Timber Beams
In some instances during the installation of roofs, partitions, or joist systems, a load bearing support is required where no vertical load bearing structure is present. In these instances, a structural timber beam can be installed between two alternative load bearing structures in order to create a suitable bearing. The implementation of these components within a larger structure can offer unique and effective building solutions, principally aiding in the creation of modern, open-plan layouts and habitable roof structures.
What is a timber beam?
Structural timber beams are large lengths of timber that extend the effective reach of existing supporting structures across inhabited spaces without intruding on the usable space within. In order to effectively support the load of other components over an unsupported span, timber beams are ordinarily larger than the regular joists that they sit amongst. Timber that is dimensionally wider than the standard joists can be used as a load bearing beam, though timber beams assembled from multiple standard lengths of timber are more common, cost effective, and easier to manuever. This type of beam sees multiple standard lengths of timber stacked side-by-side, and bolted together at regular intervals. This bolting process increases the effective load bearing capacity of the beam, far beyond the strength of the individual lengths of timber. In instances where an unusually large span must be bridged, or a particularly large load must be supported, a steel flitch plate might be introduced into the assembly of a bolted timber beam, sandwiched between the timbers to further increase its load bearing capacity. In exceptionally heavy duty applications, and those where timber is an unsuitable material (such as supporting masonry), the timber beam may be cast aside entirely in favour of a steel beam/RSJ. The location and dimensions of a beam will be specified on the engineers drawing, and may contain a detail drawing of the intended assembly.
Timber beams are a common feature within joist and roof systems alike, providing suitable support for the other components. These beams are primarily implemented to support the ends of joists around structural openings such as rooflights and stairwells, and play a crucial role in the formation of these structures. Additionally, timber beams are often installed within standard joist systems at specific locations in order to provide a suitable bearing for components that will be installed further down the line, such as stud partitions and structural studwork.
What are the criteria for the assembly and installation of a timber beam?
Bolted beams should be assembled from appropriate defect free lengths of timber in order to maximise the total load bearing capacity. The ends of the timbers should be flush with each other to ensure the correct alignment and a suitable fit during installation. All timbers should be crowned in the same direction and an attempt should be made to ensure the timbers remain flush with each other along the length of the beam. Slight discrepancies in this type of alignment are allowable due to the differing amount of crown that occurs in each length of timber, though through the use of coach screws the lengths can typically be pulled into alignment with each other. The bolts should be installed in compliance with an engineered installation pattern. Some examples of these patterns include 600mm centres with an inset of 200mm from each end, or 450mm centres staggered along the length of the beam. Timber dog tooth connectors should be incorporated on each bolt in between each timber to prevent movement over time in the beam due to shrinkage. Washers must be installed on the end of the bolt to prevent the nut from pulling through the timber. The installation criteria for timber beams are the same as those for the installation of standard timber joists.
How are bolted timber beams assembled?
To begin the process of bolting a beam together, a measurement must be taken for the intended length of the finished beam. A suitable number of timbers should be cut at this length - 2 lengths for a double timber beam and 3 lengths for a triple timber beam. Care should be taken to maintain accuracy and cut all of the timbers at exactly the same length. The timbers must then be aligned with one another in the correct orientation for bolting. By setting up a pair of benches or sawhorses, this process can be undertaken more comfortably. The very end of each length of timber should be supported via a bench in order to ensure the bearing of the beam is perfectly flush on the underside. Each timber should be bowed upwards, and arranged side by side with any cup facing inwards. Doing so will allow the tightened bolts to eliminate some of the cup present in the lengths.
With the timbers oriented properly on the bench and flushed up, temporary fixings such as coach screws cab be used to fasten them together. If the timbers are straight, a screw at each end will hold them whilst the holes are drilled. If the middle of the lengths begin to come out of alignment due to the natural curvature of the timber, a screw in the middle can be used to hold them together whilst drilling. With the timbers fixed together the beam is ready to be drilled. It is common for M12 bolts to be called for by the beam detail, which have a diameter of 12mm. A 12mm auger bit can be used to drill the holes in our timber, though this can be a bit too tight and makes it awkward to align the timber again. As such, a 13mm auger or spade bit bores a hole through the timbers with enough tolerance for the bolt to slide freely through. The often specified detail for laying out the bolts requires the first and last bolt to be 200mm in from each end of the beam, and the other bolts being set out at 600mm increments from one end. Once these have all been marked, the centre of the width of the beam can either be marked or eyeballed and then drilled.
Slow and steady pressure should be applied to the drill driver, making sure to occasionally clear the chips from the hole and bit so as not to jam the bit in the hole. Once the bit passes all the way through, another few quick vertical movements will ensure the hole is clear. This process is repeated for the other holes. Once all of the holes are drilled, the timbers can be unscrewed and the bolting process can begin. It can help to mark the correct orientation of the joists before the screws are removed so as to avoid improper reassembly.
Once unscrewed, the timbers can be pulled apart with a few inches gap between them. When the beam has been assembled, the mushroom head of the bolts can be drawn below the surface of the timber to allow for a better alignment of the beam. On the other side of the beam, the end of the bolts, the nuts, and the washers will always be protruding by at least an inch. It’s important to consider this factor before bolting the beam in order to determine which way to insert the bolts. Once decided, a bolt is inserted into each hole and tapped with a hammer until about 50mm of the bolt is protruding through the inside face of the joist. When forming a double beam from standard 50mm thick timber, 130mm bolts are used, as opposed to 190mm bolts that are used for triple beams. A timber dog is placed over the protruding end of each bolt, before the next timber is aligned with bolts and tapped into place. This process is straightforward if the individual timbers are relatively straight, though can be difficult if they posses varying degrees of bow. This process is repeated until all of the timbers are located around the bolts. With the bolts passing all of the way through the beam, a washer and nut can be placed on each bolt. Once the total assembly is held captive, an assertive blow can be made to the head of each bolts in order to ensure that it has a suitable grasp on the timber.
The nuts must be tightened until all of the timbers are pulled tightly together along the entire length of the beam. Due to the numerous teeth protruding from the handful of timber dogs sandwiched in between the timbers, the last 5mm or so to be closed up between the timbers requires a fair amount of force. As such, a large socket wrench with a deep reach 19mm socket is the perfect tool for exercising leverage on the nuts. Alternatively, an impact wrench with an impact rated socket will also work. To ensure that the timbers are being pulled together consistently, it’s good practice to tighten the nuts in increments, working up and down the beam in stages until all of the timbers come tight. If the timbers posses varying degrees of bows, or the timber dogs are causing the timbers to pull out of the desired alignment, coach screws can be used to manuever the timbers as the nuts are tightened.
Key Notes:
The individual timbers within a beam should all be bowed upwards, and cupped together appropriately.
Beams must sit with a minimum 90mm bearing on load bearing surfaces, or fully seated within a suitable hanger.