Practical Problems of Applying Performance-Based Structural Fire Engineering in

Building Design

Piotr Smardz, INBEPO Sp. z o.o.

Introduction

• The development of the structural Eurocodes was an important event from the point of view of performance-based fire safety design in Europe

• Those European standards for structural design cover the topic of fire design in terms of thermal actions (EN 1991-1-2) as well as mechanical response of most commonly used building materials (EN 1992-1-2, EN 1993-1-2 etc.)

• This presentation will focus on the issues relevant to engineering analysis of thermal actions on structural elements exposed to fire

• Such analysis is the first and fundamental step to any performance-based design of fire resistance of structural elements

Design Fires

• In order to carry out an analysis of thermal actions on structural elements exposed to fire it is necessary to select a design fire

• The design fire defines the rate of heat release of the fire as a function of time

• The design fire can either be described by a simple formulae (e.g. αt2 fire) or by a set of discrete values of HRR vs. time

• In order to accurately describe the fire in the fire analysis, it is also necessary to describe the area involved, thus an assumption must be made as to the heat release rate of fire per unit area (HRRPUA)

Design Fires – Problems:• EN 1991-1-2 provides guidance on fire growth rates, fire load densities and

maximum HRR per 1 m2 of fire only for a limited number of occupancy types including dwellings, hospitals, hotels, offices, school classrooms and shopping centres (additional values can be found in some National Annexes e.g. Irish NA to EN 1991-1-2 or PD 6688-1-2)

• No data for commonly encountered occupancies such as restaurants, sports halls, bus and train stations (platform areas) or car parks

• Guidance can be found in fire tests reports, research publications etc. however such sources are often perceived as less „reliable” by authorities approving the design

Occupancy Fire growth rate HRR per unit area SourcePicture gallery Slow - [2]Public space in rail stations, airports etc. Slow 250 kW/m2 [1]Classroom (school) Medium 250 kW/m2 [1]Dwelling Medium 250 kW/m2 [1]Office Medium 250-290 kW/m2 [1,2]Hotel reception Medium - [2]Hotel bedroom Medium 250 kW/m2 [1]Hospital room Medium 250 kW/m2 [1]Library Fast 500 kW/m2 [1]Shop Fast 550 kW/m2 [2]Shopping centre Fast 250 kW/m2 [1]Theatre (cinema) Fast 500 kW/m2 [1]Industrial (storage) Ultra-fast 86-620 kW/m2 [2]

Source:[1] EN 1991-1-2 :2002 General Actions - Actions on structures exposed to fire [2] PD 6688-1-2 :2007 Background paper to the UK National Annex to BS EN 1991-1-2

Design FiresOpen Car Parks

Design fire curve for open car parks proposed based on fire tests carried out in France in 1995 (available in report published by EC)

For a single medium-sized car (class 3) such as Ford Mondeo a design fire curve with peak HRR of 8.3MW was proposed

Question: is the above design fire curve valid and representative for modern cars?

Design FiresOpen Car Parks

Source: P. Vila Real, C. Couto, N. Lopes, Modelling of multiple localised fires and steel structural members response using the software ELEFIR-EN, ASFE, Prague 2011

Design FiresOpen Car Parks

Example of practical application: four open sided multi-storey car parks with composite concrete / steel structure designed and constructed in Poland

Design FiresOpen Car Parks

Design FiresOpen Car Parks

For the car parks discussed the CFD analysis covered time period of 30 minutes. Within this time period the cumulative HRR reaches 14,7 MW.

Design FiresOther OccupanciesExample: Analysis of thermal actions on roof elements over railway station

Design fires:• Train carriage 40 MW (FF)• Retail unit 10 MW (FF)• Small kiosk 5 MW (FF)• Equipment under roof (SS,10 min)

Design fires

Influence of Ventilation Conditions

• The amount of available ventilation is an important factor in any analysis concerning temperature within the fire compartment

• Breakage of glass in the windows or glazed facade of the building will allows inflow of fresh air to sustain combustion

• At the same time, sufficient amount of openings allows effective dissipation of heat and hence lower temperatures within the fire compartment

• The amount of glazing which fails in fire situation is difficult to predict and it depends on a number of factors including type of glazing and its fixing, uniformity of thermal exposure etc.

Influence of Ventilation Conditions

Influence of Ventilation Conditions• EN 1991-1-2 requires input information on the amount of available ventilation for

calculations based on parametric temperature-time curves (annex A) as well as estimations based on the equivalent time of fire exposure (annex F)

• The code sets minimum and maximum limits for the amount of openings in the façade which can be taken into account, relative to the floor area of the compartment

• However, it does not provide any guidance on the percentage of glazing that can be assumed to break and fall out during a fire

• The same problem exists where the analysis is carried out using an advanced model such as a two-zone model or a CDF model

Influence of Ventilation Conditions

• With the current high requirements for energy conservation it is typical that double and triple glazing systems are used in commercial buildings

• An assumption that 100% of such glazing will be destroyed during a fire is clearly unrealistic, although EN 1991-1-2 seems to suggest that this is the case, as it simply refers to the “area of windows”

• Assuming unrealistically high areas of ventilation openings during a fire can in most cases lead to un-safe results - either in terms of the temperature curve or the estimated equivalent time of fire exposure.

Influence of Ventilation ConditionsA guidance document published in Luxemburg generally relates the amount of failed glazing with the temperature reached within the compartment. It suggests two scenarios to consider:• Scenario 1 is for 90% of the glazing area to be open since the beginning of

the fire• Scenario 2 sets the following percentages of failed glazing for various

glazing types:• Simple glazing: 50% open at 100oC and 90% open at 250oC• Double glazing: 50% open at 200oC and 90% open at 400oC• Triple glazing: 50% open at 300oC and 90% open at 500oC

Influence of Ventilation Conditions

German standard DIN 18230-1 relates the maximum percentage of glazing which can be assumed to fail to the expected duration of the fire:• For single glazing up to 80% can be assumed to fail during the first 15

minutes and up to 100% if the time exceeds 30 minutes• For double glazing the values are 35%, 50% and 100% for time intervals

of 15 min, 30 min and above 30 min respectively

Influence of Ventilation ConditionsExample:Analysis of equivalent time of standard fire exposure for a 29-storey office building. Facade fully glazed except for 0,8 m high spandrel panels.Question:How much glazing will fail in fire condition?

Pessimistic – 10% of glazing fails (2,6% of floor area)

Realistic (?) – 50% of glazing fails (13% of floor area)

Results:511 MJ/m2, 10% glazing fails, sprinkler failure → 94 minutes511 MJ/m2, 50% glazing fails, sprinklers work → 36 minutes

Summary• The Eurocodes are a comprehensive set of standards for structural design

and provide a far more detailed guidance as regards structural fire design than was previously provided in the national standards of the individual EU member states

• However they do not yet provide a complete guidance on the various aspects of thermal action analysis which needs to be carried out as part of performance-based design for structural fire resistance

• Work has just commenced on a comprehensive update program of the Eurocodes, with the view of completing the process by 2020

• Ideally the second generation of Eurocodes should include more complete and consistent information as regards design parameters for calculation thermal actions on structural elements exposed to fire

Thank You for Your Attention!

piotr@inbepo.pl