Try Strutting When The Joist Is Jumping
Hugh Mansfield-Williams on strutting in timber floors
IT IS important that when we talk about timber struts, that we’re all on the same page as to the meaning of the word. This is not a feature about swaggering on a timber floor; this is instead a look at the role strutting plays in promoting composite structural behaviour of the components that form a timber floor.
Strutting helps to limit deflection and vibration of timber floors, and improves the lateral stability of individual joists. There are three main options available when considering strutting for timber floors: solid timber blocking; timber herringbone strutting; proprietary strutting devices.
Timber used for strutting should be at the appropriate moisture content and free from any major strength-reducing defects and distortion.
I-joist manufacturers have specific recommendations for blocking or strutting to provide lateral restraint to the flanges of their products where appropriate. Lateral restraint should not be provided through the web of an I-joist.
Where appropriate, metal web beam suppliers will require the use of a ‘strongback’. That is a continuous timber that is usually fixed to timber posts or nailing blocks between the flanges. A strongback will span a number of bays within a floor. The supplier will provide detailed installation instructions that should be carefully followed.
l Solid timber blocking: Solid blocking should be at least 38mm thick and extend at least three quarters of the joist depth. The blocking should be close fitting and be fixed either by skew nailing or by offsetting the blocks and end nailing through joists.
Full depth blocking should be avoided, since it could project above or below the top or bottom of the joist at some point in its length. This will result in an uneven deck surface and, since the deck or ceiling will not then be in full contact with the joists, could cause nail popping and subsequent noise from movement on the nails. TRADA’s WIS 1-36: Timber joist and deck floors, avoiding movement contains guidance.
Blocking should be tight fitting, yet not oversized as this may cause distortion in the joists. Solid blocking can loosen under joist shrinkage and localised loading, resulting in movement and noise after building completion. Joists installed at too high a moisture content will shrink across their width, causing the blocking to become loose. Therefore, moisture content control is vital to the performance of solid timber blocking.
l Timber herringbone strutting: Timber herringbone strutting should be at least 38mm x 38mm cross-section. It should not be used where the distance between joists is greater than approximately three times their depth. This is because, at shallow angles, the effectiveness of the strutting is significantly reduced.
For maximum efficiency, the timber herringbones should be nailed towards the top and bottom edges of the joists and should be in full contact. They should not stand proud of the joists as this would interfere with the fixing of decking and ceiling linings. To avoid unwanted noises in use , the struts should not be in contact with each other.
In contrast with solid timber blocking, herringbone arrangements tend to tighten up if the joists shrink, thus maintaining structural integrity. Herringbone types of strutting have therefore been found less likely to be the cause of movement and noise in service.
l Proprietary strutting devices: The most widely used types of proprietary strutting device are steel herringbone systems offered by several timber hardware manufacturers. They are pressed lengths of galvanised mild steel, usually about 1mm in thickness, and produced in a range of lengths to suit differing joist depths and spacing configurations.
Take care to ensure that there is a sufficient gap left to avoid contact at the crossover between pairs of struts in herringbone arrangements, since this may result in noise when the floor is used.
l Where to use strutting: The timber design code, Eurocode 5 makes no prescriptive requirements on the type or frequency of strutting, but it does direct the engineer to consider lateral stability not only of beams and columns but also more complex systems.
For beams, the parameter kcrit is used to penalise designs unless lateral deflection of the compressive edge of the beam is prevented and resistance to torsional rotation is provided at the supports.
Position strutting at the ends of joists and, depending upon span, along their length. Provide strutting also at intermediate bearing locations. To maintain continuity, block or solidly pack the end of each row of strutting between the outer last joist and the perimeter walls.
Provide timber strutting or blocking at the end of joists that bear on to timber, masonry or steelwork, to prevent rotation, which some types of metal herringbone strutting may not prevent.
l How to install strutting: Take care to ensure that all strutting devices are tight fitting, are securely fixed and do not protrude beyond the top and bottom edges of joists. Follow installation and fixing procedures as directed by Codes of Practice, authoritative guidance sources or manufacturers of proprietary systems.
Remember that strutting is only one component of a timber floor structure and that the functionality of the floor depends upon careful installation and fabrication of the whole floor.
It is important that blocking and strutting not be removed in order to install services such as plumbing.
This consideration might influence the strutting choice, since herringbone arrangements and strongbacks for open web joists generally occupy less space in the floor void than solid blocking, thus allowing easier installation of services. Timber herringbone strutting and strongbacks should not be notched or drilled for services. Solid blocking may be notched or drilled by a small amount. www.bmtrada.co.uk CFJ s
Hugh Mansfield-Williams is a wood floor specialist at BM TRADA
This article has been reproduced from the Contract Flooring Journal. You can find them at www.contractflooringjournal.co.uk.