Some Risky Practices in Earthquake Engineering That Need More Research and Evaluation

Abstract

Every building is designed according to its own structural system. Needs of making modifications to the structural systems of buildings may arise after the buildings are constructed. Any change in the structural components of the building by addition or omission will necessarily produce a change in its dynamic properties such as its stiffness, mass, and damping matrices, and modes of vibration. During earthquake actions, the new structural system will have different seismic responses than the original system. The new seismic responses such as shear, torsion, moment, and displacement may be reduced, or increased. Safety provisions require that before any changes in the structural system are carried out, a thorough seismic structural analysis of the new system be carried out. Linking two adjacent buildings with rigid or semi-rigid sky bridges is one example of such changes. Theoretical investigation of linking two buildings together by damping devices to mitigate seismic risks is finding its way into the literature. There have been numerous studies addressing the effects of connecting two buildings by rigid sky bridges on the seismic response of the new structural complex which comprises the two buildings and the linking beams and slabs. Dramatic changes in the seismic response of buildings are noted in most cases studied.

1. Introduction

The structural design of buildings is meant to resist the anticipated loads likely to act on a structure. The structural designer chooses the structural system to resist gravity and lateral loads. There is a variety of simple and complex structural systems used to resist combinations of vertical gravity loads and earthquake actions. The techniques available nowadays to alleviate seismic actions are numerous. The requirements of continuous load paths in the vertical and horizontal directions, stiffness and strength, and ductility are well-known requirements for the proper seismic design of structures [1].

Structural systems employing shear walls, bracing and moment-resisting frames, and combinations of them are now widely used by designers. Analysis and design of structures are guided by design codes and cover the most encountered needs and requirements of typical buildings. However, sometimes, building owners wish to make changes to their properties after the completed buildings are finished and occupied due to some unforeseen needs arising later on. Structural engineers are then consulted to execute the desired changes in the building. Some of the desired changes could produce changes to the overall structural behavior of the building. The designers are required to investigate thoroughly the new configuration of the building based on criteria and requirements of design codes originally used during the initial analysis and design stages. However, in many cases, such investigation is not performed, and a decision is finally taken to execute the desired changes without examining the structural safety of the new configuration of the buildings. The changes take many forms according to the needs arising. In one example, a new multi-story building is built beside an old and similar one and the owner decided to connect the two buildings by a corridor bridge for easy and fast circulation. In another example, a multi-story building is connected to a nearby high retaining wall in order to brace the retaining wall and provide it with ample lateral support against rotation and sliding. Other examples of adding or removing walls, partitions, beams, or even, columns, in order to suit the spaces of buildings to some new business needs are also encountered in practice. Partial or complete fatal collapses of buildings were reported due to arbitrary, non-engineered changes.

Many previous studies investigated the effects of connecting buildings together by sky bridges, sky pools, and sky gardens built of reinforced concrete or steel [2, 3, 4, 5, 6, 7, 8, 9, 10].

On the other hand, numerous theoretical studies on twin buildings linked by damping devices striving for mitigating seismic effects on the linked buildings have been conducted [11, 12, 13, 14].

In this chapter, the following cases of changes to the dynamic properties of buildings are investigated. This chapter is organized into four parts.

First: Two or more adjacent buildings connected together by bridges after the buildings are completed and occupied for the purpose of easy circulation and saving of time and effort.

Second: Buildings connected by reinforced concrete beams to nearby high retaining walls in order to provide strong lateral support to the retaining walls.

Third: Theoretical investigation and structural analysis of connecting two or more buildings together by rigidly linked sky bridges and the effects of this linking on the seismic performance of the connected buildings.

Fourth: Theoretical investigation and structural analysis of connecting two or more buildings with special devices that are designed to mitigate seismic response and reduce earthquake effects on the connected buildings.

These parts are discussed in the next paragraphs.

2. Buildings rigidly connected together long after completion without being designed as such

A need to connect two adjacent buildings together by a bridge or corridor arises sometimes long after the buildings are constructed and occupied. Fast and easy circulation of occupants and goods is a clear benefit of linking the buildings. There are uncountable examples of buildings linked together by a bridge for similar purposes. Figure 1 shows two hospital buildings connected by a steel sky bridge over a street with a span of around 22 m. The steel bridge is partly supported by the stone walls, and partly fixed to the walls using steel plates and bolts, Figure 2. The sky bridge was erected long after the two buildings were constructed when the need to use the two buildings as one hospital came into existence. Often, in such cases, a thorough seismic structural investigation of the linked buildings is not carried out.

