The Constructor Principles of Conceptual Design of Earthquake Resistant Structures

It is crucially important to take seismic hazards in seismic prone areas at early phase of conceptual design of earthquake resistant buildings. This is because not only the structural system would be acceptable and meet basic requirements stated in Eurocode 8, namely, no collapse requirement and damage limitation requirement but also the budget need for the construction would be within acceptable limit.

This article will shed light on the basic principles of conceptual design of earthquake resisting structure.

Fig.1: Earthquake Influence on Building Structure

Principles of Conceptual Design of Earthquake Resistant Structures

The basic principles of conceptual design of earthquake resistant structures includes:

• Structural simplicity
• Uniformity, redundancy and symmetry
• Bi-directional resistance and stiffness
• Torsional resistance and stiffness
• Adequacy of diaphragms at each storey level
• Adequate foundations

Structural Simplicity for Earthquake Resistant Design

Structural simplicity pertains to the provision of obvious, simple and straightforward load path to transfer seismic forces from different part of the structure to its foundation.

Not only does the load path need to be clear and simple but also its components must have sufficient stiffness, ductility and strength. This requirement should be examined by a structural designer who commonly designs the load path.

One of the significant advantages of direct load path is that it would contribute in the decrease of doubts and uncertainty in the evaluation of strength, ductility and dynamic behavior.

In contrary, complicated load path is likely to cause stress concentration and toughen the estimation of strength, ductility and dynamic response of structures. It should be bore in mind that, acceptable structures with complex load path can be designed.

Fig.2: Simple and Straight Forward Load Path of Structure

Fig.3: Seismic Load Path Component

Structural Uniformity, Redundancy and Symmetry

It is proven that, if the strength, stiffness and mass of a structure is spread symmetrically and uniformly in elevation and plan, it would show far better seismic performance in comparison with structure that does not have such properties.

As far as strength and stiffness uniformity in elevation is concerned, it prevents the creation of soft storey in the structure. It should be bore in mind that non-uniformity does not mean bad seismic performance, for instance, if such structure is isolated seismically then it would show satisfactory seismic performance.

Fig.4: Structural Uniformity in Elevation and Plan

Regarding building uniformity in plan, it would put an end to the torsional response and hence enhance dynamic performance of the structure. For the structures with irregular shape like T shape, it is recommended to introduce joints to create regular shapes as illustrated in Figure 7 Figure 8.

Regarding Figure 5 and Figure 6, they explain regular plan shapes for building in seismic prone areas which are favored plan shape and irregular plan shapes which should be avoided to in seismic areas unless proper seismic joints are provided like in Figure 7 and Figure 8.

Fig.5: Desirable Symmetrical Shape Plan for Building in Seismic Prone Areas

Fig.6: Undesired Shape Plan for Structure in Seismic Regions

Fig.7: Provision of Seismic Joints for Decrease or Eliminate Torsional Motion due to Earthquakes

Fig.8: Provision of Seismic Joints to Decrease or Eliminate Torsional Motion due to Earthquakes

However, certain design problems at joints due to seismic movements may come-up and need to be tackled. Detailing of finishes, cladding, services across joints and impacts at the joint are examples of design problems which are likely to occur due to joint provision.

A structure is said to be redundant if there are more than one load path in the building to transfer imposed seismic loads. So, if strength or stiffness of a specific load path is deteriorated, the load will be transferred though other load path. Therefore, redundancy would make the structure more reliable.

Bi-Directional Resistance and Stiffness of Structures during Earthquakes

Normally, seismic loads on both horizontal axes of structures are similar, that is why the provision of similar resistant systems in both direction is recommended. So, the structural members need to be configured orthogonally guaranteeing similar resistance property in both major directions.

Torsional Resistance and Stiffness of Structures

Lateral torsional deformation, which might stress various structural members in an un-uniform manner, could occur during earthquakes. The factor that leads to lateral torsional motion is the eccentricity between center of mass and stiffness. So, this problem need to be tackled at the design stage.

