What is Pounding? Structural and Foundation Issues of Pounding
What is pounding?
Seismic pounding between buildings is one of the most common causes of severe structural damages due to earthquake. Pounding involves movement along or transverse to separation joints provided between neighbor structures and may cause non-structural damages. From the above discussion, seismic pounding between adjacent buildings occur during earthquake when• Buildings have different dynamic characteristics
• They vibrates out of phase relative to each other
• The separation between buildings at rest is not sufficient
• Insufficient energy dissipation system, if any, to restrain movement within allowable separation distance.
Why is pounding of buildings concerned?
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| Figure 1: Wall collapse due to pounding during Loma Prieta earthquake (1989) |
Past building codes didn’t provide definite recommendation or guidelines to account pounding effect and to counteract this phenomenon. As a result in many densely populated areas to achieve maximum land usage and for economic consideration, many buildings over the world already built extremely close to neighbor and in some region even no space is left. These building are vulnerable to pounding damage during future seismic activity. A large separation distance is not expected from both technical point of view within same facility having large expansion joints and economical point of view considering loss of land. In many cities the highly congested structures become a major concern for pounding damage. This is why, it is now widely accepted that this unexpected phenomenon has to be mitigated or prevented (Abdel Raheem, 2006).
What is seismic gap?
The distance provided between two adjacent building structures is called separation joint; often same facility is divided into two wings and sometime more depending on dimension of buildings. This permits an independent movement of structures relative to each other. A seismic gap is nothing but a separation joint kept to provide room for relative lateral movement due to seismic agitation.
Considering functional continuity, building utilities have to be extended from one wing to other across the building separation and finishing is provided for architectural termination on either portion. This separation joint for older buildings may be only one or two inches. For newer buildings these separation may be as much as one foot based on desired horizontal movement or seismic drift. All details about flashing, piping, HVAC ducts, fire sprinkler facility, flooring and partitions have to be done to permit two wings to move to expected distance at these locations when two structure or separated wings more closer to each other or move apart during earthquake. Damage of such non-structural elements across seismic gaps is very common. When these gap is inadequate, pounding between buildings may cause damage to structural elements of these buildings.
History
Many events of seismic pounding have been recorded to date. Pounding event has resulted worse damage and many cases of collapse of entire building. The earthquake that rocked Mexico City (1985) has disclosed (Rosenblueth and Meli, 1986) that pounding damage was present in more than 40% of total severely damaged or collapsed building out of 330 buildings surveyed and 15% of that resulted collapse. This earthquake left behind lesson about significance of pounding by leaving maximum number of building damage till that date (Bertero, 1986).
Not only that, past and recent earthquake brought our several instances of such damages to both bridge and building structures. Some significant earthquakes are saguenay earthquake (Canada) in 1988, Cairo earthquake (Egypt) in 1992, Northridge earthquake (USA) in 1994, Kobe earthquake (japan) in 1995. Kocaeli earthquake (Izmit, Turkey) and Tohoku earthquake (japan) in 2011. Some example of these earthquake are listed below:
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Classification of pounding damage pattern
Pounding damage of building was investigated after Loma Prieta earthquake (1985) in San Francisco bay region; based on this damage patterns of pounding were classified into four types (Kasai and Maison 1991), they are
• Type-1, where major damage of buildings were reported Fig 1
• Type-2, a life threatening hazard was created by falling or failure of building accessories
• TYPE-3, building functionality is lost as important electrical,mechanical or fire protection systems become out of order.
• TYPE-4, this involves architectural and/or trivial structural damage; examples of each type will be discussed in pounding survey section.
What are the pounding vulnerable structures?
• Comprehensive pounding damages were reported in ‘unreinforced low-rise buildings’ having no building separation
• Modern buildings are well designed for seismic safety, but often may subjected to wrong architectural details which generally includes building separation is filled with rigid architectural flashings (Cole and Takewaki, 2011).
• Condition becomes worse when adjacent buildings have dissimilar dynamic characteristics and vibrate out of phase during earthquake and sufficient separation is not provided.