Figure 1.

Steel sky-bridge connecting two parts of a multistory hospital building in Amman, Jordan.

Figure 2.

The connecting steel sky bridge from below.

The connecting bridges are usually built of reinforced concrete or steel. The link may be supported directly on the beams’ or slabs’ tops of the two buildings. The concrete of the beam or slab tops may be removed and the steel bars exposed in order to splice or weld the steel bars of the linking bridge to the steel of the beams or slabs of the buildings. Steel sections (for example, Section I) may protrude into the beam or slab concrete, or be connected by plates and bolts. In all such cases of connections, the linking bridge becomes rigidly connected to the two buildings. A new complex building is, therefore, created, which comprises the two buildings and the linking bridge. The stiffness, mass, and damping matrices of this complex replace the individual matrices of the two buildings during analysis. Seismic analysis of this complex proves that its structural behavior is completely different from that of the isolated individual buildings.

In order to obtain some insight into this case, a seismic analysis of two buildings connected by sky bridges is carried out by the author. The two buildings are 18 and 13 floors in height, with different plan configurations. Sky bridges are modeled as beam columns, and hence it is a rigid connection. The buildings are connected together by one, two or three sky bridges, and seismic investigations are carried out for different cases. El Centro earthquake time- acceleration recording is utilized [15]. SAP2000 analysis results revealed that the modes of vibration of the connected buildings are quite different from those of the individual buildings. The following Figures 3–5 are the results of SAP200 analysis of two linked buildings. One of the outputs of the analysis shows that the modes of vibration are out of phase, Figure 3, and they depart from each other [15]. For example, in the structure with one sky-bridge, horizontal displacements in the right building frame U1 in Figure 3 for the two joints shown, being one to the left and the other to the right, produce a story drift of 263 + 243 = 50.6 mm. Similarly, for the left building frame, it is 66.4 mm. Some code limitations require that inter-story drift not exceed 0.015–0.02 h where h = story height. 0.015x3000 = 45 mm. These values of inter-story drift could be detrimental to the whole structure unless they are designed for such high values [15]. The linking bridge or bridges as well as some of the beams and columns of the two buildings may attract very large torsional, shear and moments values, which may well exceed their design capacities, and might render the buildings unsafe, Figures 4 and 5, [15]. For example, in the structure with three sky bridges, Figures 4 and 5, the bending moment in columns 269–270 is 93.2 kN.m as compared to 35.4, 51.6, and 60.6 kN.m for other columns in the same floor in the same plane. The same column has a shear force of 51.7 kN as compared to 25.3, 31.2, and 38.1 kN for other columns on the same floor in the same plane (see Figure 4).

Figure 3.

Out-of-phase mode of vibration and deformation of two connected buildings, a case of 1 sky bridge, with permission of the author [15].

Figure 4.

Shear forces in beams and columns and the sky bridges under seismic excitation, a case of three sky bridges, with permission of the author [15].

Figure 5.

Torsional forces in beams and columns and the sky bridges under seismic excitation, a case of three sky bridges, with permission of the author [15].

Another note-worthy example is for a column in the top short tower which shows a moment of 67.4 kN.m when it is part of the twin towers and connected to the sky beam, but it shows a reversed moment of 63.7 kN.m when it is in the unconnected single short tower under the same load combination. Similarly, some of the linking beams may attract large torsional forces that exceed their design capacities. The same note can be observed in the beams closest to the sky beams, Figure 5.

Code and safety requirements warrant that seismic analysis and redesign of the complex should be considered by the designer before connecting the two buildings.

The author suggests the connection of the two buildings be designed by having the linking bridge rigidly connected to one building while the other end has a sliding support to allow some movements during earthquake excitations, Figure 6. One important requirement for this type of connection is that the distance of the linking bridge resting on the building floors (distance L in Figure 6) should be larger than the expected maximum sum of the two displacements of the two connected floors during the design earthquake by a suitable factor of safety. The two displacements comprising the distance L should be obtained from inelastic dynamic analysis of the buildings where nonlinear plastic properties of the materials are utilized. Figure 7 shows another example of a multistory building linked to a car park building by steel I section long after the two buildings were constructed.

Figure 6.

Suggested method of connecting two building by a sky bridge.

Figure 7.