Fig.9: Center of Mass and Center of Stiffness in a Structure Subjected to Earthquakes and Suffered Displacements

The eccentricity could be decreased at design stage but it may not be completely eliminated because of number of factors which are out of designer control. For example, non-uniform mass distribution and uneven stiffness deterioration of structural elements during earthquakes.

Finally, this problem can be dealt with by arranging stiff and resistant members close to the periphery of the structure.

Adequacy of Diaphragms at Each Storey Level

The influence of diaphragms on the seismic response of a structure is considerably crucial. Not only does it transfer seismic inertia load to the vertical structure members but also prevents considerable lateral movement of the such vertical elements.

So, in order for the floors to perform their function properly, adequate in plan stiffness should be provided. In addition, to attention to the joint between floors and vertical structural members.

These measures are specifically important if there is sizable opening diaphragm or in the case of considerably long in plan floor shapes.

Finally, if the floor is constructed from precast concrete, it is necessary to provide sufficient bearing to avoid the loss of bearing during earthquakes.

Fig.10: Floor and Roof Diaphragm Action

Adequate Foundations for Earthquake Resistant Structures

It is required to design and construct foundation and its connection to the superstructure, so as for the entire structure to experience uniform excitation during earthquakes.

That is why it is advised to provide proper linkage between individual piles such as slabs or beams between piles. Added to that, when superstructure is composed of discrete walls with various stiffness then it is recommended to use tough cellular foundation.

Fig.11: Connection of individual footing with tie beams or structural slabs to prevent possible relative movement during earthquakes

The Constructor Factors Affecting Degree of Earthquake Damages to Buildings

Various factors such as brittle columns, stiffness elements, flexible ground floor, short columns, shapes, sizes, number of storeys, type of foundation, location of adjacent buildings, structural layouts etc. affects the degree of damages to buildings during earthquakes.

These factors which influence the degree of damages that a structure experience during earthquake are discussed.

Fig.1: Failure of Buildings Due to Earthquakes

Factors Affecting Degree of Earthquake Damages to Buildings

The factors that affect the extent of earthquake damages to the structures include:

1. Deviation between design and actual response spectrum
2. Brittle columns
3. Asymmetric arrangement of stiffness element in plan
4. Flexible ground floor
5. Short columns
6. Shape of the floor plan
7. Shape of the building in elevation
8. Slabs supported by columns without beams
9. Damages due to previous earthquake
10. Reinforced concrete building with a frame structural system
11. Number of storeys
12. Type of foundation
13. Location of adjacent buildings in the block
14. Slab levels of adjacent structure
15. Poor structural layout

Deviation Between Design and Actual Response Spectrum

Improper evaluation of anticipated earthquake characteristics used in the earthquake design of the structure is the most common and critical factors for the damage of buildings. However, this is not the only factor since there are certain features of the structure that could be seismic weak point.

Fig.2: Design Response Spectrum

Brittle Columns

It is reported that, the majority of structures failed during earthquake are due to column failure. The column may fail due to deterioration of concrete as a result of cyclic loading and insufficient number of ties embedded in the column at the critical locations.

Fig.3: Brittle Failure of Column

Asymmetric Arrangement of Stiffness Element in Plan

Cores in structures are the basic stiffness element and its location with regard to the building would influence the behavior of the structure during earthquakes and eventually affect the extent of the damage. However, it is reported that, small percentage of failed buildings was due to eccentricities of the core of staircase and elevators.

Flexible Ground Floor

Flexible ground floor is another factor that contributes to the damage that structure may encounter during earthquake.

When stiffness is abruptly reduced at specific level of a structure, then stresses on the structural elements of flexible story would increase and subsequently fail. Therefore, the presence of soft story would increase the extent of structural seismic failure

Flexible ground floor is made when this floor is used for commercial purposes and broad spaces need to be provided.

