The fire resistance period for a structure, or a portion of a structure, is defined as that period for which the structure must remain intact during a fire, in order that all occupants may escape. The requirements for a
Table 3.2 Factors γf for quasi-permanent and frequent load combinations
Loading type Quasi-permanent Frequent
Dwellings 0.2 0.4
Offices and stores 0.3 0.6
Parking areas 0.6 0.7
given structure are contained in the regulations, applicable to that structure.
The fire resistance of prestressed concrete members, as with reinforced concrete members, is governed by the loss of strength of the steel with increase in temperature, rather than by loss of concrete strength. Generally, failure of prestressed concrete members is only likely at temperatures above 400 °C. The high-strength prestressing steels lose a greater proportion of their strength at a given temperature than do reinforcing steels, being approximately one-half of the characteristic strength at 400 °C for strands. Thus greater fire resistance, in the form of cover to the steel, is required in prestressed concrete members than in reinforced concrete members. The cover required is usually greater than that required for protection against corrosion, and so should be considered at an early stage in the design process. For good fire resistance of all concrete members, attention must be paid to detailing, since reinforcement is required near the member faces to prevent spalling. Cracked members can withstand very high temperatures better than uncracked members, since their greater proportion of lower-strength steel is less affected by high temperatures. Lightweight aggregate concretes exhibit better fire resistance than normal-density concretes, since less spalling occurs and better insulation is afforded to the steel.
Fire resistance requirements will be given in Part 10 of EC2. In the meantime the provisions of BS8110 should be followed. The nominal covers specified in BS8110 for varying periods of fire resistance and type of structural elements are shown in Table 3.3. The lower cover values given for continuous members in all types of prestressed concrete, compared with the simply supported condition, are due to the fact that continuous members have the ability to redistribute the load if one region loses strength in a fire.
Table 3.3 Concrete cover for fire resistance (mm)
Beams Floors Ribs
Fire resistance (h) Simply supported Continuous Simply supported Continuous Simply supported Continuous 0.5 20 20 20 20 20 20 1 20 20 25 20 35 20 1.5 35 20 30 25 45 35 2 60 35 40 35 55 45 3 70 60 55 45 65 55 4 80 70 65 55 75 65
Detailed information is given in Part 2 of BS8110 on determining the fire resistance period of given structural elements, and also specified in the code are minimum overall dimensions of concrete members to provide given fire resistance periods. Further information on fire resistance of prestressed concrete members may be found in Abeles and Bardhan-Roy (1981).
3.9 FATIGUE
For prestressed concrete members subjected to repeated loading, the fatigue strength must be considered. The major areas where fatigue failure could occur are in the concrete in compression, the bond between the steel and concrete, and the prestressing steel.
The compressive stress level in concrete above which failure could occur is approximately 0.6 fck and in the life of most prestressed concrete members this
ensures that failure due to fatigue in the concrete is unlikely. Bond failures have been observed in tests on short members such as railway sleepers, but for most applications this does not present a problem.
Although the stress levels in prestressing tendons are high, the range of stress in the tendons is usually small. Fatigue failure of tendons has been observed in tests, generally associated with high concentrations of stress in the vicinity of cracks in the concrete. If the concrete remains uncracked, the range of stress in the steel is small. Uncracked members thus exhibit much better fatigue resistance than cracked members.
The fatigue strength of prestressing tendons may be taken to be between 65% and 75% of the characteristic strength for two million load cycles, and this is usually much greater than the maximum stress in the steel under total design load.
One area which has been identified as potentially troublesome is in pretensioned members where the tendons have been deflected. There are stress concentrations at the deflection points and deflected tendons should be avoided if the members are to be subjected to cyclic loading.
The stress variations in unbonded tendons are transferred to the anchorages rather than distributed to the surrounding concrete as with bonded tendons. Unbonded tendons should thus generally be avoided if fatigue is a consideration.
A method of determining the fatigue resistance of prestressed concrete members may be found in Warner and Faulkes (1979), and general information on fatigue found in Abeles and Bardhan-Roy (1981).
3.10 DURABILITY
There have been many failures of structures in recent years which can be attributed to poor durability of concrete. These failures have generally not resulted in actual collapse of a structure but serious corrosion of reinforcement has sometimes occurred, significantly weakening the structure.
Five degrees of exposure of concrete members are identified in EC2 and these are shown in Table 3.4. The minimum cover requirements for all types of steel in prestressed concrete members are given in Table 3.5. For members in exposure class 5c extra measures should be applied, such as ensuring that all sections remain in compression under all possible load combinations or providing a protective barrier to all steel in the section. The figures in Table 3.5 include a minimum construction tolerance of 5 mm. However, in a practical design other considerations, such as those outlined in the previous section and also in Chapter 9, also affect the final choice of cover.
Durability is further achieved by ensuring that the requirements of
Table 3.4 Classes of exposure
Exposure class Examples of environmental conditions 1. Dry environment Interior of buildings for normal habitation or
offices
2. Humid environment aInterior of buildings with high humidity Exterior components
Components in non-aggressive soil
bAs a above but with exposure to frost
3. Humid environment with frost and de- icing salts
Interior and exterior components exposed to frost
4. Seawater environment aComponents completely or partially submerged in seawater or in the splash zone
Components in saturated salt air
bAs a above but with frost
5. Aggressive chemical environment (in conjunction with classes 1–4)
aSlightly aggressive chemical environment Aggressive industrial atmosphere
bModerately aggressive chemical environment
Table 3.5 Concrete cover for durability (mm)
Exposure class Prestressing Reinforcement
1 25 20 2a 40 35 2b 40 35 3 45 40 4a 45 40 4b 45 40 5a 40 35 5b 40 35 5c 50 45
Table 3.6 Criteria for limit state of crack width
Design crack width under frequent load combination
Exposure class Post-tensioned Pretensioned
1 0.2 mm 0.2 mm
2 0.2 mm Decompression
3 & 4 Decompression or coating of the tendons and wk=0.2 mm Decompression Table 3.6 are met. These apply to members with bonded tendons only; those with unbonded tendons should be treated as reinforced concrete members with regard to cracking (see Section 5.11). In Table 3.6 the term ‘decompression’ is taken to mean that all of the tendons lie at least 25 mm within the compression zone. In theory, decompression can be achieved in both cracked and uncracked sections but, in practice, the 0.2 mm crack width limit applies to cracked sections and the decompression criterion applies only to uncracked sections.
3.11 VIBRATION
The fact that thinner members are used in prestressed concrete construction than in comparable reinforced concrete construction leads to the natural frequency of prestressed structures being near enough to the frequency of the applied loading to cause problems of resonance in some cases. Examples of structures where vibrations should be considered include foundations for reciprocating machinery, bridge beams (especially those in footbridges), long-span floors and structures subjected to wind- excited oscillations, such as chimneys.
A method is given in Warner and Faulkes (1979) for finding the natural frequency of most types of prestressed concrete members. Information is given in Concrete Society (1994) on the vibration of post-tensioned concrete floors.
REFERENCES
Abeles, P.W. and Bardhan-Roy, B.K. (1981) Prestressed Concrete Designer’s Handbook, Viewpoint, Slough.
Concrete Society (1994) Post-Tensioned Concrete Floors—Design Handbook, Technical Report No. 43, London.
Rowe, R.E. et al. (1987) Handbook to British Standard BS8110:1985 Structural Use of Concrete. Viewpoint, London.