Steel Sheet Piles Underground car parks: Fire resistance
Content Introduction
4
4 4 4 5 5
Aim of this brochure Steel sheet piles as permanent structural elements Economical steel solutions Specifics Fire resistance of steel sheet piles
Fire Safety
6
6 6 7 7 8 8 8 9
General fire safety concepts Active and passive protection measures Designing structures for fire loads Standardised fire curves Factors influencing the severity of realistic fires Real fire development Fire load densities Rate of heat release (RHR)
Temperature calculation for natural fires in open/closed car parks
10
10 11 12
Simulation approach Parametric calculations Safe-sided natural fire curves
Fire testing on steel sheet piles
13
13 15
Analysis of thermal properties of soils by fire tests Calibration of thermal properties of soils
Parametric temperature calculations in the sheet piles
16
16 16 17
Matrix of parameters Temperatures in the sheet piles - Fire acc. ISO-834 Temperatures in the sheet piles - Natural Fire
Fire resistance of steel sheet pile structures
18
Moment redistribution in a steel sheet pile structure Thermo-mechanical properties of structural steel Soils characteristics and fire curve
18 19 19
Computing structural behaviour during a fire event
20
20 22
Detailed calculation with SARI Results from an example computed with SARI
Technical details
23
23 24
Possible connection detail of intermediate floors Improvement of the fire resistance
Simplified verification procedure
25
26 27
Example 1: ISO-834 Fire, R90 Exemple 2: Natural Fire
Introduction Aim of this brochure This brochure provides assistance in the fire safety design of steel sheet piles to be used as permanent structural elements in underground car parks or roadworks (tunnels, underpasses, ...). It summarises the efforts that have been made on this subject and the verification services provided by Arcelor Commercial RPS. Furthermore, it includes an easy-to-use verification procedure allowing a simplified assessment of the fire safety of the steel sheet pile structure at the pre-design stage.
Steel sheet piles as permanent structural elements Steel sheet piles used as permanent structural elements in underground car parks and roadworks have a multiple role. They can: • serve as a retaining wall during the excavation phase to support the horizontal pressures • provide watertight containment for the excavation pit • form the permanent outer structural wall • carry parts of the vertical loads from the floors or even from the superstructure. Thus, avoiding the need for a temporary retaining wall outside, the permanent sheet pile wall:
(Fig.1)
Mercedes-Benz Car Park, Ghent, Belgium
• maximises the available space, an important issue especially in urban areas • shortens the construction time • reduces the total cost of the building.
Economical steel solutions The main benefit of using steel sheet piles as permanent structural elements for underground car parks and basements is the simplification of the construction sequence and the resulting substantial savings of cost and time. Economical steel solutions without any protective measures can even be defined for structures subject to fire resistance requirements. Steel sheet piles can easily be painted after completion of the works. Verifications are usually confined mainly to the design at serviceability or ultimate limit states considering the temporary execution steps as well as the final situation, and focus on the most economical solution.
Specifics Like any bearing pile, steel sheet piles can safely transmit vertical loads to the substratum through skin friction and toe resistance. Structural connection of the floors and superstructure is easy with cast in-situ concrete corbels or capping beams. Using the top-down method will even allow a further reduction in construction time, since temporary anchoring or propping of the retaining wall is not necessary.
(Fig.2)
Car Park in The Hague, The Netherlands
Fire resistance of steel sheet piles In most cases, the “Fire” load case does not have to be considered for temporary constructions. However, for permanent structures, steel sheet pile solutions are often excluded right from the design stage due to unfounded prejudices or to a lack of know-how concerning fire safety in general or the particular behaviour of steel sheet piles under the effect of a possible fire event. Arcelor has carried out extensive fire testing and numerical simulations focusing on the fire resistance of steel sheet piles and the effects of the surrounding soil. Using the knowledge gained, the fire safety of steel sheet pile walls can be verified taking the following parameters into account: • the thermal characteristics of the different soils • different fire loads • different protective measures
p = 20 kN/m2
100 kN/m
50 kN/m
Fine Sand J = 32.5°
(Fig.3)
Underground car park
100 kN/m
-1.50
-7.00
-6.70
-2.50
-2.50
AZ 26 S 390 GP
0.00
p = 20 kN/m2
0.00
-3.50
Fine Sand J = 32.5°
-6.50
AZ 26 S 390 GP
-12.50
(Fig.4)
-11.00
Road tunnel / underpass
Fire Safety General fire safety concepts The overall purpose of fire safety is to reduce the risk of both life and property losses, the main concern being the safety of lives. An appropriate fire safety concept is a package of active systems (i.e., fire detection, fire fighting) and passive systems (i.e., structural fire resistance, partitioning, etc). Factors for assessing the probability of a major fire occurring include: • the activity and combustible contents (fire load) of the building, • the type of building, • the active fire prevention. Safety precautions may differ between different types of buildings in evaluating personal fire safety as a function of:
(Fig.5)
Car Park in Scheveningen, The Netherlands
• density of human occupation • occupant mobility • size and number of storeys (escape time) • limits on the spread of smoke to remote parts of building.
