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Development of AS/NZS 2327 Section 7 System Design for Fire Resistance Presentation to The Steel in Fire Forum, 15 Sept 2015

By G Charles Clifton, University of Auckland

and

Anthony Abu

University of Canterbury

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Scope of Presentation: AS/NZS 2327 Section 7 System Design for Fire Resistance

Development of standard

Meeting the constraints of all parties

Probable scope of standard and associated industry document

New Zealand application; use with C/VM2

Future fire research needs

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Development of Standard

Meeting the constraints of all parties (if possible)

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Current Standard provisions and their shortcomings

• Fire provisions very restricted and dated

• Composite provisions Australia cover simply supported beams and only re-entrant profiles

• Composite provisions New Zealand cover solid web composite beams and all decking profiles.

• No coverage of long span cellular beam systems

• Inadequate coverage of composite columns for ambient temp design and none for fire

NZS 3404: 1997/2001/2007 Covers Bare Steel and Composite Design incl Fire

AS 2327 Part 1 covers composite beams; AS 4100 covers bare steel and fire

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• Covers latest design provisions for composite members and systems

• Covers all forms of composite steel/concrete construction and composite slabs on steel deck

• Covers system performance for serviceability, earthquake and fire

Proposed Scope of New Standard AS/NZS 2327 Composite Structures

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Section 1: General

Section 2: Design of Composite Slabs

Section 3: Design of Composite Beams

Section 4: Design of Composite Columns

Section 5: Connections

Section 6: System Design for Serviceability

Section 7: System Design for Fire Resistance

Section 8: Design for Earthquake

Section 9: Appendices

Proposed Content of New Standard AS/NZS 2327 Composite Structures

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With regard to the section on fire, the current provisions on fire cover only the fire resistance (R) of individual bare steel elements in the Standard Fire Test. They do not cover the following:

• Determining the appropriate design actions for the fire condition

• Design of composite members including slabs for fire

• Design of members and systems based on elevated temperature resistance

• Detailing for dependable performance in fire

Meeting the Demands of All Parties: 1 of 2

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To meet these requirements would greatly increase the size and scope of the standard and there is committee concern over this; therefore intention is to produce, concurrently:

Section 7 of AS/NZS 2313; and

an Accompanying Industry Document (AID)

Meeting the Demands of All Parties: 2 of 2

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Probable Scope of Fire Section (Section 7) and Accompanying Industry

Document

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• Used with the AUBRCC and the NZBC

• Covers design of steel and composite structures for fire resistance

• Performance requirements

• Definitions

• Notation

Section 7.1 Scope and General

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• Design based on Standard Fire Exposure

• Design based on Parametric Fire Exposure

• Actions during fire

• Member capacities for the Fire Limit State

Section 7.2 Basis of Design

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Comprehensive coverage of temperature dependent material properties for fire, covering:

• Mechanical properties of steel, rebar, normal weight concrete and light weight concrete, steel decking, bolts and welds

• Thermal properties of structural steel and rebar, normal weight and light weight concrete

• Suppression of spalling in normal weight concrete elements

Section 7.3 Material Properties

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• Criteria for which member based analysis is applicable

• Methods of analysis required for:

– Beams and connections

– Columns

– Braces

– Floor slabs

• One way acting slabs

• Two way acting slabs

• Allowance for localised load sharing

Section 7.4 Options for Analysis

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• General application of RfT Sf*

• Bending resistance of beams

– Section moment capacity in fire

– Member moment capacity in fire

• Temperature variation through beam cross sections

• Design based on Standard Fire Exposure for unprotected and for protected steel/composite beams

• Maximum limiting temperature for transfer beams

• Design based on elevated temperature resistance

Section 7.5 Beams

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• General application of RfT Sf*

• Temperature variation through column cross sections

• Maximum limiting temperatures and minimum load ratios

• Design based on Standard Fire Exposure for unprotected and for protected steel and composite columns

• General design method for encased composite columns

• General design method for unprotected concrete filled steel hollow section columns

Section 7.6 Columns

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• Determination of resistance in the Standard Fire by either test or calculation method

• Calculation method based on modified ECCS 82 heat path method (this method to go into Appendix).

