DESIGN GUIDE AUSTRALIA AND NEW ZEALAND DESIGN PROCEDURES FOR TIMBER ONLY COMPOSITE FLOOR SYSTEMS

Transcription

1 DESIGN GUIDE AUSTRALIA AND NEW ZEALAND DESIGN PROCEDURES FOR TIMBER ONLY COMPOSITE FLOOR SYSTEMS 1

2 Impressum Design Procedures For Timber Only Composite Floor Systems Report no: STIC Version 1-0 UTS Project no: RES First publication 2013 Copyright 2013 by Structural Timber Innovation Company (STIC), Christchurch 2013 All rights reserved. Design Guide Australia and New Zealand Design Procedures For Timber Only Composite Floor Systems First Edition 2013 Authors: Prof. Keith Crews Professor of Structural Engineering, Faculty of Engineering and Information Technology, University of Technology, Sydney Disclaimer: This guide is supplied only to authorised licensees and registered users of the EXPAN system and may only be used by them during the term of their licence or registered user agreement. If you do not have a current licence or are not a current registered user, you may not use this guide in any way (including making copies of it or supplying it to any other person). The guide will be updated from time to time. Licensees and registered users are responsible for ensuring that the version that they use is current and for obtaining updated versions and related information from EXPAN website. The authors and STIC have taken all reasonable care to ensure the accuracy of the information supplied in this guide. However, neither the authors nor STIC warrant that the information contained in this guide will be complete or free of errors or inaccuracies. By using this guide you accept all liability arising from your use of it. Neither the authors nor STIC will be liable for any loss or damage suffered by any person arising from the use of this guide, however caused. Dr. Rijun Shrestha Research Associate, Faculty of Engineering and Information Technology, University of Technology, Sydney. Structural Timber Innovation Company (STIC) 2

3 The research and development forming the foundation of this Design Guide as well as its preparation and production was proudly made possible by the shareholders and financial partners of the Structural Timber Innovation Company Ltd. 3

4 Design Guide Australia and New Zealand Design Procedures For Timber Only Composite Floor Systems First Edition

5 Table of contents Chapter Page 1. Introduction 5 2. Design requirements 5 3. Notation 6 4. Design procedure 7 5. Manufacturing provisions 8 5

6 Introduction Design requirements Notation Design procedure Manufacturing provisions 1. Introduction This design guide has been prepared to complement that produced for Timber Concrete Composite floor systems, for use in commercial and multi-residential timber buildings. Timber only floor systems are well established in Australia and New Zealand, but mainly for residential floor loads comprised of either sawn timber or Engineered Wood Products such as I-joists, in conjunction with sawn and dressed particle board or plywood flooring between 17 and 22mm in thickness. This Guide presents a design procedure based on AS for composite timber floor structures, manufactured using Engineered Wood Products such as Laminated Veneer Lumber and/or glulam, fabricated into T or box beam cassettes. The notations throughout this document are based on AS and the modification k factor subscripts should not be mixed or confused with those in NZS The Australian and New Zealand timber structures design codes have many similarities, but in some cases use different notation to describe the same modification factor for modifying the characteristic property being assessed. The table opposite provides guidance on equivalencies between the two standards. Description NZS AS Capacity factor Duration of load strength Duration of load stiffness / deflection All current Australia / New Zealand product standards for Engineered Wood products (LVL and glue laminated timber), have properties derived on the basis of the capacity factors in Table 2.1 of AS , to ensure an appropriate level of reliability. Table 2.1 Most normal applications for TCC and timber only floors will require: Φ = 0.90 for LVL Φ = 0.85 for glue laminated timber k 1 k 1 k 2 j 2 Bearing factor k 3 k 7 Parallel support k 4 k 9 Grid systems k 5 g 42 Strength sharing glue laminated timber k 6 Not applicable Stability k 8 k 12 Extensive laboratory testing has been undertaken to validate the design assumptions within this guide. The results of this testing program have confirmed that provided the flange to web connections meet the prescriptive requirements contained within this guide, the floor can be designed using the existing provisions of AS as a fully composite section, with linear elastic behaviour in resisting load actions predicted using AS/NZS Design requirements The design procedure addresses performance requirements for the strength (normative) and serviceability (advisory or informative) limit states. Load type and intensity, load combinations and modification factors for both the ultimate and the serviceability limit states have been defined in accordance with the AS/NZS 1170 standards. The limit states that require checking are: 1. Short-term ultimate limit state; where the response of the structure to the maximum load is analysed. It generally corresponds to short-term exertion of the structure. 2. Long-term ultimate limit state; This analysis focuses on the response of the structure to a quasi permanent loading and to avoid failure due to creep of the timber member in particular*. 3. Short-term serviceability limit state; This corresponds to the instantaneous response of the structure to an imposed load. 4. Long-term serviceability limit state; To identify the service life behaviour, this analysis considers time-dependent variations of the material properties; in particular creep kN serviceability limit state; The instantaneous response to and imposed load of 1.0 kn at mid-span provides an indication of dynamic behaviour. This can be replaced with a dynamic analysis if available. *Checking the end-of-life ultimate limit states correspond to analysis and assessment of the durability/reliability of the structure. 6