A. Car park building connected to a nearby college building. B. Apparent details of the connection.

3. Buildings connected to nearby walls at deep cuts

Often in mountainous terrains, a building is constructed at the foot of an excavated hill, and a high “facing wall” is constructed to support the excavation. The idea of connecting the building, at floor levels, to the “facing wall” by reinforced concrete beams attracts the designer. The author prefers to call it a facing wall rather than a retaining wall because, in fact, the wall is never designed as a retaining wall. The fact is that it is usually a reinforced concrete thin wall with constant thickness and very narrow base. Connecting such a wall to the highly stiff diaphragms of the building by beams seems to convince designers that the wall is provided with a high factor of safety against sliding and overturning. According to the geometry of the excavated hill, the building slabs may be connected to the “facing wall” in two directions (see Figures 8–10).

Figure 8.

Section of a multi-story building connected to an adjacent wall by beams.

Figure 9.

A multi-story building connected to the nearby wall by beams at several floor levels, Amman, Jordan.

Figure 10.

A building connected to a nearby wall by several beams at different floor levels, Amman, Jordan.

The structural seismic analysis of a complex a multistory building connected to a wall as described has in general a completely different structural response than the bare building alone. Shear, moment, and torsional forces prove through seismic analysis to be different from the values of the isolated building. The number of the linking beams as well as their distribution on the facades of the building have a great effect on the response. The choice of the number and distribution of linking beams at the level of each floor, that is to say, their configuration on the building elevation is in fact more arbitrary than studied by practicing engineers. As such, the response of the structural complex of the building, linking beams, and facing wall may reveal a non-safe seismic performance. Ground shaking may be imparted to the building at the base level and at all floor levels, which are connected to the wall Figure 8.

This will be more evident and may be very unsafe when the building is connected to the wall in two directions.

Time history seismic analysis of a 10-story 3-D building connected to a nearby facing wall 0.35 m thick, is carried out using the well-known El Centro acceleration recording in two directions, Figures 9 and 10. The X direction is that of the linking beams, and Y direction is perpendicular to it. The analyzed model includes the foundation soil below the building and the soil behind the facing wall attempting to include soil-structure interaction. Below foundation soil and the deep-cut soil are modeled as solid elements in SAP200. Soil properties are modulus of elasticity E = 2x10⁵ MPa, Poissons ratio = 0.2, and shear modulus G = 77 MPa. The facing wall or beams can be connected to the cut soil by Gap links according to SAP 200 modeling techniques. While the foundation of the column can be connected to the underlying soil by springs with suitable spring constants depending on the soil stiffness. The analysis results show that the behavior of the connected building with the linking beams is completely different from that of the isolated building. Behavior differences are evident in displacements, member response forces, periods of vibration, and mode shapes. Figures 11a and 12a show analysis plots of SAP2000 output for the shear, moment and torsional forces of the building with the linking beams and soil, while Figures 11b and 12b show the output of the isolated building. The differences between the results can not be overstressed. These are only examples to show the complete change of the structure behavior when the building is linked to a nearby wall.

Figure 11.

Comparison between shear and moment values of the isolated building B, and the building connected with linking beams to the wall and soil masses below and adjacent to the wall a. values are taken at 1.0 second from the application of El Centro excitation in the direction perpendicular to the linking beams.

Figure 12.

Comparison between torsional force values of the isolated building (12-B), and the building connected with linking beams to the wall and soil masses below and adjacent to the wall (12-a). Values are taken at 1.0 seconds from the application of El Centro excitation in the direction perpendicular to the linking beams.

Based on the analysis results, the following general notes can be drawn:

1. Linking a multistory building to a retaining wall or a facing wall by reinforced concrete beams at floor levels can provide lateral support to the walls.

2. The structural behavior of the connected building under seismic actions is completely different from that of the isolated building. Behaviors are manifested in the structure’s periods of vibration, mode shapes, displacements, and member forces.

3. The number of linking beams, their stiffness, and their distribution on the elevation of the building have unpredictable effects on the seismic response of the building. A complete analysis is required.

4. The shear, torsional forces, and moments in the building beams and columns as well as the linking beams and the “facing” wall may have excessive values beyond their original design capacities and could be unsafe.

4. Theoretical studies of buildings rigidly connected by sky bridges

There has been a lot of theoretical research on nearby buildings linked together by steel or reinforced concrete sky bridges. Numerous adjacent world skyscrapers are connected by sky bridges. The Petronas Twin Towers in Malaysia is a typical example (Figure 13).

Figure 13.

Petronas twin towers in Malaysia connected together by a sky bridge. Petronas twin towers | buildings, Kuala Lumpur, Malaysia | Britannica.

