Keywords: steel web, punched metal plate fastener, nail plate, joist, contact surface, finite element.

Steel webbed softwood beams are a relatively new application for floor and roof joists in North America and Europe. The beam is of composite design comprised of softwood timber flanges connected by galvanized pressed steel webs with punched metal plate fasteners. The major advantages of such a beam are

• material savings due to using discontinuous webs of small thickness
• easy fabrication due to simple structural features
• low handling costs due to light weight
• easy passage of electrical wires and ventilation ducts due to large spaces between the steel webs.

The combination of the steel web and the wood flange makes efficient use of extreme fibre stresses in the flange and shear stresses in the web. Steel webbed joists also combine the lightness of timber with the strength of the steel web, which can span far greater distances than would be possible with alternative timber products [1,2] requiring openings in their webs. This gives us unequalled design freedom across a wide range of applications for both floors and roofs in domestic, industrial and commercial applications.

The steel webbed beam consists of the steel web and the wood flanges. The web is generally punched from approximately 1mm thick galvanised steel sheet of either grade Z2 G275N to BS EN 10326: 2004 or grade G90 to ASTM standard A924. The galvanised steel is defined as steel coated with a thin layer of zinc to provide corrosion resistance such as that used in underbody auto parts, garbage cans, storage tanks, or fencing wire. Sheet steel must normally be cold-rolled prior to the galvanizing stage. The steel web has connector plates with punched nailplate fasteners on both the top and bottom for fixing to the timber flanges. The metal webs are pressed into the timber flanges using a press that incorporates external and internal clamping to ensure that a quality product is manufactured every time. When combined with the parallel stress graded timber flanges they form beams, giving an open web structure that can clear span large distances.

In the UK the timber is often Scandinavian Softwood (normally whitewood) stress graded to TR26 grade. It is available double-vacuum treated with water based preservative (e.g. Arch Aquasol). The softwood flanges may contain joints made with punched metal plate timber fasteners. The moisture content at the time of fabrication must not exceed 22% (in practice the moisture content is around 15-18%).

For such beams, web buckling is a typical failure mode. Due to complex features introduced by the punched nails at both ends of the thin-walled steel brace web, the most promising way to analyze its structural behaviour efficiently is to develop finite element models. In this way, interaction between the timber flange and the steel web can be simulated through defining the corresponding contacts. Geometric imperfections can then be introduced to obtain different buckling modes of the beam [3]. The work on the numerical modelling of the buckling failure will be dealt with separately since the joists were restrained to prevent overall buckling in the current study.

In this paper, both 2-D and 3-D finite element models, incorporating orthotropic and non-linear behaviour, have been developed to simulate the structural performance of steel webbed timber beams using the commercial code ABAQUS. Structural tests were undertaken to obtain structural performance and failure modes of the beam, by which the numerical models were validated. Material tests of the steel web were also carried out to obtain its mechanical properties which are essential for the FE modelling. Timber was treated as an orthotropic elastic material and steel as an elasto-plastic material. Interaction between the punched nail web and the timber flange was simulated by defining "TIE" connections. Load-deflection relationships obtained from FE simulations were compared with those obtained from experimental results. Reasonably good correlation was obtained. In addition, experimental failure modes were compared with simulated failure modes with detailed discussions. The developed FE models are ready to be used to undertake parametric studies with variable geometrical, material, loading and boundary conditions.

1

Zhu, E.C., Guan, Z.W., Rodd, P.D. and Pope, D.J., "FE modelling of OSB webbed timber I-beams with openings", Proceedings of WCTE, Vol. 1, Shah Alam, Malaysia, 12-15 August, 2002, pp165-172.

2

Zhu, E.C., Guan, Z.W., Rodd, P.D. and Pope, D.J., "Finite Element Modelling of Interactions between Openings in OSB Webbed Timber I-Beams", in Topping, B.H.V., (Editor), "Proceedings of the Ninth International Conference on Civil and Structural Engineering Computing", Civil-Comp Press, Stirlingshire, UK, Paper 28, 2003. doi:10.4203/ccp.77.28

3

Zhu, E.C., Guan, Z.W., Rodd, P.D. and Pope, D.J., "Buckling of OSB webbed timber I-beams", ASCE's Journal of Structural Engineering (in press).

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