Wednesday 9 November

2.00- 3.30pm Concurrent Technical Sessions: 4

4A Arawa Room Buildings

28. NZI Centre - Design of Multistorey Towers- Billings IJ, Thom CWBeca Carter Hollings & Ferner Ltd, Auckland

27. Auckland Trotting Club New Grandstand-Brown PBThorburn Davidson Ltd, Auckland

29. Mid City Towers - an Efficient Precast Concrete Framed

4B Tiri Room Concrete Properties

42. Influence of Cement Paste Flocculation on its RheologicalProperties- Nawa T., Eguchi H., Fukaya Y.Chichibu Cement Co. Ltd, Japan

4 7. Blended Cements Inhibit AAR Expansion - Kennerley RANew Zealand Cement Holdings Ltd

Building 48. Alkali-Aggregate Reaction in Concrete - a Problem inNew Zealand Too - Poole RA, Glendon JE

Holmes Consulting Group, Christchurch

4.00- 5.30pm Concurrent Technical Sessions: 5

5A Arawa Room Walls

19. Structural Walls of Limited Ductility- Paulay T, Mestyanek JM

20.

22.

Department of Civil Engineering, University of Canterbury

Experiments on Vertical Joints of Precast Concrete Wall Panel Structures Considering Restricting Effects of Horizontal Ties - Mochizuki SMusashi Institute of Technology, Japan

A Study on Failure Control and Ductility of Layered Shear Wall Frame System - Mochizuki M, Umeda MKogakuin University, Japan

21. Modelling Fire Performance of Concrete Walls- Buchanan AH, Carr AJ, Munukutla RDepartment of Civil Engineering, University of Canterbury

-• Rowe G.H., Smith L. M., Freitag S.A., Doyle R.B., St John D.A.• Central Laboratories, Works Corporation

49. Strength of Cement-Aggregate Bond-Taylor MAUniversity of California, Davis, USA

5B Tiri Room Steel-Cement Composites and Construction

50. Development and Commercialisation of AdvancedStrength Steel Cement Composites- Busck C.J.Fibre Cement Technology Ltd, Auckland

51. Ferrocement Applications in Housing- Paramasivam P, Lee SL

69.

National University of Singapore

Top-Down Construction - Construction Joints in Underground Concrete Structure - Takahei Y.Takenaka Technical Research Laboratory, Japan

70. A Top-Down Method of Constructing Permanent andTemporary Concrete Retaining Walls Incorporating SoilNailing-• Ashley A., Bird A.* Smith Lecuhars Ltd, Auckland

9

PACIFIC CONCRETE CONFERENCE

New Zealand 8-11 November� 1988

MODELLING FIRE PERFORMANCE OF CONCRETE WALLS

Andrew Buchanan, Athol Carr, Rao Munukutla

Department of Civil Engineering, University of Canterbury

SUMMARY

This paper is a progress report on a current study into the performance of reinforced concrete walls exposed to fire. A one­dimensional finite difference method of heat transfer calculation is described with typical input values and calculated results. The proposed structural calculations are described briefly, with discussion of the overall problem and overseas code requirements.

INTRODUCTION

Fire Performance of Concrete Walls

Reinforced concrete walls have generally behaved well in fires. Structural collapses of concrete walls in fire are not common, and fire spread in buildings is more often through openings and weaknesses than directly through solid walls. Most damage to concrete walls in fire has been spalling of cover concrete, instability due to fire damage of supporting structure, and occasionally permanent loss of strength due to high temperatures. This study will investigate the fire resistance of concrete walls in some detail. Amendments to MP9 will be proposed.

Functions of Wall in Fire

When exposed to a fire, reinforced perform two major functions; containment figure 1.

concrete walls are required to and load-bearing, as shown in

Woll contains

fire ond

resists load

Figure 1 - Functions of a wall in a fire

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NZS 3101

The concrete design code [14] specifies that loadbearing and non­loadbearing walls shall have minimum thicknesses of 150mm and 100mm respectively. These are thicker than some values shown in Table 1, so will govern in some cases.

Overseas codes

For non-loadbearing walls, most overseas codes specify similar thicknesses to those shown in Table 1. Many codes do not differentiate between loadbearing and non-loadbearing walls (like MP9). This includes Canadian [10] and American codes [1][17]. On the other hand some codes specify greater thicknesses for loadbearing walls, apparently following the example of FIP/CIB, [5]. This category includes the British code (2], and draft Australian code [13].

OTHER STUDIES

Walls

In the last ten to twenty years, analytical studies of fire resistance of concrete and other materials have blossomed with the help of computers. Concrete walls have received very little attention, the only study of note being tha� by O'Meagher & Bennetts (11], which is the basis for the present study. they used a Swedish heat transfer program to predict temperatures in walls, then developed a structural model using published data on the properties of concrete and steel at elevated temperatures. They calculated strains, stresses, and deflections in walls of various heights, reinforcing arrangements, and cover to reinforcing, for exposure to the standard test fire. This study will consider a more realistic range of support and restraint conditions, based on New Zealand construction practice.

Columns

Other studies of relevance are those on reinforced concrete columns, the most extensive being Canadian [8]. A recent study by Purkiss and Weeks [12] may be of more relevance to this project because it considers the case of heating on only some sides of the column.

Concrete

This project is complementary to a current project being carried out by the Building Research Association of New Zealand (BRANZ) into the fire performance of New Zealand concretes. That project is investigating the thermal and possible spalling characteristics of concrete made with a number of typical local aggregates.