Esthetic and grand appearance of similar connected towers is one architectural purpose of the connection. The sky bridges can serve as fire escape routes in addition to ease of circulation and time-saving purposes [3]. Theoretical studies of the effects of inking on the seismic performance of connected buildings have been addressed by many authors [2, 3, 4, 5, 6, 7, 8, 9, 10]. The problem of buildings linked together by sky bridges has not found its way into building design codes due to the extreme complexity of the issue. Research has shown, in general, cases of increased seismic response of the buildings, and cases of reduced seismic performance. The heights of the connected buildings varied from few floors up to more than 250 m of height. The connection was, usually, modeled as rigid beams or slabs made up of reinforced concrete or steel. The position of linking sky bridges, as they are called by researchers, was shifted from floor to floor to investigate the effects of such positions on the performance of the buildings. In general, the findings of most of those theoretical studies indicated that such rigid connections of the two buildings often resulted in dramatic changes in the seismic performance of the two isolated buildings. The seismic response (mode shapes, frequency, shear, torsion, moment) of the complex of linked buildings and sky bridges are much different for each isolated building before connection. A summary of some theoretical work is presented next.

Sayed, M [2] studied 30-floor twin buildings with sky bridges connecting them at different heights utilizing ETABS software. The sky bridge connecting the towers is 33.6 m long built of reinforced concrete and resting on the buildings’ columns on either side. As such, the connection to the towers is a rigid connection. The study did not aim at alleviating the buildings of any earthquake actions. The study conclusions described the effect of changing the height of the position of the sky bridge on the seismic response of the buildings. None of the findings of the research mentioned anything about reducing the forces or seismic responses of the buildings.

Abbood et al. [3] conducted a study on 40-floor twin buildings connected rigidly by 3 reinforced concrete links at the upper two stories. The links were 15 m long and modeled as a beam and slab. The results of the study showed that the seismic response of the two connected buildings increased or decreased compared to the single tower’s response. The study did not aim at reducing the effect of earthquake actions on the structures.

Tse et al. [4] investigated the 3-D theoretical model of two 40-floor buildings with the links connecting them modeled as rigid beams with 3 degrees of freedom at each end. The study examined the effect of changing the location of the links on the periods of vibrations.

Sayed et al. [5] carried out a 3-D dynamic (time history) analysis of the Petronas Twin Towers in Malaysia connected with a linking reinforced concrete sky bridge. The analysis used an earthquake string record scaled to 0.15 g ground acceleration in the direction of the sky bridge as well as in the orthogonal; direction. The sky bridge was positioned at the following floor heights to investigate the effect of its position on the seismic response of the connected twin towers: 22nd, 44th, 66th, and 88th floors. The analysis results were compared to the results of the bare towers, which revealed that the seismic response in the sky bridge was amplified. It was also found that the location of the sky bridge on the upper floors had a detrimental effect on the seismic response of the twin towers when the earthquake excitation was in the direction of the sky bridge.

Hu et al. [7] theoretically investigated two linked buildings for the wind-induced response. The effects of the mass, stiffness, and location of the links on the acceleration of the linked buildings revealed that the torsional acceleration of the linked buildings was always larger than that of the unlinked buildings. This is in agreement with other research [15]. The author concluded that designers should be careful of this fact when designing linked buildings [7].

Based on the general findings of previous research, the following notes can be drawn:

1. Many studies were interested in the effect of changing the location and height of the linking sky bridges on the structural behavior of the connected buildings. The sky bridges, built of reinforced concrete, were rigidly connected to the buildings. All studies had the evident conclusion that the connected buildings showed different seismic responses than the isolated buildings. Some studies revealed detrimental effects or amplified response on the buildings during seismic analysis.

2. These studies were not aimed at relieving the buildings of earthquake actions by linking them by sky bridges.

3. The door is wide open for re-considering those same studies searching for solutions of relieving the connected buildings of any risks caused the connections during earthquake excitations.

5. Theoretical studies of buildings connected together by damping devices

The three previous parts of this chapter show cases of connecting building together or to adjacent walls. The purposes of such connections are to make easy and fast circulation between connected buildings in the former case and produce strong lateral support for the walls in the latter case. Previous studies of those cases have not tried to investigate means of mitigating earthquake effects on the new structural complexes produced by the connections. Numerous new studies have been carried out adopting newly invented technical devices to control and alleviate seismic effects on buildings. These techniques could be suitable methods of balancing the risks arising from connecting buildings to each other or to nearby walls. The door is open for more theoretical and experimental research in this regard.