Active and passive protection measures Possible protection measures include: Active protection measures, like: • fire and smoke detectors, • automatic sprinklers, will normally limit the spread of fire and ensure that fire-fighting services are called to the scene as quickly as possible. Passive protection measures are used to prevent the build-up of high temperatures in the load-bearing structural elements. These measures may include: • specially applied protection materials (i.e., insulation boards, coating) • natural passive protection of the steelwork by concrete fill or the surrounding soil
Designing structures for fire loads According to the relevant standards, any structure subject to the risk of a possible fire event has to be designed taking the effects of such a fire into account.
kS ; kE
Different aspects have to be considered: • the effects of smoke development
1.10 1.00 0.90 0.80 0.70
kS=ST/S20 kE=ET/E20
0.60 0.50 0.40 0.30 0.20 0.10 0.00
0
(Fig.6)
100
200
300
400
500
600
700
800
900 1000
Temp [°C]
Thermo-mechanical characteristics of steel
Depending on the available combustibles and on the ventilation conditions, the fire compartment may be filled with toxic gases. In order to prevent intoxication and human losses, a certain amount of ventilation has to be achieved.
• the effects of high temperatures
Although steel will not burn, its mechanical properties will be strongly influenced by elevated temperatures. Thus, a thorough design of the structural elements is quite an important aspect.
Indeed, the mechanical properties of steel decrease with increasing temperature to about 10% of their nominal values for temperatures of 800°C. Structural design is mainly based on standardised fire curves. However, consideration of natural fire loads on the basis of the available combustibles is gaining more widespread acceptance.
Temp [°C]
Standardised fire curves The predominant fire resistance assessment methods all over the world are generally based on standardised temperature/time curves, such as ISO-834, ASTM-E119, Hydro-Carbon, RABT, EBA, etc.
Standardised Fire curves
1400 1200
The traditional definition of fire resistance is the time expressed in minutes that a structural element is able to support the design loading when exposed to the standard fire before a specific condition of failure is reached. Accordingly, the elements or structures are classified into fire resistance categories R30, R60, etc.
1000 800 600
ISO-834 ASTM-E119 HydroCarbon RABT EBA
400 200 0
0
(Fig.7)
30
60
90
Standardised fire curves
120
150
180
Time [min]
These standardised curves, however, do not describe natural fires in a realistic way. A more rational approach to fire engineering design of buildings is based on the behaviour of a structure in a real or natural fire. Such design procedures have been progressively developed in recent years and are increasingly accepted by authorities in many countries.
Factors influencing the severity of realistic fires The intensity of a fire and the duration of the phases, as measured by the temperature/time curve of the gases in the fire compartment, depend on many parameters: • • • • •
the the the the the
amount and distribution of combustible materials, characteristics, i.e., the burning rate of these materials, ventilation conditions of the room, compartment geometry, thermal properties of the walls and ceiling.
Real fire development In a natural fire, three different phases can be identified: • In the first phase, the combustible begins to burn; temperature varies from one point to another with significant gradients within the compartment, and there is a gradual propagation of the fire. In this first step, there is normally no risk of structural failure, although some local damage to contents may occur.
Temp [°C]
Real fire development failure of active measures
pre-flashover phase
ISO-834
fully developed fire
cooling phase
Flashover
success of active measures
time [min]
(Fig.8)
Realistic fire development
• In the second phase, the average temperature rises in the compartment; if it reaches about 300°C to 500°C, the upper layers are subjected to sudden ignition, called “flashover”, and the fire develops fully. The transition from a local fire to a fully developed fire is an essential step in evaluating structural fire safety. After flashover, the gas temperature increases very rapidly to a peak value, often in excess of 1000°C, and becomes practically uniform throughout the compartment. • In the third phase, the available combustibles begin to decrease and the gas temperature necessarily falls.
Fire load densities The fire load is defined as the sum of all combustible materials of a compartment, expressed in [MJ] or in [kg] wood equivalent. It may be also expressed by the fire load density qf in [MJ/m2] or [kg/m2], which is the ratio of total fire load to the floor area. Characteristic values of the fire load density qf for buildings according to EN1991-1-2, “Actions on structures exposed to fire” are: (Table 1)
Characteristic values of fire loading
Type of fire compartment
fire load density qf [MJ/m2] mean 80% fractile
Dwellings
780
948
Hospitals
230
280
Hotels (bedrooms)
310
377
Libraries
qf for buildings
1500
1824
Offices (standard)
420
511
Schools
285
347
Shopping centres
600
730
Theatres (cinemas)
300
365
Transport (public space)
100
122
Characteristic fire loads for cars are about 5000 to 7000 MJ.
