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Design Strategies for Fitting the Beam into the Space Available |
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Timber beams tend to be deeper than steel beams given the same spanning and load situation. As a result, strategic positioning of beams is required to ensure they fit in the space available. The main scenarios are described below. For each, the issues that adversely effect timber beams are stated, followed by strategies for solving or avoiding the problem.
Bulkhead beams
- Issue: The amount of beam projecting downwards into a room is limited by the head height for windows, doors and other openings.
- Strategy: If beam depth poses a problem then consider moving part of the beam up into the joist plane but in doing so, check there is enough joist depth to provide for shear requirements.
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Roof void beams
- Issue: In some instances, the ends of a roof beam (e.g. strutting/hanging/counter beam) may need to be cut on a splay so as not to protrude above the rafter line. This can create problems in terms of the shear and bearing resistance at the ends of the beam.
- Strategy: Specific end detailing can be used to assist this situation - typically blocking connected to the underside of the beam ends. Refer to beam manufacturer for further guidance on this issue.
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Beam in the wall above
- Issue: A lack of space to hide a timber beam (e.g. a bearer) in the joist depth, can sometimes be solved by allowing that beam to protrude above the joists it supports.
- Strategy: The beam can be hidden in a wall above, as long as there are no doors or other openings in the wall. External walls often present a strong possibility for this option.
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Hidden (in-plane) floor beam
- Issue: For spatial design reasons there is often a need to hide a beam (e.g. a bearer) in the same plane as the floor joists it supports.
- Strategy:
Consider the depth to span ratio of the various timber beam options offered in this publication and choose the one with the least impact on depth.
Check the cost impact of using floor joists deep enough to hide the required bearer depth. In making the calculation bear in mind that deep joists can be used to conceal plumbing stack-work thus saving the cost of constructing a false ceiling for this purpose.
Consider allowing the beam to rise above the floor level if it can be housed inside a wall above (also see following point)
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Inter-storey beams (dividing old and new construction)
- Issue: Upper storey extensions often involve using deep beams (e.g. bearers) to lift the new floor construction clear of the old ceiling construction (of the storey below). The beams also serve to simplify the new floor joist setout.
- Strategy: Box beams are potentially useful in this situation as the extra depth can easily be attained without significant changes to fabrication requirements. Careful detailing can also make it possible for the new floor joists to be sound isolated from the existing ceiling joists.
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