Pros/Cons for Transverse rebar in structural fire response ...fire-research.group.shef.ac.uk/steelinfire/downloads/IC_2014.pdf · Pros/Cons for Transverse rebar in structural fire

Pros/Cons for Transverse rebar in structural fire response of a composite structure

Steel in Fire Forum 2014 Iris Chang

2

Introduction • Shear studs allow beams and concrete slab to act

together

• Transverse rebar designed into the slab structure

above particular beam spans

• Transverse rebar (loose bars) used to assist in

composite behaviour between composite slab and

underlying steel frame


• Transverse rebar not designed to provide direct load bearing capacity for the slab

so generally ignored in structural fire analysis

3

Introduction • Some steel members may be left unprotected

• In fire, exposed members will undergo large vertical deflection

• Tensile membrane action typical

• Compressive ring can cause hogging moments over protected beams resulting

in high strains – integrity of compartmentation

• Ropemaker Place and The Kings Place

have added loose bars over protected

beams to decrease reinforcement strain

• Recent project included finite element

models with transverse rebar added into the

input

• Results showed that structural response can

be effected by the use of transverse

reinforcement

4

Project background


Not included

in assessment

13 storey office building:

• Retail on ground and first floors

• Plant on levels 10 to 13

18 storey office building:

• Retail on ground and first floors

• Plant levels on levels 16 to 18

Both Buildings 6b and 7a are:

• Sprinkler protected

• 120mins fire rating separating the retail from office

• 90mins fire rating between office levels

Nova Victoria Development - London

5

Sub model analysis – Idealised geometry


Typical office floor plate

6

Sub model analysis – Material properties


• Concrete behaviour modelled in line with Eurocode 2 approach

• Concrete cracking modelled in line with the Model Code 1990 (T. Telford, 1993)

Material Model

Steel members S355

Light weight concrete Grade C28/35

Reinforcing mesh A252 mesh

Transverse rebar H16 bars at 150mm centres centred over the relevant beams

• Steel behaviour modelled in line with Eurocode 3 approach

7

Sub model analysis – Geometry and sections


8

Sub model analysis – Steel beam elements


Steel frame is constructed of nodes and beam elements.

A beam member is created:

• Element is assigned with a defined Material

• Element is assigned with a defined Part

• 2 noded beam elements with a 3rd node dictating its orientation

• Beam releases at the nodes can be specified to the required DOFs

• Each beam element is 250mm in length

• Adjacent beam elements are connected by shared nodes

9

Sub model analysis – Composite slab


Nova Victoria is a composite structure, the slab is modelled without the steel decking

0.141m

2.52E-4m

Slab structure modelled

with its effective

thickness without the

profiled decking

Steel reinforcement is

modelled as a smeared sheet of

steel within the floor slab

Floor slab

• Constructed of 5 layers (8 layers where transverse rebar is modelled)

• Each layer is assigned a thickness and a defined Material

• The elements representing the slab is meshed and assigned with this defined Part

0.141m

1.34E-3m

Transverse reinforcement

within the slab element

10

Sub model analysis – Composite slab


Composite behaviour of slab modelled by assuming full shear connection between

slab and beams.

The nodes in the shell elements (the slab elements) are made to share nodes with the

previously created beam elements

The shell elements and beam elements are OFFSET so that the centroid of each

element are not passing through each other at the reference axis but at the right

distance from the reference axis

• Each shell element is offset +0.075

• Each beam elements are offset -0.309 at both nodes

• The -0.009 offset in the beam is to account for the omitted profile decking

11

Sub model – Boundary Conditions


12

Sub model – Loading


• Self-weight of structure

• Typical office floor design load

• Column loads obtained by structural engineers

• Façade load (line load)

• Loads from adjacent floor plates (line load)

13

Sub model – Thermal loading


Applied to the relevant shells/beams

Create the temperature to be experienced by the relevant beam or in the slab

• Heat transfer analysis method

• Lumped mass – steel

• 1D finite difference model - concrete

• Design fire scenarios

Design fires:

• Standard fire

• Parametric fire

• Travelling fire – Monte

Carlo Analysis Law, A.,

Stern-Gottfried, J., &

Butterworth, N. (2013). Solving

design challenges: revisiting

time equivalence. In 13th

International Conference and

Exhibition on Fire Science and

Engineering (Interflam).

14

Sub model – Thermal loading


• The calculated temperature time curves are inputed as a thermal load

• Temperature - time curves are assigned to relevant sections

• For shell elements:

• Temperature history can be applied to multiple temperature lines throughout the

thickness of the shell

• Each temperature line is assigned with a defined load curve and the depth in the

shell where this temperature lies

• Temperature at the element integration points determined by interpolation of input

• For beam elements:

• Multiple temperature histories can be

defined for a beam section

• It was assumed that the top of the beam

cross section will be cooler

s

t

s = -1 t = -1 s = 1 t = -1

s = -1 t = 0.9 s = 1 t = 0.9

s = -1 t = 0.91

s = 1 t = 1 s = 1 t = 1

15

Sub model – Results


First checks:

• Global stability

• Slab runaway

• Column stability

• Reinforcement strain

Check against:

• Structure stability

• Integrity of floor

compartmentation

16

90 minutes standard fire – Reinforcement strain


Without transverse rebar:

• Reinforcement experienced

high strain – 60 minutes

• Stability of the structure lost

before the 90 minutes

With the addition of transverse rebar:

• Strain over the protected beams

decreased

• Higher level of strains at transverse

bar edges

• Structural stability maintained

throughout

17

Addition of transverse rebar


• Transverse rebar provided added

stiffness and strength to the structural

system

• Sections of slab with transverse rebar

less inclined to deflect

• Slab centre capable of deflecting to a

span to deflection ratio of 11

• Increased strain within reinforcement

at locations where there is a

difference in stiffness - deflection

18

Worst case heating regime – fast burning fire


• Different heating regimes can result in

different structural responses

• For this particular arrangement, fast

growing fire is worst case

• High reinforcement strain over

protected beams developed during the

heating phase

• The added transverse rebar relocated

high strains in the reinforcement to bar

edges

• Reinforcement strain over protected

beams are relatively low

19

Storage loading – long slow burning fire


Before first beam

element failed

When first beam

element failed

• Increased rebar over the primary span

• High difference in stiffness at the edge of

the transverse rebar

• High localised straining within the slab

transferred into the underlying beams

• This caused yielding in the lower flange

and web of beam elements underneath

• Beam element written off when top flange

of the beam failed

• Released energy transferred into overlying

slab

20

Concluding remarks


• Transverse rebar can give added strength and stiffness to the composite slab,

in fire

• The difference in stiffness within the slab system at the transverse rebar edge

can lead to an increase in reinforcement strain, in fire

• High reinforcement strain over the protected beams or transverse rebar edges

can lead to reinforcement rupture – integrity of floor compartmentation

• The response of the structure will vary dependent on the heating regime

experienced

• Transverse rebar can provide benefit but may also lead to reinforcement

rupture at the rebar edges

• Transverse reinforcement designed into the structure should be included in

structural fire modelling

• Scope for physical testing into relationship between transverse rebar location

and quantity against the structure stability and floor compartmentation

• Can the addition of transverse rebar be considered as a means to increase

robustness of structure or introduce unconservative structural response at fire

limit state

21 Pros/Cons for Transverse rebar in structural fire response of a composite structure

Questions?

Thank you!

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Pros/Cons for Transverse rebar in structural fire response ...fire-research.group.shef.ac.uk/steelinfire/downloads/IC_2014.pdf · Pros/Cons for Transverse rebar in structural fire