Rate of heat release (RHR) The fire load defines the available energy but the gas temperature in a fire depends on the rate of heat release, which is governed by the available oxygen supply. The same fire load burning very quickly or smouldering can lead to completely different gas temperatures.
RHR [MW]
Rate of heat release high ventilation important heat release during short time weak ventilation lower heat release but during longer time
time [min]
(Fig.9)
Different rates of heat release
Temperature calculation for natural fires in open/closed car parks Simulation approach Calculation according to EN 1991-1-2 Annex C 4
2
(Fig.10) Plan
1
3
This method (based on the heat fl ux according to the HASEMI method) gives the temperatures at the top level of the SSP wall as a function of the distance to the fi re source, (see fi g. 10) for a single or multiple car fi re scenario.
5
The heat release of the different single cars is based on experimental curves, derived from several car burning tests performed in the frame of the ECSC project “Development of design rules for steel structures subjected to natural fi res in closed car parks”.
view of car park and fire scenario
3)3
)FBUSFMFBTF4JOHMFDBS
It is assumed that the fi re starts in car 1 and may spread to the neighbouring cars 2 and 3 after 12 minutes and then to cars 4 and 5 after 24 minutes.
3)3
(Fig.11) Standardised
The Rates of Heat Release from the second and following cars have a slightly different increasing phase as they are already heated by the fi rst car (see fi g. 12).
UJNF
heat release curve
In cases where a sprinkler system is available, it could be considered that only 1 car is burning, as the sprinklers cool it down and avoid a spread over.
)FBUSFMFBTF.VMUJQMFDBST
DBS DBS DBS DBS DBS UPUBM3)3
The calculation of the temperature distribution takes into account the location of the respective individual heat releases and the height of the compartment.
Infl uencing parameters for this curve are:
UJNF
(Fig.12) Heat
• the fi re load of the single cars • the numbers and position of the cars with respect to the SSP. • the compartment height
Gas temperatures
1000
Temperature [°C]
release for multiple car fire scenario
It shall be noted that the temperatures are calculated at the top level, just below the ceiling, and thus are safe sided.
800 600 400 200 0
0
15
30
45
60
Local gas temperatures computed with HASEMI
(Fig.13)
0
75
90
time [min]
3)3
)FBUSFMFBTF.VMUJQMFDBST Av [m2]
H = 2,5 m
UPUBM3)3
TJNQMF3)3
B = 15 m
L = 10 m
Ac=150/300/600m2
L = 20 m
L = 40 m
(Fig.14)
Modelling for OZONE
(Fig.15)
UJNF
Simplified RHR for OZONE calculation
Simulation with OZONE V2.2
Fire characteristics RHR design curve
OZONE is a two zone calculation model that gives gas temperatures in both the hot and cold layers. (available on Arcelor’s homepage free of charge)
2-zone simulation c1 : Tu>500°C or c2 : HiTign
modif. RHR curve
This simulation takes into account the total fire as well as the compartment properties with the ventilation conditions.
c3 : Hi0.5∑Af
The simulation is based on a simplified RHR curve with the same fire load as the HASEMI calculation for the 5-car fire scenario.
1-zone simulation
O2
OZONE takes into account the ventilation conditions (c1, c2, c3, c4) of the fire compartment and defines the possible heat release accordingly.
