Precast Concrete And Seismic Issues

DAMAGE RESISTANT DESIGN

The Christchurch earthquakes have shown the need for buildings that will not only protect life, but will also survive a major event without loss of functionality for an extended period.

Two technologies used in buildings that survived the Christchurch earthquakes with no loss of functionality and minimal if any repair required to finishes or services showed that we already know how to construct better buildings and there is little if any added cost.

CCANZ has produced a series of short videos covering Damage Resistant Design building on the experience and lessons learned from these earthquakes.

Prestressed Seismic Structural Systems (PRESSS). This technology allows buildings to accommodate seismically induced movements without damage and without permanent deformation.

Base Isolation. This technology has been refined over the years and is now accepted as a major contributor to seismic survivability of structures in many countries.

Non tearing Joints. Deformations forced by seismic activity can cause permanent elongation to structural members that cause further damage to connected elements. Research and testing at Canterbury University covers a method of overcoming this.

GENERAL

The following sections are based on information available at the time of writing. They are comments based on observations and close inspections of many damaged buildings immediately following the earthquake, and later involvement in remedial work. They do not constitute design guides and suitably qualified and experienced professionals should be used for all design and assessment work. The following information may be superseded as new codes are written and research undertaken.

The 22 Feb 2011 Christchurch earthquake caused widespread damage and led to intensive assessment of the performance of all parts of buildings.

Structures designed to 1995 and later codes generally performed as intended.

Building codes are being revised and research projects are underway to improve our knowledge and understanding leading to better building performance. The following is based on early observations by experienced industry professionals, but it must be emphasised that this whole subject is continually under review and changing as research is undertaken and our understanding improved.

Knowledge of constructing buildings to resist seismic forces and accommodate the imposed deformations has increased greatly over the past couple of decades. Modern cars are much safer than those constructed before 1995, and similarly buildings constructed after 1995 performed much better than older buildings because of improvements in design and understanding.

Buildings are designed to resist a level of force determined by probability of events likely to occur during its lifetime. It is not realistic to design every building to accommodate extremely rare events that may occur only once in 10,000 years just as it is accepted that cars are not designed to ensure occupants experience no injury in the most extreme crashes.

There was little damage observed to buildings following the 22 Feb 2011 earthquake that caused surprise to engineers experienced in seismic design and geotechnical issues.

In many instances, building structures that performed as intended by their designers, were subsequently demolished because of permanent ground movements rather than due to failure of the building structure.

In other cases the costs of repairs to services and finishes, insurance constraints and time issues made demolition the economic option.

Design criteria were for building structures to survive a 1 in 500 year event and continue to function with only minor repairs, or survive an extremely rare 1 in 2,500 year event without causing loss of life through collapse. Non structural items such as ceilings, finishes services etc may have different criteria.

The majority of collapses were of older unreinforced masonry buildings. Two high profile “modern” buildings that collapsed with tragic loss of life were not designed to post 1995 codes.

Damage to precast concrete was investigated. Precast concrete designed and installed in accordance with modern codes performed well. The importance of correct detailing and the dangers of post construction work being carried out without proper understanding were highlighted

STAIRS AND RAMPS

Stairs and ramps require special consideration because the horizontal displacement between different levels of a building creates a significant change to the length of the diagonal line taken by stairs or ramps. Stairs or ramps are not designed to resist the horizontal movement of the buildings, but instead they should be constructed in such a way that they are isolated from the movement of the building to allow the building to deform without imposing unsustainable forces or causing loss of support.

The movements to be accommodated by stairs or ramps can be quite extreme in some buildings, particularly multistorey frame buildings.

Where the amount of movement exceeded that allowed for, one end could drop off the support as the building moved. Movement in the other direction could cause the stair string to be subject to extreme compression and in some cases that caused a bending failure which in turn shortened the length of the stair string so it dropped off its support as the building moved in the other direction.

Despite numerous and well publicised failures, there were no problems observed with stairs or ramps that would not be eliminated by proper design and detailing.

Problems associated with stairs and ramps fell into three main categories :-

  • Stairs seated into recesses to allow movement had these recesses filled in after construction when floor coverings were being replaced. In these cases, the stair string was prevented from sliding and was subject to extreme compression forces that deformed the stairs by bending at intermediate landings. This caused the stair string to shorten, and then to drop off the support when the building moved in the other direction.
  • Deformations caused by extreme earthquake forces exceeding the amount allowed for in the design.
  • There were instances, particularly in parking buildings, where ramps were not detailed to permit sliding. Parking buildings are not typically high rise flexible frame buildings and the forces or deformations imposed were seldom sufficient to cause a failure, but in extreme instances the propping effect of the ramp caused damage to adjacent structural components as well as damaging the ramp itself.

Some considerations when designing stair strings

  • Stair strings should be designed to accommodate compression forces imposed by friction and interference from floor coverings that resist sliding of the stair string.
  • Stair strings should be detailed to eliminate the possibility of inadvertent or inappropriate subsequent work restricting the ability of the stair string to move freely to accommodate inter storey movement. For this reason, it is generally not advisable to seat into pockets.
  • A very conservative approach should be taken when estimating the inter storey movement to be allowed for.

TILT UP WALLS

Generally tilt up walls as commonly used in single storey factories performed well. There was damage from ground deformation causing adjacent panels to move independently and the resultant clashing between adjacent panels caused spalling at the joints.

Generally this was minor and may not justify changes to design approaches.

Deformations will have caused damage to fixings that were not detailed to accommodate the amount of movement imposed. We are not aware of any collapses resulting from this.

