Structural fire resistance

Describe (with the drawings) the fire resistance.

Connections are vital parts in the performance of precast concrete structures.

Performance requirements :

I. The load bearing function the capacity of the connection to take up the different actions in the structure,

II. Separating function, namely the ability to prevent fire spread (propagation)

I. The load bearing function

Sd,fi(t) ? design effect of actions in the fire situation at time

Rd,fi(t) ? corresponding design resistance at elevated temperatures.

Indirect actions resulting from the effects of thermal expansion and deformations, mainly affecting the connections.

1. Increase of the support moment for restrained continuous structures (at the colder top side of the member). Precast structures are generally designed for simple supporting conditions where the rotational capacity is large enough to cope with this action.

2. Forces due to hindered (entravé, empêché) thermal expansion. Not critical, take account of the phenomenon at the design stage.

3. Large deformations due to cumulated thermal dilatations (fire covers a wide building surface and lasts for a long period). Rotational capacity of the connection between beams and columns at the edge of a building is a critical parameter for the stability of the entire structure.

4. Local damage at the support. The deflection of beams and floors may have an influence on the support connection. The problem can be solved by increasing the thickness of the bearing pad (patin).

5. Cooling effect (after a long fire) may introduce tensile forces on the connections between long structural members. These effects are not taken into account in the design.

Structural fire resistance

The principles and solutions valid for the fire resistance, is based on the large fire insulating capacity of concrete. The most important analysis concerns the resistance against indirect actions due to thermal dilatations and deformations.

1. Bolted connections.Simply supported connections. Solution to transfer horizontal forces in simple supports. Have to be well protected by concrete. Provide additional stiffness (rigidité).

2. Connections between superposed columns.Portal frames composed of columns and beams ensure horizontal stability. The horizontal blocking of possible large deformations depends on the stiffness of the column and the rigidity of the restraint. Behave rather well in a fire. Inmulti-floor precast structures, columns generally transfer only vertical forces, the horizontal rigidity being assumed by central cores and shear walls. Choice between a single storey columns orcontinuous columns over several storeys (étage). The blocking in itself is not so dangerous, since it provides a kind of prestressing (may lead to shear failure of the column).

3. Floor-beam connections.The connections between precast floors and supporting beams are situated within the colder zone of the structure, and hence less affected by the fire. The position of the longitudinal tie reinforcement (longitudinal means in the direction of the floor span) should preferably be in the centre of the floor thickness, or a type of hairpin connecting reinforcement (Fig. 5). In case of restrained support connections, the Eurocode (pr EN 1992-1-2) prescribes to provide sufficient continuous tensile reinforcement in the floor itself to cover possible modifications of the positive and negative moments.

4. Floor-wall connections.Walls exposed to fire at one side will curve because of the differential temperature gradient. At the same time the supported floor will expand in the longitudinal direction. Both phenomena will lead to an increasing eccentricity of the load transfer between floor and wall, with a risk of collapse (Fig. 6). A possible solution is to increase the rigidity of the wall. The situation is most critical for masonry walls since they cannot resist large bending moments.

5. Hollow core floor connections.Experience during fire tests in laboratories has shown that the structural integrity and diaphragm capacity of hollow core floors through correctly designed connections, which as a matter of fact constitutes the basis for the stability of the floor at ambient temperature, are also essential in the fire situation. Due to the thermal dilatation of the underside, the slab will curve. As a consequence, compressive stresses appear at the top and the bottom of the concrete crosssection and tensile stresses in the middle (Fig. 7). The induced thermal stresses may lead to internal cracking. In principle, cracked concrete sections can take up shear as good as non-cracked sections on condition that the cracks are not opening. In fact, the crack borders are rough (rugueux) and shear forces can be transmitted by shear friction and aggregate interlocking (Fig. 8). The figure 8 illustrates the generation of transverse forces due to the wedging effect. In hollow core floors, this transversal force is taken up by the transversal tie reinforcement at the support. The decrease of the concrete strength at higher temperatures is hardly playing a role. Such temperatures appear only at the bottom part of the concrete section, and much less in the centre. From the foregoing, it appears that at the design stage, provisions are to be taken to realize the necessary connections between the units in order to obtain an effective force transfer through cracked concrete sections. The fact that shear failures have not been observed in real fires shows that there exist enough possibilities to realize the connections between the units. (This has also been proven in numerous fire tests in different laboratories.) The possible design provisions are explained hereafter.

6. Steel connections.Steel connections, such as steel corbels (encorbellement) and similar, shall be protected against the effect of fire, either by encasing (encastrer, recouvrir) them into concrete/mortar (containing an expansive agent) or by an adequate fire insulation. The concrete cover should be at least 30 mm for 1 hour of fire resistance and 50 mm for two hours. Precautions are to be taken to prevent spalling (écaillage) of the concrete cover by adequate reinforcement. In case of partially encased steel profiles, for example in thin floor structures, the temperature rise in the steel profile will be slower than in non-encased unprotected profiles, due to the effect of the thermal conductivity of the surrounding concrete. However, it is recommended to protect the exposed steel flange (bride, rebord) by a fire insulating material, Fig. 9.

II. Separation function

Requirements, with respect to the separating function, are expressed as limit states of thermal insulation and structural integrity against fire penetration. They apply mainly for joints between prefabricated floors, walls, or walls and columns, which should be constructed to prevent the passage of flames or hot gases. Longitudinal joints between precast floor elements generally do not require any special protection. The precondition for thermal isolation and structural integrity is a minimum section thickness (unit plus finishing) according to the required fire resistance. Minimum dimensions are given in Table 1, according to CEB (1991). The joint should also remain closed. The latter is realized through the peripheral tie reinforcement. When the section is too small, for example due to the limited thickness of flanges of TT-floor elements, a special fire insulating joint strip can be used. Joints between walls and columns can be made fire tight by either providing connecting reinforcement at half height, or through a special profile of the column cross-section (Fig. 10).

Date: 2016-06-13; view: 5