Design Temp.curve
(Fig.16)
Calculation approach of OZONE
Parametric calculations Number of burning cars: 5 (Table 2)
Ventilation conditions for closed car parks
Ventilation conditions
total 8 cars 150 m2 total 16 cars 300 m2 total 32 cars 600 m2
high ventilation 60 m3/m2/h
Lv
Mv
Hv
Sa
0.4 m3/s
1.5 m3/s
2.5 m3/s
Ma
0.8 m3/s
2.9 m3/s
5.0 m3/s
La
1.7 m3/s
5.8 m3/s
10.0 m3/s
Temperatures - Large car park
3)3DBST-BSHFDMPTFEDBSQBSL
1200 TJNQMF3)3 3)3-B)W 3)3-B.W 3)3-B-W
Hasemi T°-La-Hv T°-La-Mv T°-La-Lv des-LargeComp
1000 800 600 400
medium ventilation 35 m3/m2/h
Temperature [°C]
3)3
Compartment Area Ac
low ventilation 10 m3/m2/h
200 0
0
15
30
45
60
75
(Fig.17)
Heat release in large car parks
90
105
120
time [min]
UJNF
(Fig.18)
Temperatures in large car parks
11
TJNQMF3)3 3)3.B)W 3)3.B.W 3)3.B-W
Hasemi T°-Ma-Hv T°-Ma-Mv T°-Ma-Lv des-MedComp
1000 800 600 400
Temperatures - Medium car park
1200
Temperature [°C]
3)3
3)3DBST.FEJVN$MPTFEDBSQBSL
200 0
0
15
30
45
60
75
90
105
(Fig.20) Temperatures in medium sized car parks
3)3DBST4NBMM$MPTFEDBSQBSL Temperature [°C]
3)3
(Fig.19) Heat release in medium sized car parks
TJNQMF3)3 3)34B-W 3)34B.W 3)34B)W
Temperatures - Small car park
1200
Hasemi T°-Sa-Hv T°-Sa-Mv T°-Sa-Lv des-SmallComp
1000 800 600 400
120
time [min]
UJNF
200
0
UJNF
(Fig.21) Heat release in small car parks
0
15
30
45
60
75
90
105
120
time [min]
(Fig.22) Temperatures in small car parks
Note: most standards specify a minimum ventilation for closed car parks of about 10 m3/m2/h which is considered here as low ventilation (Lv curves in the following fi gures) From fi g. 17 it can be clearly seen that for low ventilation conditions the heat release will be less than 50% of the initial design values, however, the fi re duration will be doubled. From fi g. 18 it can be seen that the calculated mean temperatures in the closed fi re compartment are quite lower than for fi res in open car parks with full ventilation during the fi rst hour approximately. Only after about 45-70 minutes (depending on the ventilation conditions and the compartment size), the temperatures in the closed fi re compartment will be higher than those in the open compartment. A safe sided approach for the defi nition of design temperatures will be to consider the temperatures from open compartments in the fi rst phase and the temperatures from enclosed compartments in the second phase. From these fi gures, it appears that the heat release is also governed by the size of the fi re compartment.
Temperature [°C]
Safe-sided natural fi re curves Calculation of gas temperatures for realistic fi re loads
Design temperatures - Closed car parks
1400
For any verifi cation of steel sheet pile structures in enclosed car parks where natural fi re design will be accepted, one of the three design fi re curves shown will be applied.
ISO RABT des-SmallComp des-MedComp des-LargeComp
1200 1000 800 600 400 200 0
0
30
60
90
120
150
180
time [min] (Fig.23) Comparison of fi re curves
2
In comparison with the standardised curves, both the fi re development and extinction phases can be observed to be considered in a more realistic way, but are still safe sided.
Fire testing on steel sheet piles Analysis of thermal properties of soils by fi re tests A series of fi re tests has been carried out at the laboratories of the MSM department at the University of Liège, Belgium. The aims of these tests were to determine, experimentally, the temperature fl ow in the steel sheet pile and in the soil and to defi ne the thermal characteristics of the soil for use in FE-simulations.
mSF
Test set-up
. (-
7-
(3
The tests were carried out using a test furnace, the front side of which was closed by the test specimen. The test specimen consisted of a caisson built up from PU 6sheet piles and plates and fi lled with different soil types with different water contents. A series of 4 tests was carried out:
73
UFNQFSBUVSFHBVHFT DBJTTPOCVJMUVQGSPN441BOEQMBUFT mMMFEXJUIEJGGFSFOUTPJMUZQFT (Fig.24) Schematic test set-up
Test N°
Soil
Water content
1
Clay
Dry / moist
2
Clay
Saturated
3
Sand
Dry / moist
4
Sand
Saturated
Fire curve The heating of the oven was controlled in such a way as to simulate a fi re curve according to ISO-834 up to a fi re duration of 3 hours, except for the tests with saturated sand, where the heating control could not be maintained due to a shortage of gas supply. Fig. 26 shows the temperatures in the furnace in comparison with the relevant standard curve. (Fig.25) Test furnace opened after the test
Gas Temperatures
1200
Temperature [°C]
The temperatures were recorded in the heating phase as well as in the cooling phase up to 6 hours (360 min).
1000
Cooling phase
Heating phase
800
End of heating for test N° 4(SS)
600 t-ISO f-SH f-LH f-LS f-SS
400 200 0
0
30
60
90
120
150
180
210
240
time [min]
(Fig.26) Measured gas temperatures in the furnace
1000 800
end of heating phase
600 400
1000 800 600
f-SH pm-SH pv-SH pg-SH
400
SH (SandDryWet) LH (Clay(DryWet)
200 0
Steel Temperatures
1200
Temperature [°C]
Temperature [°C]
Mean steel temperatures
1200
200
LS (ClaySaturated) SS (SandSaturated)
0
30
60
90
120
0 150
180
210
240
0
30
60
90
120
150
180
(Fig.27) Comparison of measured steel temperatures
for the different tests
210
240
time [min]
time [min]
(Fig.28) Measured temperatures on steel for test N°3
(Sand Dry/moist)
Temperatures measured on steel A series of 9 temperature gauges was disposed on the steel surface at different locations. The infl uence of soil type and water content on the steel temperatures is quite substantial, as shown by fi g. 27. The temperature in the steel may differ by up to 200°C due to the concave/convex shape of the sheet pile and to the different steel thickness. The highest temperatures can be measured at the outer fi bre (PM series, cf. fi g. 28) and the lowest temperatures at the location of the interlocks (high massivity).