PRECAST CLADDING PANELS

The major problem associated with precast cladding panels was from connections not being able to accommodate the imposed deflections. In extreme cases, this resulted in total failure of the connections and precast panels falling.

Besides this very serious issue, there were few other unexpected problems observed with precast panels.

Some damage occurred to panels as a result of highly irregular shape or irregular arrangement of openings.

Some panels at corners of buildings were damaged because the joints between the panels on different faces of the building were unable to accommodate the movements imposed. This could be exacerbated by beam elongation on flexible frame buildings.

Instances were observed of slotted connections detailed to accommodate building deformation being rendered inoperative by post construction work. In one particular case thick washers were welded over the slots to prevent movement. These caused the bolts to shear as the building deformed in the earthquake. Panels and fastenings which would otherwise have performed well, experienced catastrophic failure with potentially fatal results principally due to inappropriate post construction work.

Fixings are available that perform well and reduce the possibility of post construction work affecting their ability to function as intended.

Performance of all types of cladding panels is subject to a review, and it is expected that more detailed recommendations on fixings will be issued.

Inter storey deformations can be accommodated by ensuring the total weight of the panel is supported at only one level, and that fastenings at all other levels can accommodate the expected deformations and forces.

PRECAST FLOOR SYSTEMS

As far as we are aware, there were no instances where failure of any precast flooring system initiated a collapse. Precast floors fell in the earthquake, but only as a result of failure of the structural support system, not as a result of any failure of the floor or its immediate seating detail.

This is despite concerns expressed earlier following some testing, various papers, and theoretical issues considered by various review groups.

In a few cases, extreme support rotation combined with beam elongation caused damage that would have led to collapse if the earthquake had been of longer duration. Compliance with current codes requiring low friction bearing strips and seating lengths to allow for beam elongation would have reduced the possibility of collapse.

It is strongly recommended that a conservative approach be taken to seating lengths as the consequences of loss of seating are severe.

Following research and large scale testing, code changes addressed a number of issues identified to improve performance of precast floor systems and these changes were validated by observation of precast floors following the Christchurch earthquake.

The main concerns are issues that become more severe with ductile frames and flexible buildings. Large rotations of the supports can impose additional positive or negative bending moments on the floor at the support which may not have been considered when designing the floors as simply supported systems.

Associated with ductile frames is beam elongation from formation of plastic hinges subjected to cycles of bending reversal. This results in the supports for the floor being pushed further apart with potential loss of seating.

While both these effects were observed in the extreme events, we are not aware of any building collapses initiated by loss of seating of flooring units. Recent changes to codes have addressed these issues and designers should feel comfortable designing in accordance with NZS3101:2006 and amendments.

Particular issues covered in the code

  • Seating of precast flooring units on low friction bearing strips. This reduces the positive bending that can be imposed on the floor at the support as a result of rotation of the support and reduces the damage at the interface from concrete to concrete sliding.
  • Specific requirements for designers to allow for the effects of beam elongation on seating lengths of precast units. A conservative approach should be taken.
  • Hollowcore floor units to have additional reinforcing in the bottom of cores at the support. This provides additional positive bending capacity over the development length of the prestressing tendons, and provides shear capacity where other means of support is lost.
  • Hollowcore units to be separated from adjacent structural elements where damage could be caused by the stronger stiffer structural elements forcing the precast floor units to deflect in a way that they can not accommodate without damage.

Structures that will not induce beam elongation and support rotation will obviously not require the same level of consideration. Some systems are less susceptible to damage from support rotation. The effect of support rotation on seating length is reduced with thinner floors and flange supported tees.

FLOOR TOPPINGS – DIAPHRAGMS AND REINFORCING

At each level of a building, floors act as diaphragms to transfer the horizontal loads into the resisting elements, either frames or walls. There is a concentration of forces where the loads are transferred into the resisting elements. Greater spacing of load resisting elements results in higher diaphragm stresses and larger forces where the diaphragms connect to the load resisting elements.

Observations from the Christchurch earthquake :-

  • Buildings having a regular shape with lateral load resisting elements in each direction sustained less damage.
  • Distribution of the load resisting elements throughout the floor plan in each direction resulted in improved performance.

These factors reduced torsion effects and the diaphragm forces to be transferred.

These considerations are not specific to precast floor systems.

Extreme movements caused by the Christchurch earthquakes caused cracking in the reinforced concrete topping to floors. As cracks were forced to open wider, welded wire reinforcing mesh inevitably failed as it exceeded its ability to yield.

Cracks of 20 mm and more were opened up in concrete floors by the earthquake.

With welded wire mesh reinforcing, the movement must be accommodated within the distance between cross wires. A 20 mm stretch within the 150 mm wire spacing commonly used was beyond the capacity of the wires and caused the mesh to fail, often for the full length of the building. This could result in a loss of diaphragm capacity with potentially catastrophic consequences.

It is recommended that welded wire mesh not be used in diaphragms that may be subject to inelastic deformations. This is the case with all concrete floors and is not confined to toppings on precast floors.

All concrete shrinks, and as it does it will normally crack. All suspended floors will have shrinkage and flexural cracks under normal conditions. Movements during earthquakes cause these cracks to widen, and often people only become aware of their existence when looking for damage after an earthquake and incorrectly attribute the cracks to the earthquake.

Design of concrete flexural members assume that the concrete does not contribute tensile strength to the load carrying capacity of the member. Tension cracks in floors will not prevent the floor from being able to support its design load. What does occur when the cracks become more extreme, is that the section properties change and the floor can become more flexible and feel more “lively” but this in itself does not indicate any lessening of load carrying capacity as the ultimate strength design is based on the assumption that the concrete has cracked.