Seismic Pounding Effects In Buildings

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By
Prof. A. B. Kawade, Mr. Abhijeet A. Sahane
Amrutvahini College of Engineering, Sangamner

Abstract
Major seismic events during the past decade such as those that have occurred in Northridge, Imperial Valley (May 18, 1940), California (1994), Kobe, Japan (1995), Turkey (1999), Taiwan (1999) and Bhuj, Central Western India (2001) have Continued to demonstrate the destructive power of earthquakes, with destruction of engineered buildings, bridges, industrial and port facilities as well as giving rise to great economic losses. Among the possible structural damages, seismic induced pounding has been commonly observed in several earthquakes. As a result, a parametric study on buildings pounding response as well as proper seismic hazard mitigation practice for adjacent buildings is carried out. Therefore, the needs to improve seismic performance of the built environment through the development of performance-oriented procedures have been developed. To estimate the seismic demands, nonlinearities in the structure are to be considered when the structure enters into inelastic range during devastating earthquakes. Despite the increase in the accuracy and efficiency of the computational tools related to dynamic inelastic analysis, engineers tend to adopt simplified non-linear static procedures instead of rigorous non-linear dynamic analysis when evaluating seismic demands. This is due to the problems related to its complexities and suitability for practical design applications. The push over analysis is a static, nonlinear procedure that can be used to estimate the dynamic needs imposed on a structure by earthquake ground motions. This project entitled “Seismic Pounding Effects in Buildings.” aims at studying seismic gap between adjacent buildings by dynamic and pushover analysis. A parametric study is conducted to investigate the minimum seismic pounding gap between two adjacent structures by response Spectrum analysis for medium soil and Elcentro Earthquake recorded excitation are used for input in the dynamic analysis on different models. Pounding produces acceleration and shear at various story levels that are greater than those obtained from the no pounding case, while the peak drift depends on the input excitation characteristics. Also, increasing gap width is likely to be effective when the separation is sufficiently wide practically to eliminate contact. The results of pushover analysis viz. pushover curves and capacity spectrum for three different lateral load patterns are observed to study the effect of different lateral load pattern on the structural displacement to find out minimum seismic gap between buildings.

1. INTRODUCTION
1.1 General:
Investigations of past and recent earthquake damage have illustrated that the building structures are vulnerable to severe damage and/or collapse during moderate to strong ground motion. An earthquake with a magnitude of six is capable of causing severe damages of engineered buildings, bridges, industrial and port facilities as well as giving rise to great economic losses. Several destructive earthquakes have hit Egypt in both historical and recent times from distant and near earthquakes. The annual energy release in Egypt and its vicinity is equivalent to an earthquake with magnitude varying from 5.5 to 7.3. Pounding between closely spaced building structures can be a serious hazard in seismically active areas. Investigations of past and recent earthquakes damage have illustrated several instances of pounding damage (Astaneh-Asl et al.1994), Northridge Reconnaissance Team 1996, Kasai& Maison 1991) in both building and bridge structures. Pounding damage was observed during the 1985 Mexico earthquake, the 1988 Sequenay earthquake in Canada, the 1992 Cairo earthquake, the 1994 Northridge earthquake, the 1995 Kobe earthquake and 1999 Kocaeli earthquake. Significant pounding was observed at sites over 90 km from the epicentre, thus indicating the possible catastrophic damage that may occur during future earthquakes having closer epicentres.

Pounding of adjacent buildings could have worse damage as adjacent buildings with different dynamic characteristics which vibrate out of phase and there is insufficient separation distance or energy dissipation system to accommodate the relative motions of adjacent buildings. Past seismic codes did not give definite guidelines to preclude pounding, because of this and due to economic considerations including maximum land usage requirements, especially in the high density populated areas of cities, there are many buildings worldwide which are already built in contact or extremely close to another that could suffer pounding damage in future earthquakes. A large separation is controversial from both technical (difficulty in using expansion joint and economical loss of land usage) views. The highly congested building system in many metropolitan cities constitutes a major concern for seismic pounding damage. For these reasons, it has been widely accepted that pounding is an undesirable phenomenon that should be prevented or mitigated zones in connection with the corresponding design ground acceleration values will lead in many cases to earthquake actions which are remarkably higher than defined by the design codes used up to now. The most simplest and effective way for pounding mitigation and reducing damage due to pounding is to provide enough separation but it is sometimes difficult to be implemented due to detailing problem and high cost of land. An alternative to the seismic separation gap provision in the structure design is to minimize the effect of pounding through decreasing lateral motion (Kasai et al. 1996, Abdullah et al. 2001, Jankowski et al 2000, Ruangrassamee & Kawashima 2003, Kawashima & Shoji 2000), which can be achieved by joining adjacent structures at critical locations so that their motion could be in-phase with one another or by increasing the pounding buildings damping capacity by means of passive structural control of energy dissipation system or by seismic retrofitting. The focus of this study is the development of an analytical model and methodology for the formulation of the adjacent building-pounding problem based on the classical impact theory, an investigation through parametric study to identify the most important parameters is carried out. The main objective and scope are to evaluate the effects of structural pounding on the global response of building structures; to determine the minimum seismic gap between buildings and provide engineers with practical analytical tools for predicting pounding response and damage. A realistic pounding model is used for studying the response of structural system under the condition of structural pounding during elcentro earthquakes for medium soil condition at seismic zone V. Two adjacent multi-story buildings are considered as a representative structure for potential pounding problem.