Temperatures measured in the soil A total of 30 temperature gauges in 5 series of 6 gauges were placed in the soil body to record the temperatures at different distances (2, 4, 6, 10, 15 and 20 cm) from the sheet pile. The temperature gradient is quite regular for non-saturated soils as shown in fi g. 30.
(Fig.29) Temperature gauges in the soil body
The temperature evolution is governed by the water content of the soil.
Temperature [°C]
Temperatures in Dry/wet Sand 1200
sSH-p sSHv-2 sSHv-4 sSHv-6 sSHv-10 sSHv-15 sSHv-20
1000 800 600
It can be seen from the fi gure that the heating in the different locations is delayed at 100°C due to the energy input required to evaporate the residual water.
400 200 0
Influence of water in permeable soils
100°C
0
30
60
90
120
150
180
210
240
time [min] (Fig.30) Temperatures measured in the soil for test N°3
Temperature [°C]
(Sand Dry/moist)
For saturated sand, it can be observed that the heating of the steel is not regular due to the development of a water fl ow inside the soil body (fi g. 32), and that the heating is considerably reduced.
Soil Temperatures in Saturated Sand
1200
441
sSS-p.Th.5 sSSm-2 sSSm-4 sSSm-6 sSSm-10 sSSm-15 sSSm-20
1000 800 600
mSF
400
QFSNFBCMF TPJM
200 0
0
30
60
90
120
150
180
210
240
time [min] (Fig.31) Temperatures for test N°4 (Sand Saturated)
Calibration of thermal
(Fig.32) Induced water fl ow in permeable soils caused
by temperature gradient
Calibration of thermal properties of soils
Spec.Heat [J/kgK]
Lambda [W/mK]
A series of extensive FE-simulations (back-calculation of tests) allowed for a calibration of the thermal properties (fig. 33 & 34) of some typical soils.
3.00 2.50 2.00
Gravel
GravelSat
Sand
SandSat
SdClay
SdClaySat
Clay
ClaySat
1.50
1600 1400 1200 1000
1.00
Gravel Sand SdClay Clay
800
0.50 0.00
1800
600 0
100
200
300
400
500
600
700
800
0
100
200
300
GravelSat SandSat SdClaySat ClaySat
400
500
(Fig.33)
600
700
800
Temperature [°C]
Temperature [°C]
Thermal conductivity
(Fig.34)
Specific heats
Effects of the water content In order to take into account the water flow (fig. 32) the calibration allowed to define equivalent water contents as a function of the permeability of the soils (fig. 35 & 36)
1FSNFBCJMJUZ 1FSNFBCJMJUZ
&
&
&
&
&
&
&
&
&
&
1FSNFBCJMJUZ (Fig.35)
$PBSTFHSBWFM
&
.FEJVNHSBWFM
&
'JOFHSBWFM 4BOE6-H $PBSTFTBOE .FEJVNTBOE 'JOFTBOE
& &
&
&
$PCCMFT
&
&
Equivalent water contents for calculation
4PJM5ZQF
&
&
&
8BUFS$POUFOU
&RVJWBMFOU8BUFS$POUFOU
& & &
(Fig.36)
4JMUZTBOE 4JMU -PFTT -PBN $MBZFZTJMU 4JMU6-H 4JMUZDMBZ $MBZ
Permeability of soils
15
Parametric temperature calculations in the sheet piles Matrix of parameters Fig. 37 - Fig. 44 show the mean temperature in the steel for different parameters: 4 types of soils Gravel Sand Silt Clay
2 water contents
3 steel plate thicknesses
dry/wet saturated
2 fire curves
5 mm 10 mm 20 mm
ISO-834 NatFire
For cohesive soils, the effect of the water content is less important than for sandy soils, due to the lower permeability.