• Or insufficient energy dissipation system to restrict building to move within designed gap between adjacent facilities.
In the past earthquake, it was noticed that significant damage was encountered within the perimeter of 90 km around epicenter which is an indication of disastrous damage under earthquake event at sites close to epicenters. A details description of vulnerable structures are presented below:
Why are URM buildings most vulnerable to pounding?
Special attention to unreinforced masonry (URM) buildings is required as pounding damage of such buildings found sufficiently frequently and a common damage pattern is outlined from experiences in Christchurch earthquake (2011). The cracking pattern was found The cracking pattern was found to extend through masonry walls typically from topmost point of contact of two building to lintel or window arch whatever found nearer in either building. Then cracks often extend from window opening and reached up to top of parapets of buildings. But parapets damage were not generally linked to effect of pounding. URM building not having ductile steel skeleton is susceptible to pounding damage.
Damage pattern in multistoried buildings
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Figure :Load path is idealized for damage of masonry building(Cole
et al. 2012) a) Typical effect of pounding of masonry building
b)Strut damage
of masonry building is idealized presenting collision points as arrow where shaded area are more vulnerable to damage. |
Multistoried buildings, though not frequent also subjected to damage especially at lower floor; the damage was found to be lack severity with increase in distance from topmost point of contact. The concentration of cracks was reported in stiff lateral components like wall sections discontinued by wide windows.
Sometimes local crushing in masonry units was found at the point of collision of two adjacent floors. When buildings of different height collide, the floor right above topmost point of contact frequently subjected to significant cracking. In the above figure a load path is idealized, presenting also a typically found distribution of damage in masonry structure. In actual situation, the damage struts were reported not to align to 45 degree. Interestingly the angle is governed by discontinuation of walls by any types of opening.
Are modern buildings safe against pounding?
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| Figure: Architectural flushing is rigid enough to transfer force causing damage the neighbor buildings due to pounding |
In general modern buildings are suffered less damage due to pounding. The reasons behind this are providing greater separation distance in modern buildings and the presence of neighbor weaker buildings; as an example, consider an old URM building collided with concrete building having ductile RCC frame. The URM building will suffer more damage due to inherent brittle, weak properties.
Influence of rigid flashing elements
Some pounding damage are observed when building separation provided to avoid pounding is influenced by cosmetic flashing (fig). The actual purpose of providing gap is not achieved as this distance is covered with architectural detailing provided some type of flashing which produce a strong and stiff elements becoming a media to transfer enough force to the neighbor structure to result pounding damage.
In some cases, flashes resulted failure of neighbor building elements; on the other case, the entire flashing comes out from its at rest position and remain detached from both facilities. Detached flashing can produce hazard of falling objects of significant height when flashings are disheveled form multiple storied buildings.
Pounding damage due to presence of pedestrian bridges
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| Figure: Pounding damage of Christchurch polytechnic institute of technology |
The most usual mitigation measure of pounding is to provide sufficient gap between buildings, will not work for some special cases like presence of pedestrian bridges. Pedestrian bridges are used to connect to buildings or two wings of same facility to achieve more functionality. Take an example of Christchurch earthquake, Christchurch polytechnic institute of technology madras street campus. In the above figure lower and upper pounding locations are indicated by red circles.
In the above figure lower and upper pounding locations are indicated by red circles. Close up views of pounding damages of the upper and lower floors of this bridge are shown in green border. In upper floor relative movement of residual opening is clear and from view of lower floor it can be noticed that URM wall was damaged due to pounding at lower reinforced concrete floor of these bridge.
According to Chouw (2002), two adjacent building having different configuration will have different fundamental periods which means there have dissimilar oscillation patterns and movement will be out of phase of adjacent one. Previous experiences show that behavior of buildings under pounding underestimated as buildings connected by pedestrian bridges were designed not considering soil-structure interaction. Thus during near source earthquake such structures may be damaged due to pounding.