Dynamic and pushover analysis is carried out on the structures to observe displacement of the buildings due to earthquake excitation. The behavior of the structures under static loads is linear and can be predicted. When we come to the dynamic behaviors, we are mainly concerned with the displacements, velocity and accelerations of the structure under the action of dynamic loads or earthquake loads. Unpredictability in structural behaviors is encountered when the structure goes into the post-elastic or non-linear stage. The concept of push over analysis can be utilized for estimating the dynamic needs imposed on a structure by earthquake ground motions and the probable locations of the failure zones in a building can be ascertained by observing the type of hinge formations. The strength capacity of the weak zones in the post-elastic range can then be increased by retrofitting. For the purpose of this study, SAP2000 has been chosen, a linear and non-linear static and dynamic analysis and design program for three dimensional structures. The application has many features for solving a wide range of problems from simple 2-D trusses to complex 3-D structures. Creation and modification of the model, execution of the analysis, and checking and optimization of the design are all done through this single interface. Graphical displays of the results, including real time animations of time-history displacements, are easily produced.

1.2 Seismic Pounding Effect between Buildings:
Pounding is one of the main causes of severe building damages in earthquake. The non-structural damage involves pounding or movement across separation joints between adjacent structures. Seismic pounding between two adjacent buildings occur during an earthquake different dynamic characteristics adjacent buildings vibrate out of phase at-rest separation is insufficient. A separation joint is the distance between two different building structures – often two wings of the same facility – that allows the structures to move independently of one another.

A seismic gap is a separation joint provided to accommodate relative lateral movement during an earthquake. In order to provide functional continuity between separate wings, building utilities must often extend across these building separations, and architectural finishes must be detailed to terminate on either side. The separation joint may be only an inch or two in older constructions or as much as a foot in some newer buildings, depending on the expected horizontal movement, or seismic drift. Flashing, piping, fire sprinkler lines, HVAC ducts, partitions, and flooring all have to be detailed to accommodate the seismic movement expected at these locations when the two structures move closer together or further apart. Damage to items crossing seismic gaps is a common type of earthquake damage. If the size of the gap is insufficient, pounding between adjacent buildings may result in damage to structural components the buildings.

1.2.1 Required Seismic Separation Distance to Avoid Pounding:
Seismic pounding occurs when the separation distance between adjacent buildings is not large enough to accommodate the relative motion during earthquake events. Seismic codes and regulations worldwide specify minimum separation distances to be provided between adjacent buildings, to preclude pounding, which is obviously equal to the relative displacement demand of the two potentially colliding structural systems. For instance, according to the 2000 edition of the International building code and in many seismic design codes and regulations worldwide, minimum separation distances (Lopez Garcia 2004) are given by ABSolute sum (ABS) or Square Root of Sum of Squares (SRSS) as follow:

S = Ua + U b ABS (1)
S = (Ua2 + U b2 ) SRSS (2)
Where S = separation distance and Ua, Ub = peak displacement response of adjacent structures A and B, respectively.

Bureau of Indian Standards clearly gives in its code IS 4326 that a Separation Section is to be provided between buildings. Separation Section is defined as `A gap of specified width between adjacent buildings or parts of the same building, either left uncovered or covered suitably to permit movement in order to avoid hammering due to earthquake . Further it states that for buildings of height greater than 40 meters, it will be desirable to carry out model or dynamic analysis of the structures in order to compute the drift at each storey, and the gap width between the adjoining structures shall not be less than the sum of their dynamic deflections at any level. ` Thus it is advised to provide adequate gap between two buildings greater than the sum of the expected bending of both the buildings at their top, so that they have enough space to vibrate. Separation of adjoining structures or parts of the same structure is required for. Structure has different total heights. This is to avoid collision during an earthquake. Minimum width of separation gaps as mentioned in 5.1.1 of IS 1893: 1984, shall be as specified in Table 1.1 . The design seismic coefficient to be used shall be in accordance with IS 1893: 1984

Table 1.1: Minimum width of separation gaps as mentioned in 5.1.1 of IS 1893: 1984

NOTE — Minimum total gap shall be 25 mm. For any other value of ?h the gap width shall be determined proportionately.

