JAN 2016 Beam Lamination
Timber Beams are commercially supplied in standard thicknesses. This is normally 35/45mm for solid timber, 35/45/63/75mm for LVL and 65/85mm for Glulam beams. There are wide variations to this from other manufacturers and products such as pre-primed beams but in this factsheet we will concentrate on common structural beams. When these beams are designed for high loads or long spans a single thickness is often not suitable and they need to be doubled or tripled up. This could result in specification of say a 2/240×63 hySPAN or a 3/240×45 hySPAN beam to support the loads. The multiple thinner laminations are required to be secured together to obtain the final built-up section.
Before we look at the specifics of lamination, there are some factors that help determine the importance of how the beams are joined together. The main purposes of lamination are to hold the beam together, transfer loads across the beam, and to prevent moisture getting trapped between the laminations.
As mentioned in a number of previous factsheets, the type and location of the loads will help to determine lamination details. If the beam is uniformly loaded from above across all laminations then the load is inherently put onto all pieces.
This is the simplest case. If the beam is loading on one side (a bearer with joists coming into the side using hangers) the loading may only be onto one piece. In this case the lamination detail needs to be able transfer the load into the other plies so the beam acts as a single unit. There are iterations between these two where the load is placed onto some but not all the plies and in these cases an even loading of all sections is still essential.
The type of loading is also important. Heavy point loads normally need some consideration if they are not acting on all plies as the load needs to be distributed evenly through the final section. They normally need additional fixings around them to do this.
AS1684.2 – 2010 has general lamination details for nails and screws in Clause 2.3.1 and Figure 2.8. This uses minimum 2.8mm diameter nails or 10g screws staggered at a spacing of twice the beam depth. Figure 2.8 notes additional nails at point loads and supports as was described above but does not give specific details. hySPAN LVL also has lamination details H1 and H2 (shown above) for double and triple beams. This includes elastomeric adhesive and a waterproof membrane over to help prevent moisture between the plies. Trapped moisture could result in beam cupping or long term rot of the member.
People often ask about replacing the nails with 14g bugle head screws. For beams supplied by Meyer Timber I can recommend if using 14g Type 17 Bugle head screws of the same length as Detail H1/H2 then the spacing can tripled. If using 10mm diameter bolts with timber washers the spacing can be increased to 600mm centres top and bottom. These configurations will allow for adequate shear transfer for loads other than heavy point loads coming into one side of the beam. Heavy point loads should be dealt with using a specific design and fastener configuration for this load which is installed in addition to the standard lamination pattern described above.
Beam lamination has been commonplace in the building industry for a long time. There have been details in standards to deal with simple cases but with an increase in complexity of structures and load transfer there needs to be consideration in lamination details. Specifiers must watch out for beam laminations that support heavy point loads, especially when they do not act directly on every lamination. If there are doubts then you should consult a suitably qualified engineer for advice.