Behavior of set-back building
Among different geometrical irregularities in structure, set-back is very common. In this case the lateral dimension of the structure at a specific elevation is reduced abruptly. The detail of behavior of such building is out of the scope of this article. When a facade of one building is set-back from adjacent one, the collision under pounding is further amplified and damage at this critical location is also increased. The observation of Darfield earthquake (2010) also reported such type of damage. The building subjected to preceding set-back damage in Darfield earthquake were subsequently damaged in the Christchurch earthquake, but no catastrophic failure were reported.
What are the factors influencing pounding?
Physical aspects of this phenomena have to be understood to set a rational basis upon which mitigation measures are adopted to avoid potentially disastrous outcome. Many researchers and code recommendations have figured out many factors that may affect the pounding. These are
• Height of buildings
• Separation distance between neighbor building
• Type of the lateral load resisting system incorporated in the structure
• The location and point of collision
• The stiffness of structure
• The peak ground motion at the building site during earthquake
• The fundamental period of these structure
• The material used in expansion joint or fill materials (if any)
• The construction material like concrete, masonry, steel etc.
• Story height
• Location of building in seismic zonation map
• Type of vibration induced during earthquake (in-phase/out of phase)
• The condition of buildings like new, old or retrofitted
• Damping mechanisms
• The mitigation measure or methods adopted to mitigate pounding
• Earthquake induced torsion
Some of the factors listed above are considered to have trivial influence on the structural pounding; but others factors are crucial and have strong influence on pounding. The provisions in code to control or diminish pounding mainly based on separation distance between neighbor building and drift limitation of structure. According to available provisions in different codes the main factors that have impact of pounding is drift which is mainly depends on
• Building height
• Seismic zones
• Separation between neighbor buildings
• Lateral load resisting system
Structural characteristics resulting pounding hazard
A. Buildings having largely variable mass
When a building is surrounded by others of significantly differing mass, they subjected to pounding damage. Unfortunately such damage can not be separated from other forms of pounding as no noticeable differences were observed.
B.Floor to wall or floor to column pounding
This form of pounding is considered most severe damage to vertical elements of structures. It was found that one third of building surveyed for pounding had collision between adjacent structures having different floor height. Such building configuration results collision between building’s floor with neighbour building’s walls or columns. Floor means slab (if projected from beam) or slab with beam (T beams).
C. Building with highly variable total height
Large difference in overall height of buildings was found to enhance damage when they become in contact with others. Fortunately, it was found that in general buildings with such configuration have greater separation distance which significantly reduce this hazard.
D. Identical buildings constructed in a row having no separation
Interaction between more than two structures were found comparatively common in Christchurch earthquake than Darfield earthquake. Such type of damage was observed in buildings with significantly varying dynamic properties. When identical type of buildings are concerned, it is less frequent to suffer damage. But when similar buildings are constructed in a row, they are vulnerable to pounding damage (Jeng & Tzen 2000). The buildings on the ends of row may suffer additional damage as momentum of central structures are finally transferred through them. However, in the Christchurch earthquake no clear indication of amplification of damage of end buildings due to pounding was noticed.
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Figure: An example
of torsional pounding of building due to plan irregularities.
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E.Torsional action under pounding of buildings
To identify torsional pounding in buildings was found difficult under external inspection; an example of torsional pounding is shown in following figure in Christchurch earthquake.
F.Brittle materials used in building construction
It is discussed earlier in this post that URM buildings are the most common structural system that were subjected to pounding damage during earthquake. More or less 75% of pounding damage was found in connection with URM buildings and moderate to severe pounding damages were found in such brittle.
General comments from pounding survey
• Modern buildings having enough separation distance relatively less susceptible to pounding. However, when expansion joints are provided with small separations in such buildings, many case of pounding were reported.• There have some evidence of correlation between pounding and soft soil condition under foundation. Soft foundation without adequate treatment may enhance shaking during earthquake. In addition, settlement and rocking may be accompanied in building supported on soft soil.