Unintended restraint ? unintended restraint of hollow core floor units

Hollow core floor units are normally designed to be simply supported and to resist shear and bending mainly in the longitudinal direction. However, the design and detailing of the support connections often result in restraining effects. Since hollow core elements normally have no top reinforcement and no shear and transverse reinforcement, it is necessary to carefully consider the risk that cracks caused by unintended restraint reduce the shear capacity of the element. Some possible causes for restraint at floor-wall connections are shown in Fig. 29.

The causes can be :

o clamping (verouillage) between wall elements at the end of the hollow core element,

o frictional forces developing between the floor and wall elements,

o bond stresses between the end face of the hollow core unit and the joint grout/joint concrete,

o dowel action from the concrete that is allowed to fill the cores at the end of the hollow core unit,

o tie bars.

As described in (Fig. 24), the end displacement of a prestressed slab varies with the amount of prestressing, dead loads, live loads, slab thickness, span length etc. It should be noted that the result is often as shown in Fig. 24 and not as shown in Fig. 29.

Fig. 29. Possible causes for unintended restraint of hollow core units at the support connection

The actual restraint moment Mrestr in the connections depends on the actual need for deformation (rotation and overall shortening) and the stiffness of the elements involved. The restraint moment can never exceed the capacities of the connection details. To estimate the restraint moment, the following expressions (1) and (2) are recommended, EN 1168 (2005). It is not necessary to consider the effect of friction or of the tie bar when the joint interface is still uncracked. The total normal force is used as a conservative assumption.

where:

?μt - frictional coefficient at top edge

?Fctj - tensile strength of joint concrete

?Nt - total normal force in the wall at the top edge

?Wcj - sectional modulus of the joint interface (the whole joint interface including

the cores)

To estimate the maximum axial restraint force Nrestr that can develop, the following expressions are recommended.

where:

?μb - frictional coefficient at bottom edge

?Nb - total normal force in the wall at the bottom edge

?Acj - full area of joint interface (including the cores)

The end zone of the hollow core unit should be analysed with regard to the risk of cracking taking into account the restraint effect, ordinary loading and the effective prestressing force. Within the transfer length the development of the prestressing force must be considered. Near the support the prestressing force may be insufficient to prevent a possible crack, starting from the top, from propagating deeply into the section. A deep crack may result in a considerable reduction of the shear capacity.

There are in principle three ways to handle this risk:

o the connection can be designed to be effectively simply supported (Fig. 40)

o the restraint moment can be reduced or limited so that restraint cracks are avoided

o a restraint crack in the preferred location (Fig. 27b), is permitted and the connection zone is strengthened so that the shear capacity is sufficient in spite of this crack

In the two last cases the following measures should be taken to reduce the risk of cracking in unfavourable positions (Fig. 27b) within the critical region where there is a significant tensile stress in the top of the section:

o transverse slots (fente) or other weaknesses should be avoided

o the concrete fill in cores without tie bars should not extend outside the wall in case of straight ends of the hollow core units (Fig. 30a), or not outside the transverse recess (embrasure, alcove) in case of slanted (incliné) ends (Fig. 30b)

o tie bars anchored in cores or longitudinal joints should not be cut off within the critical region

The restraint moment can be reduced by limiting the load on the wall, i.e. limiting the number of floors or providing soft joint fill that prevents wall load from entering the end zones of the hollow core unit (Fig. 30a). To avoid direct loading from the wall and facilitate crack formation in the preferred location, the hollow core unit could be provided with slanted ends as shown in Fig. 30b.

In case of high wall loads restraint cracks can not be avoided and the elements need to be strengthened by placing additional reinforcement bars in concreted cores or grouted joints (Fig. 31). The purpose of the tie bars is to increase the shear capacity of a cracked section in the favourable position (Fig. 27b).

Fig. 30. Measures to limit or avoid restraint moment at the support connection of hollow core elements: a) use of soft joint fill, b) slanted ends .

The following aspects should be considered :

o the reinforcement used for strengthening should be placed in the upper part of the section (above mid-depth) in addition to the ordinary tie bars, which are placed in the lower part of the section;

o the additional reinforcement bars should be anchored so that they are able to transfer their yield load in the assumed crack section;

o the amount of reinforcement in the upper part should be limited so that no further flexural cracks (from a negative bending moment) appear in unfavourable locations after formation of the first crack in the preferred location.

Fig. 31. Strengthening of the end zones of the hollow core elements by tie bars placed in concreted cores

Recent test results show that there is a considerable shear capacity of a cracked section when the crack is in a favourable position. This can be explained by the following mechanisms:

o shear capacity of the bottom compressive zone below the tip of the crack

o dowel action of the bottom strands balanced by the support reaction

o friction and interlocking effects along the crack as long as the crack is kept together by reinforcement in the upper part of the section.

Another case of unintended restraint of hollow core floors appears when the floor is connected to a stabilising unit, e.g. an elevator shaft, along the longitudinal edge of a hollow core unit. Then the hollow core units near this connection will act as supported on three sides and the restraint may result in unexpected cracking.

Date: 2016-06-12; view: 157