Temperatures for Gravel
1000
Temperatures for Sand Temp [°C]
Temp [°C]
Temperatures in the sheet piles - Fire acc. ISO-834
800 600
800 600
400 200 0
1000
5mm
sat-5mm
10mm
sat-10mm
20mm
sat-20mm
400
T-ISO
0
30
60
90
0
120
Time [min]
(Fig.37)
5mm 10mm 20mm T-ISO
200
Steel temperatures for ISO-fire Gravel (wet/saturated)
0
(Fig.38)
800 600
90
5mm 10mm 20mm T-ISO
200
Steel temperatures for ISO-fire Sand (wet/saturated)
1000 800
0
30
60
400
sat-5mm sat-10mm sat-20mm
90
5mm 10mm 20mm T-ISO
200 0
120
0
30
60
(Fig.39)
16
Steel temperatures for ISO-fire Silt (wet/saturated)
sat-5mm sat-10mm sat-20mm
90
120
Time [min]
Time [min]
120
Time [min]
600
400
0
60
Temperatures for Clay
1000
Temp [°C]
Temp [°C]
Temperatures for Silt
30
sat-5mm sat-10mm sat-20mm
(Fig.40)
Steel temperatures for ISO-fire Clay (wet/saturated)
Temperatures in the sheet piles - Natural Fire The following figures have been derived from the design fire curve as defined for a medium-sized fire compartment (cf. fig. 23). As the maximum temperatures will be obtained after 30-45 minutes, the slight differences for the fire curves in the extinction phase have no impact on the fire resistance of the structure.
Temperatures for Sand 5mm 10mm 20mm T-NAT
800
Temp [°C]
Temp [°C]
Temperatures for Gravel 1000
sat-5mm sat-10mm sat-20mm
1000
600
600
400
400
200
200
0
0
(Fig.41)
30
60
90
0
120
Time [min]
Steel temperatures for NAT-fire Gravel (wet/saturated)
5mm 10mm 20mm T-NAT
800
0
(Fig.42)
sat-5mm
10mm
sat-10mm
20mm
sat-20mm
Temp [°C]
Temp [°C]
5mm
T-NAT
600 400
200
200
30
60
90
0
120
Time [min]
(Fig.43)
120
Time [min]
Steel temperatures for NAT-fire Sand (wet/saturated)
Steel temperatures for NAT-fire Silt (wet/saturated)
5mm 10mm 20mm T-NAT
800
400
0
90
1000
600
0
60
Temperatures for Clay
Temperatures for Silt 1000 800
30
sat-5mm sat-10mm sat-20mm
0
(Fig.44)
30
60
90
sat-5mm sat-10mm sat-20mm
120
Time [min]
Steel temperatures for NAT-fire Clay (wet/saturated)
For a simplified verification of the fire safety of a steel sheet pile structure, the maximum steel temperature may be interpolated
from Fig. 37 - Fig. 40 for a fire curve according to ISO-834, or from Fig. 41 - Fig. 44 for a natural fire curve.
17
Fire resistance of steel sheet pile structures Moment redistribution in a steel sheet pile structure Generally, the design of a steel sheet pile section is governed by the maximum bending moment at the service stage or possibly during an intermediary construction stage. For a sheet piling structure, the bending moment shape is generally not characterised by (negative) hogging moments at the fixed supports. #FOEJOH.PNFOUT
%JTQMBDFNFOUT
400
2.0
300
1.5
200
1.0
100
0.5
'JSF *40
MEd,fire maxMserv
U U U U U U
0
Time [min] 0
-100
30
60
90
120
0.0 150 180
M1 M2 M3 D2
-200 (Fig.45)
Moment redistribution in a SSP wall
(Fig.46)
-0.5
Displacements [cm]
Bending Moments [kNm]
-1.0
Moment redistribution in a SSP wall
Hence, for such hyperstatic structures, local plastification with a resulting redistribution of moments could allow an increase in the maximum loading or a decrease in the cross sectional resistance. There is an advantage associated with this ability, because during a fire event, as a consequence of increasing temperatures, the cross sectional resistance will decrease without necessarily losing overall stability. The ultimate state will be reached if both the hogging and sagging moment MEd,fire equal the moment resistance MRd,hot. Moment MEd,fire depends on the earth and water pressures involved.
18
Thermo-mechanical properties of structural steel
kS ; kE
The mechanical properties of steel will decrease with increasing temperature according to the following figure. 1.10 1.00 0.90 0.80 0.70
kS=ST/S20 kE=ET/E20
0.60 0.50 0.40 0.30 0.20 0.10 0.00
(Fig.47)
0
100
200
300
400
500
600
700
800
900 1000
Temp [°C]
Thermo-mechanical steel characteristics
This leads to a loss of cross-sectional resistance at elevated temperatures: M Rd,Hot = W pl × fy × ks
Soils characteristics and fire curve The thermal properties of the materials and the partial heat absorption by soil leads to a reduction and delay in the heating-up of steel sheet pile. The influence of soil type and water content (especially for saturated soils) is considerable. However, it shall be noted that for any type of natural fire, temperatures will be lower than those for the ISO curve.
Temperature [°C]
Note: this figure is only a rough estimation; see Fig. 37 - Fig. 44 for greater accuracy.
