design guide australia and new zealand design...

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DESIGN PROCEDURES FOR TIMBER ONLY COMPOSITE

FLOOR SYSTEMS

DESIGN GUIDE AUSTRALIA AND NEW ZEALAND

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Structural Timber Innovation Company (STIC)

Authors:

Prof. Keith CrewsProfessor of Structural Engineering, Faculty of Engineering and Information Technology, University of Technology, Sydney

Dr. Rijun ShresthaResearch Associate, Faculty of Engineering and Information Technology, University of Technology, Sydney.

ImpressumDesign Procedures For Timber Only Composite Floor Systems

Report no: STIC- 2013-44

Version 1-0

UTS Project no: RES 08 - 244

First publication 2013

Copyright © 2013 by Structural Timber Innovation

Company (STIC), Christchurch 2013

All rights reserved.

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.

Design Guide Australia and New Zealand

Design Procedures For Timber Only Composite Floor SystemsFirst Edition 2013

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.

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Design Guide Australia and New Zealand

Design Procedures For Timber Only Composite Floor SystemsFirst Edition 2013

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Table of contents

Chapter Page

1. Introduction 5

2. Design requirements 5

3. Notation 6

4. Design procedure 7

5. Manufacturing provisions 8

6

1. IntroductionThis 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 AS1720.1 –

2010 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 AS1720.1 – 2010

and the modification “k” factor subscripts should not be mixed or

confused with those in NZS 3603.

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 3603 – 1993 AS 1720.1 - 2010

Capacity factor All current Australia / New Zealand product standards for Engineered Wood products (LVL and glue laminated tim-ber), have properties derived on the basis of the capacity factors in Table 2.1 of AS 1720.1 – 2010, 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

Duration of load – strength k1 k1

Duration of load – stiffness / deflection

k2 j2

Bearing factor k3 k7

Parallel support k4 k9

Grid systems k5 g42

Strength sharing – glue laminated timber

k6 Not applicable

Stability k8 k12

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 AS1720.1 as a

fully composite section, with linear elastic behaviour in resisting load

actions predicted using AS/NZS 1170.1.

2. Design requirementsThe 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.

5. 1.0-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.

Introduction Design requirements Notation Design procedure Manufacturing provisions

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3. NotationUnless noted otherwise in the figures below, all symbols and letters

used in the design procedure conform to those in AS1720.1 – 2010:

Figure 1: Notation for a typical composite timber only floor system

Figure 1: Notation for a typical composite timber floor system

Figure 2: Notation for an individual “cassette” in a typical composite timber floor system

A Cross-sectional area of the entire section

Afc Cross-sectional area of the top (compressive) flange

Aft Cross-sectional area of the bottom (tension) flange

As Shear area of the web = 2/3 bw hw

Aw Cross-sectional area of the web

bf.c Width of the top (compressive) flange (equals c/c spacing of webs)

bf.t Width of the bottom (tension) flange

bw Width (thickness) of the web

h Overall depth of floor

hc Distance from the centroid to the top of the top flange

ht Distance from the centroid to the bottom of the bottom flange

hf.c Height of the top (compressive) flange

hf.t Height of the bottom (tension) flange

STIC 2013-44 Timber Design Procedures - ver May 16 2013 Rev 2.pages

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Figure 2: Notation for an individual “cassette” in a typical composite timber only floor system

Figure 1: Notation for a typical composite timber floor system

Figure 2: Notation for an individual “cassette” in a typical composite timber floor system






bf.c Width of the top (compressive) flange (equals c/c spacing of webs)





ht Distance from the centroid to the bottom of the bottom flange




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bf.c Width of the top (compressive) flange (equals c/c

spacing of webs)





ht Distance from the centroid to the bottom of the

bottom flange



hcentroid Distance from the bottom of the bottom flange to the

centroid = ht

I Second moment of inertia of the composite section

Z top Section modulus above the centroid (top flange)

Z bot Section modulus below the centroid (bottom flange)

E value of the modulus of elasticity of the timber members

hcentroid Distance from the bottom of the bottom flange to the centroid = h f.t





characteristic strength in bending

characteristic strength in compression

characteristic strength in shear

characteristic strength in tension

j2 stiffness modification factor – load duration

k1 duration of load (timber)

k4 moisture condition (timber)

k6 temperature (timber)

k7 length and position of bearing (timber)

k9 strength sharing between parallel members (timber)

k11

size factor (timber) – this is normally applied to the characteristic

strength property by the manufacturer

k12 stability factor (timber)

M* Moment action resulting from applied loads

M d_top Design moment capacity – top flange

M d_bot 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 Design axial capacity (compression) – top flange

N d_bot Design axial capacity (tension) – bottom flange

Q top First moment of shear area for top flange

Q bot First moment of shear area for bottom flange

q top Shear flow at interface between web and top flange

q bot Shear flow at interface between web and bottom flange

V* Maximum shear effect

Vd Design shear capacity of the web

Capacity factor


5

















k11

















Capacity factor


5

















k11

















Capacity factor


5

















k11

















Capacity factor


5








k11 size factor (timber) – this is normally applied to the

characteristic strength property by the manufacturer





N*c Axial force (compression) induced in top flange from

bending

N*t Axial force (tension) induced in bottom flange from

bending
























k11

















Capacity factor


5

Capacity factor

EIeff effective stiffness of timber floor cross-section

G self-weight


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4. Design procedureThe 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 characteristicsIn 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.