EFFECT OF TEMPERATURE ON MATERIAL PROPERTIES

Introduction

From experimental observations, the effects of high temperature on the properties of Concrete under certain known conditions, e.g. moisture content, type of aggregates etc., have been well documented. The materials of interest here are concrete and steel. In the heat flow model described below, it is assumed that reinforcement has an insignificant effect on temperatures in the walls. Hence the present study is concentrated on

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temperature may have a thermal conductivity which is 10 to 20% higher than the corresponding value of the "non-preheated" specimen.

Concrete Type

Temp1 (° C)

K1 (\l/m'' K)

Temp2 CC)

K2(W/m"K)

Tl CC)

Cl (J/Kg° K)

T2 (° C)

c2 (J/Kg° K)

Table 2

Porous Expanded clay Sand Heavyweight concrete lightweight concrete concrete with

foam concrete limestone aggregate

200 200 200 200

0.130 0.259 0.930 1.030

800 800 800 800

0.242 0.339 0.582 0.702

0 0 0 0

921 837 770 712

200 200 200 200

1047 955 895 879

Effect of temperature on thermal conductivity and specific heat of concrete

Figure 3 shows the general temperature, for concrete. The are given in Table 2.

relationship between specific heat and specific values defining the relationship

Figure 3

C2

c, t,

·-

t,

r,

Effect of temperature on specific heat of concrete

Moisture in Concrete

The effect of moisture is described by Bushev et al (4) as: "concrete is a porous material whose pores always contain free water. This water evaporates upon the action of heat on concrete, thus reducing its rate of heating, by absorbing the latent heat of evaporation. Zones of dry and wet concrete (Figure 4), separated by a moving boundary (evaporation front) on which the water evaporates, are formed on heating concrete.

23,3

Subdivision into Elements

The entire surface of one face of the wall is subjected to the same fire environment. A unit length of the wall is therefore considered and is divided into a number of segments top to bottom as shown in Figure 7. Each segment is identically divided into a number of transverse elements across its thickness. Temperatures are considered only to vary through the thickness of the wall, allowing a one-dimensional heat transfer model to be used.

The reinforcement is assumed to have an insignificant effect on the temperatures in the walls and is therefore ignored in the heat transfer model. The reinforcement temperature used in the structural analysis is taken as the temperature in the concrete at the same location.

Fire Test Standard

International standard ISO 834 [6) specifies standard heating and pressure conditions, a test method and criteria for the determination of the fire resistance of elements of building construction of various categories. The furnace time - temperature curve is shown in Figure 5.

Figure 5

---

u •

'-

Cl) � ti � Cl)

� 800

�

Cl)

600

400::---��--L----1...-L-_J 0 1 2 3 4 5 6

Duration (hours)

Standard time-temperature curve

Heat Flow

We can simulate a fire In steady state heat flow, the vary with time. In transient laws of heat flow by conduction

test using the principles of heat flow analysis. temperature at any point in the system does not

heat flow, temperature varies with time. The are

(1) The quantity of heat in a body is proportional to its mass andtemperature.

(2) Heat flows from higher to lower temperatures.

(3) The flow of heat across an area is proportional to the area and therate of temperature change with respect to distance.

Using one-dimensional analysis and applying law 1, the change ininternal energy with respect to time of an element of thickness dx is

235

oT

ot

T' Tm)( m - -At (7)

Where Tm+l' Tm and Tm-l are temperatures at adjacent node points, andT' is the temperature at node m after time increment At.

m

Equation (4) can be expressed in the finite difference notation of equations (6) and (7).

(T' T) a(Tm-1 2T T m+l)m - m m + At =

(�)2

(8)

Solving for T'm

T' = T [ 1 - 2a.At ] [ Tm-1 T ] a.At

(�)2

+ m m + m+l (�/

(9)

Using equation (9) and knowing the temperature of the wall at the beginning of the test, we can compute the temperature T' of a typical interior

m node at each succeeding time increment throughout the test period.

is

Convection

The rate of heat flow at a boundary between a solid and a fluid or gas

where

h = convection coefficient (W/m2 -°K)

T = temperature of body. TA= temperature of surrounding fluid or gas.

The coefficient h varies according to the turbulence of the fluid orgas, roughness of the body surface, and distrance from the body. If a nodepoint m lies on the surface of a wall, then the heat flow equation is :

Assuming the cross-sectional area of the wall is equal to the exposed surface area for one-directional flow, the transient energy balance at the node m gives

T T

k ( m-1 - m) �

Solving for T'

At T'm = 2c.p(�)

or

m

- h (T -T)m A

T' Tm) ( m - -= C .p • � .6.t

237

( 11)

Figure 7

Elements

tw

WALL DIVIDED

INTO SEGMENTS

SEGMENT DIVI DED

I NTO ELEMENTS

Division of wall into segments and elements for thermal and structural analysis

Variables

The principal variables to be considered are: thickness, slenderness, reinforcing content, cover, load, eccentricity of load, and structural continuity. The main applications to be considered are isolated walls near the bottom of multi-storey concrete buildings, where axial loads may be high.

CONCLUSIONS

This paper is a progress report on a continuing project. Conclusions to date are :

1 Heat flow in concrete walls can be modelled with a simple one­dimensional finite difference method.

2 MP9:1987 offers well accepted walls, but may be unsafe compression axial loads.

guidance for walls

for non-loadbearing concrete with significant in-plane

3 Concrete made from lightweight aggregates has much better insulating properties than normal weight concrete.

4 Moisture content does not have a significant influence on the heat transmission through concrete walls.

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