1200
Temperatures in steel sheet pile
1000 gravel
800
sand
ISO-fire curve
600
Dry(wet) Soils clay clay sand gravel
400
NAT-fire curve
200 0
0
Saturated Soils
Dry(wet) Soils Saturated Soils
30
60
90
120
150
180
Time [min] (Fig.48)
Temperatures in steel sheet piles
19
Computing structural behaviour during a fire event
Our engineers from Arcelor Commercial RPS can provide assistance in verifying the fire resistance of your steel sheet pile structure, taking into consideration: • • • •
your basic solution and retaining wall calculations additional loads, especially vertical loads from intermediate floors and superstructure different fire scenarios, i.e., standard fire curves (ISO, HydroCarbon,...) or a natural fire curve possible protection measures
This will allow them to propose safe and economical steel solutions.
Detailed calculation with SARI Calculation approach Input: - Soil Type - Fire Curve - Protections - etc
Input: - SSP type/size - etc
Thermal Calculation Disc. of cross-sections T°SSP = f ( t )
Output: - Sections - Temperatures - Structural behaviour - etc
(Fig.49)
E y,SSP = f (T° SSP ) = f ( t ) Sy,SSP = f (T° SSP ) = f ( t )
Input: - structural system - supports - geotechnical data - earth/water pressures - vertical loads - steel grade - etc
Structural Calculation - Discr. of structure - Action effects (t=0) - Thermal elongation = f ( t ) - Plastic redistribution = f ( t ) - Action effects = f ( t ) - etc
Calculation approach
Required input In order to allow our technical staff to verify the fire resistance by calculation, the following project information is required:
20
SSP: Structure:
section type and steel grade length all relevant levels:
head and toe level of SSP floor levels, temporary anchors / strut levels (if applicable) vertical loads from floors, superstructure, …
Soil:
levels of soil layers, water, … characteristic geotechnical data: soil type, friction angle, cohesion, permeability, …
(Table 3:) Verification of the fire safety of a SSP structure
Retaining wall calculation Subgrade Reaction Model Interaction of: •
Structural Steel
•
Geotechnics
Specific data Input of missing data: •
Steel grade
•
Protections
•
Thermal soil characteristics
•
Vertical loads
•
Fire scenario
SAFIR Calculation Structure: beam elements Discretisation
Cross section: finite elements (U and Z-sections)
Thermal calculation
FE-model For each different section = f(SSP, protection, soil, water,…) Beam model. Consideration of:
Structural calculation
Results
•
Subgrade reaction pressures
•
Vertical loads
•
Thermo-mechanical laws for SSP
•
Sections
•
Structure
•
Temperatures
•
Structural behaviour during fire event
21
Results from an example computed with SARI %JBNPOEGPS4"'*3
%JBNPOEGPS4"'*3
'*-&"J@- /0%&4 #&".4 53644&4 4)&--4 40*-4
'*-&"J@- /0%&4 #&".4 53644&4 4)&--4 40*-4
#&".41-05 40*-41-05 #&/%*/(.0.&/51-05
#&".41-05 40*-41-05 #&/%*/(.0.&/51-05
5*.&TFD
5*.&TFD
:
: 9
;
& /N
;
moments at service stage
5JNF#FBN#FOEJOH.PNFOU1MPU
#FBN #FBN #FBN #FBN #FBN
& /N
(Fig.51) Bending
%0'
.
(Fig.50) Bending
9
moments after redistribution 5JNF%JTQMBDFNFOUT1MPU
/PEF /PEF /PEF /PEF /PEF
5JNF
&JHFO7BMVF
5JNF&JHFO7BMVFT1MPU
5JNF
(Fig.54) Rigidity of system
22
5JNF
(Fig.52) Evolution of bending moments during fi re event
(Fig.53) Evolution of displacements during fi re event
Technical details Possible connection detail of intermediate fl oors
(Fig.55) Schematic view of fl oor connection
4FDUJPO""
#
"
4FDUJPO##
#
"
(Fig.56) Connection details: concepts
2
Improvement of the fire resistance Protection measures to improve the fire resistance in case of high axial loading include: a) protective coating (intumescent paintings or sprays) b) insulating panels c) masonry d) concrete fill of the SSP inner pans e) complete concreting The fire verification calculations with the software SARI can take these protective measures into account. Alternatively, the sheet pile wall could be protected locally only, in that any x-th pile is being reinforced by concrete fill or as a box pile. In case of a fire event, the non-protected piles will be considered only with respect to bending from horizontal pressures. However, these reinforced piles will have to resist both the bending and the total vertical loads.