Newly developed techniques for changing the dynamic properties of buildings are linking two nearby buildings by damping devices. Previous sections of this chapter considered buildings rigidly connected to each other or to walls, without aiming at reducing seismic responses during earthquake excitations. This section considers previous theoretical studies conducted to enhance buildings’ performance during seismic actions. The purpose was to reduce story drifts, member moments, shear, and torsional forces. This is done by connecting two adjacent buildings with damping devices to dissipate energy imparted to the buildings by ground acceleration.

The buildings were considered a few stories to more than 50 floors in height. Many types of damping devices were used. Friction pendulum bearings are seismic isolators used usually between the structure and its foundation. In one study [12], they are used between the sky bridge and the buildings on both sides of the sky bridge. Viscous, viscoelastic, and friction dampers were also used in another theoretical study [12] to connect two buildings at different heights to reduce the effects of earthquake actions. Viscous damper reduces the vibrations induced by earthquakes, while viscoelastic dampers dissipate the building’s mechanical energy by converting it into heat. Friction dampers operate by dissipating kinetic energy through friction. Friction dampers are installed diagonally between floors to reduce the story drifts. In one study, they were used between two buildings to reduce the earthquake effects by dissipating energy through frictional forces as shown in Figure 14. A summary of these studies is included here.

Figure 14.

Friction dampers used to connect two buildings at floor levels.

Bhaskararao et al. [11] conducted a theoretical investigation of two adjacent buildings connected with various types of dampers namely viscous, viscoelastic, and friction dampers. With all types of dampers used, it was concluded that the earthquake response of the two buildings was reduced. It was also concluded that it was not necessary to connect the buildings at all floor levels to achieve such a reduction. Optimum location of dampers at specific floor heights can be obtained.

Xiaohan et al. [12] considered four similar towers, around 240 m high, connected with a sky corridor bridge 300 m long at the rooftop. SAP200 software was used in the analysis. Friction pendulum bearings are installed on the buildings’ tops to support the sky corridor bridge. Several damping devices connected the towers and the sky bridge. The purpose of the study is to reduce the earthquake response and the member forces using the used devices. The authors concluded that connecting towers with friction pendulums with tuned mass dampers reduced effectively the structure seismic responses. Best seismic reduction was obtained when the frequency of the dampers is close to the frequency of the primary structure. On the other hand, larger interaction forces between sky bridges and towers were noted when towers and bridges were rigidly connected, a finding consistent with other research conclusions [15].

Rouzbeh et al. presented a state-of-the-art review of buildings connected together by various types of links. The authors classified the buildings coupling as rigid, passive, semi-active, active, and hybrid connections. These coupling links proved to be effective in preventing the pounding of adjacent buildings and reducing their seismic response. Damage to buildings due to pounding is very common between nearby or adjacent buildings during earthquakes, Figure 15.

Figure 15.

Pounding of two adjacent buildings and damage produced. Full article: Effects of pounding on adjacent buildings of varying heights during earthquake in Pakistan (tandfonline.com).

Uz et al. [14] used fluid viscous dampers to connect two buildings and concluded that they improve the response of buildings to earthquakes, Figure 16.

Figure 16.

Typical fluid viscous dampers. Viscous damper - an overview | ScienceDirect topics.

Huaxiao et al. [16] carried out a theoretical study of two unequal buildings connected together by varying number of passive control devices at different floor levels. These devices can be metal friction dampers or fluid viscous dampers. The buildings are 59 stories (268 m height) and 55 stories (210.2 m height). The buildings are connected by a corridor with rigid support at one terminal and sliding support at the other terminal, the same way as suggested by the writer. The study considered wind forces and earthquake excitation at the same time. Tuned Liquid Column Damper Inerter (TLCDI) and tuned mass damper with an inerter (TMDI) are used to connect the two buildings to mitigate the wind and earthquake responses. The authors concluded that these devices are effective in reducing the wind and earthquake responses, and they are more effective in the wind case than in the earthquake case.

Based on these and other studies concerning the earthquake mitigating devices linking two buildings, the flowing can be concluded:

1. Friction pendulum bearings, viscous, viscoelastic, and friction dampers are control devices used in theoretical studies to connect two adjacent buildings in order to mitigate earthquake responses. They proved through dynamic analysis of the linked buildings to be effective in reducing earthquake responses.

2. Linking two buildings by a rigid connection on one building and sliding connection on the other is considered effective in avoiding the transfer of building response to the other building.

3. Pounding of adjacent buildings can be prevented by using suitable linking devices, and more effective developments are finding their way into the smart building industry.