Fig.4: Flexible ground floor of multi storey structure which could cause structural failure during earthquake

Fig.5: Soft storey problem during earthquake

Short Columns

The failure of short columns due to earthquakes is less common compare ordinary column failure. However, short column may fail in shear in explosive manner and eventually lead to the collapse of the structure.

Fig.6: Short Column in a Building

Shape of floor plan

It is demonstrated that, square shape floor plan exhibit the best seismic behavior compare with other shapes such as X, I, and +. Therefore, the extent of building seismic damage is influenced by floor shape plan.

Shape of Building in Elevation

It is proven that, structures with regular upper storeys show superior seismic response compared with buildings which their upper storeys are in form of setbacks.

Slabs Carried by Columns Without Beams

Flat slab system is considerably vulnerable structural system and does not have satisfactory resistance against seismic effects. Such structural system is considerably flexible and has low ductility.

That is why EC8 prevents the use of flat slabs unless other seismic resistant structures like shear walls and flexible frames are used as well.

Fig.7: Flat slab system

Damages of Structures due to Previous Earthquakes

Building that suffered certain type of damages from previous earthquakes would experience the same mode of failure if the repair technique had not been conducted properly.

It is found that, structures that repaired long time ago would undergo the same damage but this is less common in most recent repaired structures. This is because repairing methods have developed and hence their effects are more profound.

Reinforced Concrete Buildings with Frame Structural System

Frame structural system is the source of vulnerability is buildings. This is because it undergoes considerable inter-storey drift during seismic excitation. Such large displacement damage infill walls that their repair is substantially costly.

So, the major point that makes frame structural system vulnerable is the high cost needed to repair damaged infill walls.

Number of Storeys

It is reported based on statistical data that, the vulnerability of structure to earthquake forces decreases as the number of storeys are increased.

It shown in several earthquakes like those of Bucharest in 1997 and Mexico City that the extent of damages was more severe in high rise building (more than three storeys) compare with low rise buildings.

It is known that, the presence of masonry infill in building will not only increase the strength of the structure but also it improves its stiffness. These improvements are more obvious and effective in low rise buildings compare with high rise structures.

Type of Foundation

Type of foundation used influence the degree of earthquake damages in two forms including direct and indirect effect.

With regard to direct effect of foundation form, it manifest itself in number of characteristics such as fracture of foundation soil, failure of foundation member like foundation beam fracture, ground differential settlement which is the most common effect , soil liquefaction that is rarely occur but has catastrophic effect, and general or partial landslide of foundation soil.

The indirect influences of foundation type include out of plane movements of individual columns base in the case of isolated foundations which are not connected together, or when beams between foundations are flexible.

Therefore, isolated foundation would increase the severity of the earthquake damages.

Location of Adjacent Buildings in the Block

The location of the adjacent buildings on the block affects seismic response of the structure considerably. For example, buildings at the corner of the block would experience greater damage and more sensitive to earthquakes compare with free standing structures.

Factors such as asymmetric distribution of stiffness in building plan and transfer of kinetic energy through poundings are commonly increase the vulnerability of the corner buildings.

Slab Levels of Adjacent Structures

It is found that, the impulse loading that a structure gets from adjacent buildings have a considerable influence on the extent of the earthquake damages. It is reported that, the extent of damages in structures with different floor level slabs are significantly greater than buildings with the same floor slab level.

This can be clearly observed in Mexico City earthquake which occurred in 1985 and Thessaloniki earthquake occurred in 1978.

Poor Structural Layout

By and large, poor collaboration between architectural engineer and structural engineer at conceptual design phase results in poor structural layout.

Examples of poor structural layout includes cut off of columns, asymmetric arrangements of stiffness elements in plan and elevation, and irregularities in form in plan and elevation.

It is reported that around one third of structures collapsed in Athen earthquake occurred in 1999 were due to poor structural layout.