7 Introduction Design requirements Notation Design procedure Manufacturing provisions 3. Notation Unless noted otherwise in the figures below, all symbols and letters used in the design procedure conform to those in AS : Figure 1: Notation for a typical composite timber only floor system h t h f.c h f.t h centroid I Z top Z bot Distance from the centroid to the bottom of the bottom flange Height of the top (compressive) flange Height of the bottom (tension) flange Distance from the bottom of the bottom flange to the centroid = h t Second moment of inertia of the composite section Section modulus above the centroid (top flange) Section modulus below the centroid (bottom flange) q bot Shear flow at interface between web and bottom flange V* Maximum shear effect V d EI eff G Design shear capacity of the web Capacity factor effective stiffness of timber floor cross-section self-weight Figure 2: Notation for an individual cassette in a typical composite timber only floor system A A fc A ft A s A w b f.c b f.t b w h h c Cross-sectional area of the entire section Cross-sectional area of the top (compressive) flange Cross-sectional area of the bottom (tension) flange Shear area of the web = 2/3 b w h w Cross-sectional area of the web Width of the top (compressive) flange (equals c/c spacing of webs) Width of the bottom (tension) flange Width (thickness) of the web Overall depth of floor Distance from the centroid to the top of the top flange E value of the modulus of elasticity of the timber members characteristic strength in bending characteristic strength in compression characteristic strength in shear characteristic strength in tension j 2 k 1 k 4 k 6 k 7 k 9 k 11 k 12 stiffness modification factor load duration duration of load (timber) moisture condition (timber) temperature (timber) length and position of bearing (timber) strength sharing between parallel members (timber) size factor (timber) this is normally applied to the characteristic strength property by the manufacturer stability factor (timber) M* Moment action resulting from applied loads M d_top M d_bot Design moment capacity top flange Design moment capacity bottom flange N* c Axial force (compression) induced in top flange from bending N* t Axial force (tension) induced in bottom flange from bending N d_top N d_bot Q top Q bot q top Design axial capacity (compression) top flange Design axial capacity (tension) bottom flange First moment of shear area for top flange First moment of shear area for bottom flange Shear flow at interface between web and top flange 7