STIC 2013-44 Timber Design Procedures - ver May 16 2013 Rev 2

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4 Design procedureThe 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 characteristicsIn 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.

𝑏𝑏𝑓𝑓.𝑡𝑡 ≤ 𝑀𝑀𝑀𝑀𝑀𝑀 (𝑏𝑏𝑤𝑤 + 0.1 × 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠), �𝑏𝑏𝑤𝑤 + 20 × ℎ𝑓𝑓.𝑡𝑡� for bottom flange (1a)

𝑏𝑏𝑓𝑓.𝑐𝑐 ≤ (𝑏𝑏𝑤𝑤 + 0.1 × 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠) for top flange (1b)

4.2 Design for flexural effectsThe 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 AS1720.1 –2010, for combined bending and axial load effects. The equations below apply to a simplysupported beam and would need to be interpreted correctly for use with continuous beams.

Bending capacity of the section above the centroid is given by:

𝑀𝑀𝑑𝑑 = ∅𝑘𝑘1𝑘𝑘4𝑘𝑘6𝑘𝑘9𝑘𝑘12𝑓𝑓′𝑏𝑏𝑍𝑍𝑡𝑡𝑡𝑡𝑡𝑡 (2a)

Bending capacity of the section below the centroid is given by:

𝑀𝑀𝑑𝑑 = ∅𝑘𝑘1𝑘𝑘4𝑘𝑘6𝑘𝑘9𝑘𝑘12𝑓𝑓′𝑏𝑏𝑍𝑍𝑏𝑏𝑡𝑡𝑡𝑡 (2b)

Where: k4, k6, k9 and k12, will all normally equal 1.0

Axial capacity (compression) of the top flange is given by:

𝑀𝑀𝑑𝑑_𝑡𝑡𝑡𝑡𝑡𝑡 = ∅𝑘𝑘1𝑘𝑘4𝑘𝑘6𝑘𝑘12𝑓𝑓′𝑐𝑐𝐴𝐴𝑓𝑓.𝑐𝑐 (3a)

Axial capacity (tension) of the bottom flange is given by:

𝑀𝑀𝑑𝑑_𝑏𝑏𝑡𝑡𝑡𝑡 = ∅𝑘𝑘1𝑘𝑘4𝑘𝑘6𝑘𝑘11𝑓𝑓′𝑡𝑡𝐴𝐴𝑓𝑓.𝑡𝑡 (3b)

for bottom flange (1a)


6


















for top flange (1b)

4.2 Design for flexural effectsThe 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 AS1720.1 – 2010, 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.



6


















(2a)



6


















(2b)




6


















(3a)



6

















𝑀𝑀𝑑𝑑_𝑏𝑏𝑡𝑡𝑡𝑡 = ∅𝑘𝑘1𝑘𝑘4𝑘𝑘6𝑘𝑘11𝑓𝑓′𝑡𝑡𝐴𝐴𝑓𝑓.𝑡𝑡 (3b) (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:


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The axial force induced in each flange as a result of the bending action is calculated using the following equations:


𝑁𝑁𝑐𝑐∗ = 𝑀𝑀∗ × �ℎ − ℎ𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 − ℎ𝑓𝑓.𝑐𝑐/2� × 𝐴𝐴𝑓𝑓.𝑐𝑐 ÷ 𝐼𝐼 (4a)

Axial load induced (tension) in the bottom flange is given by:

𝑁𝑁𝑐𝑐∗ = 𝑀𝑀∗ × �ℎ𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 − ℎ𝑓𝑓.𝑐𝑐/2� × 𝐴𝐴𝑓𝑓.𝑐𝑐 ÷ 𝐼𝐼 (4b)

Combined bending and compression – top flange:

�𝑀𝑀∗

𝑀𝑀𝑑𝑑�2

+ 𝑁𝑁𝑐𝑐∗

𝑁𝑁𝑑𝑑_𝑡𝑡𝑡𝑡𝑡𝑡≤ 1.0 (5a)

and

𝑀𝑀∗

𝑀𝑀𝑑𝑑+ 𝑁𝑁𝑐𝑐∗

𝑁𝑁𝑑𝑑_𝑡𝑡𝑡𝑡𝑡𝑡≤ 1.0 (5b)

Combined bending and tension – bottom flange:

𝑘𝑘12𝑀𝑀∗

𝑀𝑀𝑑𝑑+ 𝑁𝑁𝑡𝑡

∗

𝑁𝑁𝑑𝑑_𝑏𝑏𝑡𝑡𝑡𝑡≤ 1.0 (5c)

