within the timber structural design. For example, >> a truss – which will by definition have connections at nodes – will be materially the most efficient way to span a space, but it may well be cheaper to use a simply supported single spanning glulam beam. This solution will use more timber but has fewer connections.

Be cautious of indeterminate structures

Indeterminate structures with multiple possible load paths such as a propped cantilever beam, for example, make internal force calculation complex. This type of timber structure has uncertain relative stiffness of load paths due to the high flexibility, and variation within that flexibility, of its timber connections.

Timber elements and connections generally fail in a brittle way. Hence governing failure modes of timber structures may be brittle, and the lower bound theory of plasticity will not hold. This means that if a load path of an indeterminate timber structure with multiple load paths becomes overloaded, the load may not successfully redistribute to alternative load paths even if they have the capacity to withstand that additional load. In such a case, internal forces will not remain in equilibrium with the applied load, resulting in structural collapse.

Engineers need to be much more cautious about design than they would be with more ductile and forgiving materials. The simplest solution is to design timber structures with determinate load paths. Where this is not possible, the sensitivity of load distribution to variability of stiffnesses of different load paths should be tested, despite additional costs.

Dynamics governs floor depth

Because timber is lightweight, it is susceptible to higher accelerations and so floor depth can be governed by dynamics. So, it is important to either carry out a dynamic analysis early in the design process or inform the client that dynamics should be checked later and that results may affect the design. Engineers should note that modelling the entire floor plate is significantly beneficial when compared with modelling a few bays, as it will accurately capture the positive contribution of the larger mass of global modes.

Resistance to disproportionate collapse

Since timber is a brittle material, it does not have the ductility required for catenary action. Therefore, where local regulations require resistance to disproportionate collapse, tying is not a viable approach and element removal should be used instead.

It should be noted that element removal as means of achieving disproportionate collapse requirements is much more viable for timber than for more traditional materials because:

• Timber member design is governed by serviceability limit state (SLS) as opposed ultimate limit state (ULS) requirements. This leads to spare capacity at ULS, provided connections are also designed for the accidental load case.

• When compared with steel and concrete structures, the self-weight of timber is a smaller proportion of the total load, so the percentage reduction of total load for the accidental disproportionate loading case is much higher for timber structures. >>