Mandos máquinas con doble altura de trabajo (OPCIONAL) (B-C-D)

Similar to the strengthening of a slab-column connection there are certain basic points that need to be addressed when repairing a damaged structure.

It should be kept in mind that most structural repairs are fairly to very expensive and will be disruptive to the normal use of the structure or part thereof. The old adage stating prevention is better than cure is especially true when it comes to the design for punching shear.

5.2.1. Proposed Classification of Damage

The risk of collapse and the extent of damage on a structure must be

assessed by means of visual inspection. When dealing with punching shear failure the following should be considered (Wood et al 1998):

- Prior to collapse no signs of distress will be evident on the soffits of the slab under question

- Star / radial cracking on top of the slab is merely an indication of the redistribution of permissible flexural stresses and does not give an indication of the shear behaviour of the slab-column connection

Fig 5.7 Typical Star Cracking

- Circumferential cracking around the column is usually evident at about 80% of the ultimate punching shear capacity of slabs without shear reinforcing.

- Due to deterioration of the cover concrete and numerous other reasons the shear crack may never extend to the surface of the slab. The shear crack can develop along the longitudinal tension reinforcing causing delamination.

In order to determine the type and extent of remedial works necessary, a system of grading the inflicted damage on the structure is needed. The following grading is proposed:

5.2.1.1.

Damage Level 1

 Minor radial cracking originating from the column corners and concentric cracks forming around the column

 Minor deflection of the slab around the column and a possible crack pattern around the column indicative of a flexural failure

5.2.1.2.

Damage Level 2

 Extensive radial and concentric cracking around the column, with the possibility of shear cracks having developed through the full depth of the slab

 Crushing of the concrete at the column face

5.2.1.3.

Damage Level 3

o A myriad of diagonal shear cracks, radial and concentric cracking on the slab surfaces and general disintegration of the concrete. o Excessive deflection of the slab surrounding the column

o The slab being suspended on longitudinal reinforcing bars going through the column core.

In essence the structure / slab could collapse at any instant if additional loading is introduced or if the structure is damaged any further.

5.2.2. Proposed remedial works for the different levels of

damage

In order to apply the correct remedial measures to a specific structure, quite a number of factors must be taken into account. Some of these factors are:

o Age and condition of the structure

o The use of the structure – i.e. the ratio of the level of permanent loads to level of live / repetitive loads

o The importance of the structure – e.g. is the structure used as a hospital or as a unimportant storage facility

o The expected service-life of the repaired structure

o Possible alteration in the use of the structure – e.g. conversion of offices into apartments

o The extent and severity of the damage to the structure

The interpretation of theses factors, especially the latter one, depends on sound engineering judgement, sufficient understanding of the failure mechanism and sufficient knowledge of the strengthening or structural principles used in the remedial measures.

The proposed remedial works presented in the following sections serve only as a guide and need to be adapted to suit each specific repair or strengthening application using sound engineering judgement.

5.2.2.1. Repair of Damage Level 1

Typically the repair of a connection suffering a relatively minor level of damage will be less involved or invasive as for more extreme levels of damage. The repair will most likely consist of methods similar to proposed strengthening procedures:

 Epoxy crack injection

 Installation of additional shear reinforcing

 Plate bonding to provide additional longitudinal reinforcing  Additional drop panels or concrete overlays

 Increasing the column cross-section or adding column capitals

5.2.2.2. Repair of Damage Level 2

Repair of such a structure will at least entail the following procedures:

 Removal of all loose and unsound concrete

 Cleaning of the concrete substrate and cracks by means of high pressure water jetting

 Epoxy injection of all structural cracks, i.e. cracks deemed to impair the structural capacity of the connection

In addition to this, some or all of the following measures could be implemented:

 Installation of additional shear reinforcing

 Increasing the slab thickness by means of added drop panels or concrete overlays

 Increasing the shear perimeter by means of increased column cross-section or the addition of bolt-on steel column heads

 Replacing the upper portion of the original concrete with a fibre reinforced concrete infill / overlay portion

5.2.2.3. Repair of Damage Level 3

Repair of a structure damaged to this extent will comprise of extreme remedial measures, disruption of use and intervention. The repair process will at least incorporate the following steps:

 Propping of the slab adjacent to the damaged slab-column connection(s)

 Demolition of the slab surrounding the column, with special care being taken not the damage the existing reinforcing  Jacking of the surrounding intact slab portions to an

appropriate level if necessary

 Installation of dowels into the edge of the original concrete slab

 Epoxy injection of all the remaining visible cracks  Enlarging of the slab-column interface by means of

enlarging the column or addition of a column capital

 Casting of a new slab with sufficient longitudinal reinforcing, shear reinforcing and/or drop panels to resist the required load capacity

5.3. Retrofitting of slab-column connections for improved