𝑀𝑀∗

𝑀𝑀𝑑𝑑− 𝑍𝑍

𝐴𝐴× 𝑁𝑁𝑡𝑡

∗

𝑀𝑀𝑑𝑑≤ 1.0 (5d)

4.3 Design for shear effectsShear is generally not a limiting state for strength in these types of floor beams. However, a check of the web for shear is recommended:

𝑉𝑉𝑐𝑐 = ∅𝑘𝑘1𝑘𝑘4𝑘𝑘6𝑓𝑓′𝑠𝑠𝐴𝐴𝑠𝑠 (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:

𝑄𝑄𝑐𝑐𝑐𝑐𝑡𝑡 = 𝐴𝐴𝑓𝑓.𝑐𝑐�ℎ𝑐𝑐 − ℎ𝑓𝑓.𝑐𝑐/2� (7a)

𝑄𝑄𝑏𝑏𝑐𝑐𝑐𝑐 = 𝐴𝐴𝑓𝑓.𝑐𝑐�ℎ𝑐𝑐 − ℎ𝑓𝑓.𝑐𝑐/2� (7b)

(4a)



7







�𝑀𝑀∗




and

𝑀𝑀∗






∗


𝑀𝑀∗



∗

𝑀𝑀𝑑𝑑≤ 1.0 (5d)






(4b)



7







�𝑀𝑀∗




and

𝑀𝑀∗






∗


𝑀𝑀∗



∗

𝑀𝑀𝑑𝑑≤ 1.0 (5d)






(5a)

and


7







�𝑀𝑀∗




and

𝑀𝑀∗






∗


𝑀𝑀∗



∗

𝑀𝑀𝑑𝑑≤ 1.0 (5d)






(5b)



7







�𝑀𝑀∗




and

𝑀𝑀∗






∗


𝑀𝑀∗



∗

𝑀𝑀𝑑𝑑≤ 1.0 (5d)






(5c)


7







�𝑀𝑀∗




and

𝑀𝑀∗






∗


𝑀𝑀∗



∗

𝑀𝑀𝑑𝑑≤ 1.0 (5d)






(5d)

4.3 Design for shear effectsShear is generally not a limiting state for strength in these types of

floor beams. However, a check of the web for shear is recommended:


7







�𝑀𝑀∗




and

𝑀𝑀∗






∗


𝑀𝑀∗



∗

𝑀𝑀𝑑𝑑≤ 1.0 (5d)






(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:


7







�𝑀𝑀∗




and

𝑀𝑀∗






∗


𝑀𝑀∗



∗

𝑀𝑀𝑑𝑑≤ 1.0 (5d)






(7a)


7







�𝑀𝑀∗




and

𝑀𝑀∗






∗


𝑀𝑀∗



∗

𝑀𝑀𝑑𝑑≤ 1.0 (5d)





𝑄𝑄𝑏𝑏𝑐𝑐𝑐𝑐 = 𝐴𝐴𝑓𝑓.𝑐𝑐�ℎ𝑐𝑐 − ℎ𝑓𝑓.𝑐𝑐/2� (7b) (7b)STIC 2013-44 Timber Design Procedures - ver May 16 2013 Rev 2

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𝑞𝑞𝑡𝑡𝑡𝑡𝑡𝑡 = 𝑄𝑄𝑡𝑡𝑡𝑡𝑡𝑡 × (𝑉𝑉∗ ÷ 𝐼𝐼) (8a)

𝑞𝑞𝑏𝑏𝑡𝑡𝑡𝑡 = 𝑄𝑄𝑏𝑏𝑡𝑡𝑡𝑡 × (𝑉𝑉∗ ÷ 𝐼𝐼) (8b)

4.4 Design for deflection & dynamicsLimits 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 Bof AS1720.1 – 2010.

5 Manufacturing provisionsThe 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

(8a)


8

𝑞𝑞𝑡𝑡𝑡𝑡𝑡𝑡 = 𝑄𝑄𝑡𝑡𝑡𝑡𝑡𝑡 × (𝑉𝑉∗ ÷ 𝐼𝐼) (8a)

𝑞𝑞𝑏𝑏𝑡𝑡𝑡𝑡 = 𝑄𝑄𝑏𝑏𝑡𝑡𝑡𝑡 × (𝑉𝑉∗ ÷ 𝐼𝐼) (8b)

4.4 Design for deflection & dynamicsLimits 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 Bof AS1720.1 – 2010.




(8b)

4.4 Design for deflection & dynamicsLimits 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

AS1720.1 – 2010.

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:

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 AS1720.1 – 2010. 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: EIeff 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.

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. Ammendment to Notations: EIeff - effective stiffness of timber floor cross-section G - self-weight

where: EIeff 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.


9

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.


4.4 Design for deflection & dynamicsLimits 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 AS1720.1 – 2010.





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