8 Introduction Design requirements Notation Design procedure Manufacturing provisions 4. Design procedure The design procedure has three fundamental stages: 1. Identifying the geometric characteristics of the cross-section of the composite beam. 2. Evaluation of the strength capacity. 3. Assessment of the serviceability limit states. 4.1 Cross-section characteristics In cases where the flanges and webs have differing properties (such as the use of cross laminated timber), it is necessary to determine the modular ratio and apply this to determine effective widths of members, prior to determination of the section properties. For irregular sections (e.g. where the top and bottom flanges are different) the location of the centroid must be determined, in order to calculate the relevant section properties. It is strongly recommended that the c/c web spacing be such that shear lag effects do not occur in the flanges. This is normally met by satisfying Equation 1. bb ff.tt MMMMMM (bb ww ssssssss), bb ww + 20 h ff.tt for bottom flange bb ff.cc (bb ww ssssssss) for top flange 4.2 Design for flexural effects (1a) (1b) The imposed UDL induces flexure in the webs and a combination of flexural and axial load effects in the flanges. This requires satisfying the requirements of Clause 3.5 of AS , for combined bending and axial load effects. The equations below apply to a simply supported beam and would need to be interpreted correctly for use with continuous beams. Bending capacity of the section above the centroid is given by: MM dd = kk 1 kk 4 kk 6 kk 9 kk 12 ff bb ZZ tttttt (2a) Bending capacity of the section below the centroid is given by: MM dd = kk 1 kk 4 kk 6 kk 9 kk 12 ff bb ZZ bbbbbb (2b) Where: k 4, k 6, k 9 and k 12, will all normally equal 1.0 Axial capacity (compression) of the top flange is given by: NN dd_tttttt = kk 1 kk 4 kk 6 kk 12 ff cc AA ff.cc (3a) Axial capacity (tension) of the bottom flange is given by: NN dd_bbbbbb = kk 1 kk 4 kk 6 kk 11 ff tt AA ff.tt (3b) The axial force induced in each flange as a result of the bending action is calculated using the following equations: Axial load induced (compression) in the top flange is given by: NN cc = MM h h cccccccccccccccc h ff.cc /2 AA ff.cc II (4a) Axial load induced (tension) in the bottom flange is given by: NN tt = MM h cccccccccccccccc h ff.tt /2 AA ff.tt II (4b) Combined bending and compression top flange: MM NN cc MM dd NN dd_tttttt MM + MM dd NN cc and NN dd_tttttt 1.0 Combined bending and tension bottom flange: MM kk 12 + NN tt 1.0 MM dd NN dd_bbbbbb MM (5a) (5b) (5c) ZZ NN tt 1.0 (5d) MM dd AA MM dd 4.3 Design for shear effects Shear is generally not a limiting state for strength in these types of floor beams. However, a check of the web for shear is recommended: VV dd = kk 1 kk 4 kk 6 ff ss AA ss (6) Connection details recommended for achieving fully composite design behaviour are specified in Section 5. The first moment of shear area and hence the shear flow at the interface between web and the flanges can be checked using the following equations: QQ tttttt = AA ff.cc h cc h ff.cc /2 (7a) QQ bbbbbb = AA ff.tt h tt h ff.tt /2 (7b) qq tttttt = QQ tttttt (VV II) (8a) qq bbbbbb = QQ bbbbbb (VV II) (8b) 4.4 Design for deflection & dynamics Limits on the deflection and dynamic behaviour need to be determined to suit the functional requirements of the flooring system, in accordance with Guidelines presented in Appendix B of AS A more rigorous dynamic assessment can be carried out based on the fundamental frequency of the timber floor noting that this formula predicts the behaviour of an individual beam element, which will generally be conservative as a prediction of the floor system behaviour. Prediction of the first fundamental frequency of simply supported timber floor beam is based on an empirically derived methodology, which is summarised in the formula below: Nat Freq (Hz) = where: EI eff is the effective stiffness of the timber beam cross-section and will need to be derived based on the modular ratio if the web and flanges have different properties, G is the self-weight and all units are in N mm. 8

9 Introduction Design requirements Notation Design procedure Manufacturing provisions Currently accepted design methods for timber floors such AS 1684, are generally based upon the assumption that acceptable performance of the floor is considered to occur when the fundamental frequency exceeds 8 Hz. However, this is a simplification and recent studies (Hamm, et al. 2012) indicate that lower frequencies in the 3.5 to 5.5 Hz range, may also be acceptable. A more comprehensive assessment of the dynamic performance of the floor where the dynamic performance is deemed to be critical can be undertaken based on quantifying a Response Factor (Willford and Young 2009 and Smith et al. 2006). This method is based on concrete and steel-concrete composite floor design but is considered to be equally applicable to timber floors. However, the method will normally require the use of finite element modelling to establish the dynamic parameters of the floor such as natural frequencies, mode shapes and damping. 5. Manufacturing provisions The recommended procedure for connecting flanges to webs is gluing and screwing. The design philosophy behind this is that the glue creates an infinitely stiff bond to resist serviceability load events, whilst the screws provided a mechanical connection that can ensure composite action occurs at the design ultimate load events. In the beams tested, the glue bond was a PURBOND polyurethane glue, fastened using 14G Type 17 screws (as indicated in Figure 3) at nominal centres of 400mm c/c along the entire length of the web. Figure 3: Dimensions of the Type 17 screws used for manufacture 9