Section 7.7 One Way Spanning Slabs

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• Performance criteria given

• Must use a rational method which is supported by experimental testing

– Method itself not given, this will be in AIDocs

Section 7.8 Two Way Spanning Composite Floor Systems

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• Insulation requirements around connections

• Determination of connection temperatures

• Connection strength

Section 7.9 Connections

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• General detailing requirements: full details given

• Fire ductile detailing requirements: performance criteria given; details will be in AID

Section 7.10 Constructional Details

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• What shall be included in an advanced analysis

• Validation of advanced calculation models

Section 7.11 Framework for Advanced Analysis

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New Zealand Application: use with C/VM2

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• Introduced in April 2012

• Mandatory for FED from April 2013

• Being used in majority of multi-storey buildings (low rise and above)

• Used with Loadings Standard AS/NZS 1170.0

• Provides the “Loadings Standard” provisions for Fire Engineering Design

• Needs materials standards written for use with it

Verification Method C/VM2

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C/VM2 requires FED for 10 fire design scenarios

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• BE = blocked exit

• UT, CS, SF = different types of developing fire for life safety occupants in enclosure of origin

• HS, VS, IS = horizontal, vertical, internal spread of fire

• FO = firefighting operations (principal scenario for structural robustness)

• CF = challenging fires (high growth or high toxicity)

• RC = robustness check on fire safety features (failure to operate scenarios)

Rest of C/VM2 slides focus only on FO

Design Scenarios

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These are the mandatory requirements. Paraphrased they are to:

• Protect Other Property (Neighbour)

– Limit radiation flux at boundary

– Limits on opening size

– External openings to be stable

• Facilitate Fire Fighting and Rescue

– Control fire fighter tenability conditions on arrival to firecell in unsprinklered firecells

– Requirements for access

– Structural systems required for firefighter safety must be stable during and after fire

NZ Building Code Requirements for FO

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For buildings with escape height > 10m

a) Safe paths to resist burnout

b) Structure supporting floors to resist burnout

For buildings with escape height ≤ 10m

a) Safe paths can access all floors for 60 mins from ignition or burnout whichever less

b) Floor systems to resist collapse for at least 30 mins from ignition

c) From ignition to full development to be calculated (or taken as 10 mins)

FO Scenario Building Stability Requirements from C/VM2

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1. Use a time equivalent formula and ensure FRR ≥ te

2. Use a parametric time versus gas time temperature formula to generate gas time – temperature conditions for input into a structural response model

3. Construct a Heat Release Rate versus time design option then generate gas time – temperature conditions for input into a structural response model

More now on 1 and 2 which are the most commonly used methods of FED in New Zealand

Burnout Design Fires – 3 Options

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Option 1: Time Equivalence Formula

ef,mod = fire load energy density (MJ/m2 floor area) modified

kb = thermal inertia of firecell factor

km = thermal inertia of firecell factor

wf = ventilation factor

and

Relement ≥ te required

fmbfe wkket mod,

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• Use either Eurocode or Modified Eurocode to get gas time temperature conditions, g, then

• Get steel temps, s, from gas temps; for unprotected downstand beams:

– bottom flange, web = 0.95g

– top flange = 1.0g – 150C

• Compare s with limiting steel temperature, Tl or do elevated temperature capacity check

– if s ≤ Tl or S* ≤ Rf,T design is OK

– otherwise need to increase insulation

• Typically used with steel beams above permanent linings which act as radiation barriers

Option 2: Parametric Curve with Limiting Temperature

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Flowchart Example from the New Standard; general FED procedure

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Flowchart Example from the New Standard; Beams

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Flowchart Example from the New Standard; Columns subjected to Standard Fire Exposure

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Future fire research needs

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• This limits the tensile membrane capacity

• What are fracture strain limits for:

– Rebar crossing tension crack in the strongly heated condition

– Rebar over internal supporting beams subject to high negative curvatures when relatively cold

• Reinforced concrete prism tests at elevated temperatures?

Fracture Strain of Rebar Crossing Crack

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Determining the Adequacy of Slab Panel Detailing Provisions

• Determine by large scale experimental testing and by modelling the adequacy of the current SPM detailing provisions; validate the tensile membrane stable vertical deflection limit (critical to design procedure)

• Three large scale fire tests have recently supported the need for verification with premature failures when details not included:

• Mokrsko: slab pulled off slab panel edge support beam due to lack of edge and anchor bars around shear studs

• Fracof: fracture of mesh where not adequately lapped within slab panel

• VUT: shear failure at interior support where interior support bars too short and wrongly placed

• Second planned VUT test deferred; lack of funding

• NIST/NFRL test can/should? target this

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Contribution of Long Span Beams with Continuous Web Openings to Slab Panel Resistance

• These are becoming more common

• Status:

– web contribution currently ignored

– bottom flange laterally buckles

– is this accurate?

• Need student and funding

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Slab Panel Performance with Alternative Material Specifications

• General determination following on from 2011 research

• Status:

– Linus Lim in 2000 undertook PhD 6 slab panel tests and procedure verification

– Repeat tests with steel fibres instead of general mesh

– These used in conjunction with additional support reinforcement?

– Influence of cement types and mix design on panel performance

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Slab Panel Performance with Steel Fibre Reinforcement

• General determination following on from 2011 research

• Status:

– Linus Lim in 2000 undertook PhD 6 slab panel tests and procedure verification

– Repeat tests with fibres instead of general mesh

– These used in conjunction with additional support reinforcement?

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