(Fig.58)
(Fig.57)
24
Passive protection measures
Local reinforcement of the steel sheet pile wall
Simplified verification procedure
or
p1
p1 = 0
pw pe
MEd
MEd L
MEd
pm = p2 / 2
p2
pm = p1 = p2
p2
MEd 2
MEd = pm · L2 / 16
MEd = pm · L / 12 (Fig.59) Equivalent
MEd
system for simplified verification
Step 1 Determine: • the relevant earth and water pressures (this earth and water pressures may be the result of a previous cold analysis using a limit earth pressure method, a subgrade reaction model or a fi nite element method) for the considered fl oor level:
p1 = pw1 + pe1 • the mean pressure: pm = • the factor β: β =
and
p2 = pw2 + pe2
p1 + p2 2
p1 p2
Step 2 Determine the bending moments acting at ultimate limit state: M Ed =
k=
where: for:
12 1+ 0.03 ⋅ β
k=
Free rotation at top level
€
pm ⋅ L 2 k
16 2 1+ 0.025 ⋅ ( 1− β )
Fixed support at top level
€ 2
Step 3 Determine the axial load ratio: n =
N Ed
and the relative slenderness: λ =
N pl
N pl N cr
where: NEd = acting axial load
N pl = A ⋅ f y
A = cross sectional area of sheet pile
fy = yield stress
N cr =
EI ⋅ π 2 L 2b
EI = rigidity of sheet pile Lb = buckling length
Step 4
Step 5
1.1
Reduction factor ks [-]
Determine the maximum temperature in the sheet pile section from one of the figures (see Fig. 37 - Fig. 44). The figures show the steel temperatures for different soils and water contents.
n ⋅ λ , determine the strength reduction factor kS as a function
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.1 0.0
of the temperature.
Note: The proposed verification method is applicable for € Z- and U-sections. continuous
0
(Fig.60)
100 200 300 400 500 600 700 800 900 1000
Reduction factor for elevated temperatures
Step 6 Verification of the design moment resistance. The design moment resistance in the hot SSP-section can be defined as:
M Rd , Hot = WPl ⋅ σ red with:
σ red = fy ⋅ kS => M Rd ,Hot = WPl ⋅ fy ⋅ kS
And it shall be verified:
M Ed ,0 ≤ M Rd ,Hot
€ € Fire, R90 Example 1: ISO-834 € Given the following data: Section: AZ 26 in S 355 GP A = 198 cm2/m Iy = 55510 cm4/m Wpl = 3059 cm3/m Soil: Saturated Silt, below the water table Location: second underground floor with L = 3.20 m => Lb = 1.60 m Axial load: Ned = 350 kN/m Earth and water pressures:
p1 = pw1 + pe1 = 25kN / m2
26
€
€
= 0.00 = 0.01 = 0.02 = 0.04 = 0.06 = 0.10 = 0.15
0.2
With
€
n n∙lam n∙lam n∙lam n∙lam n∙lam n∙lam
1.0
p2
= pw2 + pe2 = 50kN / m2
Temperature [°C]
Step 1 pm =
p1 + p2 = 37.5kN / m2 2
β=
p1 = 0.50 p2
Step 2 €
k=
16 €15.9 2 = 1+ 0.025 ⋅ ( 1− β )
M Ed
pm ⋅ L 2 = = 24.15kNm/ m k
Step 3 €
€
EI ⋅ π 2 Ncr = = 449418kN / m L 2b
N pl = A ⋅ fy = 7029kN / m€ λ=
N pl Ncr
= 0.125 => €
n=
N Ed = 0.05 N pl
n ⋅ λ = 0.006
€
€ For a required Fire resistance F90 acc. ISO-834 => maxT ≈ 850 °C€ Step 5:
Temp [°C]
Step 4 1000 800 600
Reduction factor: ks ≈ 0.04
400
T-ISO LS 20-ISO
200
Step 6:
0
Verification: MRd,Hot = WPl × fy × ks = 43 kNm/m > MEd = 24 kNm/m
0
(Fig.61)
30
60
90
120
Time [min]
Temperature - time curve, ISO-834 fire
Example 2: Natural Fire Given a section AZ 13 in S 355 GP with A = 137 cm2/m Iy = 19700 cm4/m Wpl = 1528 cm3/m with the same horizontal and axial loading.
Step 3
€
λ=
N pl Ncr
Step 4 €
Step 5
Ncr =
EI ⋅ π 2 = 159494kN / m L 2b
N = 0.175 => n = Ed = 0.07 N pl €
For a Natural fire -> max T€≈ 575°C
€
n ⋅ λ = 0.013
Temp [°C]
N pl = A ⋅ fy = 4863kN / m
1000 800
T-NAT LS 10-NAT
600
Reduction factor: ks ≈ 0.50
Step 6
400 200
Verification: MRd,Hot = WPl × fy × ks = 270 kNm/m >> MEd = 24 kNm/m
0
0
(Fig.62)
30
60
90
120
Time [min]
Temperature - time curve, Natural fire
27
66, rue de Luxembourg L-4221 Esch-sur-Alzette (Luxembourg) Tel.: (+352) 5313 3105 Fax: (+352) 5313 3290 E-mail:
[email protected] www.arcelor.com/sheetpiling
3-21-06-1-E Edition 2006
Sheet Piling Arcelor Commercial RPS S.à r.l.