1.3 Methods of Seismic Analysis of A Structure:
Various methods of differing complexity have been developed for the seismic analysis of structures. The three main techniques currently used for this analysis are:
1. Dynamic analysis.
• Linear Dynamic Analysis.
• Non-Linear Dynamic Analysis.
2. Push over analysis.

1.4.1 Capacity-Curve:
It is the plot of the lateral force V on a structure, against the lateral deflection d, of the roof of the structure. This is often referred to as the ‘push over’ curve. Performance point and location of hinges in various stages can be obtained from pushover curves as shown in the fig. The range AB is elastic range, B to IO is the range of immediate occupancy IO to LS is the range of life safety and LS to CP is the range of collapse prevention.

1.4.2 Capacity-Spectrum:
It is the capacity curve transformed from shear force vs. roof displacement (V vs. d) co-ordinates into spectral acceleration vs. spectral displacement (Sa vs. Sd) coordinates.

1.4.3 Demand
It is a representation of the earthquake ground motion or shaking that the building is subjected to. In non-linear static analysis procedures, demand is represented by an estimation of the displacements or deformations that the structure is expected to undergo. This is in contrast to conventional, linear elastic analysis procedures in which demand is represented by prescribed lateral forces applied to the structure.

1.4.4 Demand Spectrum
It is the reduced response spectrum used to represent the earthquake ground motion in the capacity spectrum method.

1.4.5 Displacement-Based Analysis
It refers to analysis procedures, such as the non linear static analysis procedures, whose basis lies in estimating the realistic, and generally inelastic, lateral displacements or deformations expected due to actual earthquake ground motion. Component forces are then determined based on the deformations.

1.4.6 Elastic Response Spectrum
It is the 5% damped response spectrum for the seismic hazard level of interest, representing the maximum response of the structure, in terms of spectral acceleration Sa, at any time during an earthquake as a function of period of vibration T.

1.4.7 Performance Level
A limiting damage state or condition described by the physical damage within the building, the threat to life safety of the building’s occupants due to the damage, and the post earthquake serviceability of the building. A building performance level is that combination of a structural performance level and a non-structural performance level

1.4.8 Performance Point
The intersection of the capacity spectrum with the appropriate demand spectrum in the capacity spectrum method (the displacement at the performance at the performance point is equivalent to the target displacement in the coefficient method). To have desired performance, every structure has to be designed for this level of forces. Desired performance with different damping ratios have been shown below:
Reduced demand Spectrum Performance Point Capacity spectrum Elastic response spectrum 5% damped.

1.4.9 Yield (Effective Yield) Point
The point along the capacity spectrum where the ultimate capacity is reached and the initial linear elastic force-deformation relationship ends and effective stiffness begins to decrease.

1.4.10 Building Performance Levels
A performance level describes a limiting damage condition which may be considered satisfactory for a given building and a given ground motion. The limiting condition is described by the physical damage within the building, the threat to life safety of the building’s occupants created by the damage, and the post earthquake serviceability of the building.

1.4.11 Immediate Occupancy
The earthquake damage state in which only very limited structural damage has occurred. The basic vertical and lateral forces resisting systems of the building retain nearly all of their pre- earthquake characteristics and capacities. The risk of life threatening injury from structural failure is negligible.

1.4.12 Life Safety
The post-earthquake damage state in which significant damage to the structure may have occurred but in which some margin against either total or partial collapse remains. Major structural components have not become dislodged and fallen, threatening life safety either within or outside the building. While injuries during the earthquake may occur, the risk of life threatening injury from structural damage is very low. It should be expected that extensive structural repairs will likely be necessary prior to reoccupation of the building, although the damage may not always be economically repairable.

1.4.13 Collapse Prevention Level
This building performance level consists of the structural collapse prevention level with no consideration of nonstructural vulnerabilities, except that parapets and heavy appendages are rehabilitated.

2. CONCLUSION
• Constructing separated buildings is the best way of preventing structural poundings.
• In this study, it is concluded that constructing adjacent buildings with equal floor heights and separation distances reduces the effects of pounding considerably.
• Existing adjacent buildings which are not properly separated from each other can be protected from effects of pounding by placing elastic materials between them.
• As the PGA value increases, the minimum separation between the structures also increases.
• The separation distance between the two structures decreases, the amount of impact is increases, which is not applicable in all cases. It is only applicable when the impact time is same. It may also decreases when separation distance decreases, which leads to less impact time.
• At resonance condition the response of the structure is more and may lead to collapse of the whole structure.
• The duration of strong motion increases with an increase of magnitude of ground motion.

We at engineeringcivil.com are thankful to Prof. Amol B. Kawade and Mr. Abhijeet A. Sahane for submitting the research paper on “Seismic Pounding Effects In Buildings” to us. We are hopeful that this will be of great help to all those who are studying the seismic effect on buildings.

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