Earth Quake Engineering

By

What is Earth Quake Engineering?

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.
Earthquake Influence on Building 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.
Structural Simplicity for Earthquake Resistant Design
Fig.2: Simple and Straight Forward Load Path of Structure
Seismic Load Path ComponentFig.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.
Structural Uniformity in Elevation and Plan
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.
Desirable Symmetrical Shape Plan for Building in Seismic Prone Areas
Fig.5: Desirable Symmetrical Shape Plan for Building in Seismic Prone Areas
Undesired Shape Plan for Structure
Fig.6: Undesired Shape Plan for Structure in Seismic Regions
Provision of Seismic JointsFig.7: Provision of Seismic Joints for Decrease or Eliminate Torsional Motion due to Earthquakes
Provision of Seismic Joints in Building Structures
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.
Center of Mass and Center of Stiffness in a Structure
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.
Floor and Roof Diaphragm Action
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.
Connection of individual footing with tie beams
Fig.11: Connection of individual footing with tie beams or structural slabs to prevent possible relative movement during earthquakes

Comments

Post a Comment

Popular posts from this blog

Concrete Floor Slab Construction Process

By
Image
Concrete floor slab construction process includes erection of formwork, placement of reinforcement, pouring, compacting and finishing concrete and lastly removal of formwork and curing of concrete slab. Table of Contents Concrete Floor Slab Construction Process 1. Assemble and Erect Formwork for Slab 2. Prepare and Place Reinforcement for Slab 3. Pour, Compact and Finishing Concrete Floor Slab 4. Curing Concrete and Remove Formwork Concrete Floor Slab Construction Process Assemble and Erect Formwork Prepare and Place Reinforcement Pour, Compact and Finish Concrete Curing Concrete and Remove Formwork 1. Assemble and Erect Formwork for Slab The formwork shall be designed to withstand construction loads such as fresh concrete pressure and weight of workers and operators and their machines. Guide to Formwork for Concrete ACI 347-04 shall be followed for the design of formworks. Moreover, there are various construction aspects that need to be considered during...
Read more

Steel Pile Foundations – Types, Design and Connections

By
Image
Precast piles and driven cast in piles make use of steel pipes. In the case of precast or totally performed piles, there are two classification such as hollow small displacement piles and solid piles. The hollow small displacement piles make use of  steel pipes  when steel pile foundation is recommended. When it comes to solid piles,  steel H- piles  are used. In the case of driven cast in place piles the main classification is concrete tube steel tube. In case of steel tubes, we make use of  closed ended tube  and  open-ended tubes. Generally, the steel piles can be classified as: Screw Piles Disc Piles H-piles Disc Piles Commonly used steel piles are rolled steel H section piles or pipe piles. The pipe piles have either an open or a closed end that is driven into the ground. I-section or wide flange piles can also used as pile foundation. The H-sections are preferred more over I-sections, as the H-section have same thickness for the w...
Read more

How to Construct Concrete Stairs?

By
Image
Construction of concrete stairs includes steps such as designing, preparing foundation, building formwork, placement of reinforcement steel bars, concreting, finishing and curing. Construction of concrete stairs is a difficult task that requires an engineer to study all the aspects and design it and a skilled labour to construct it.In this article we explain you the detailed method of how to construct a concrete stairs from start to end. Fig 1: Concrete Stairs Table of Contents Steps for Construction of Concrete Stairs   1. Designing of Concrete Stairs 2. Foundation and Support for Concrete Stairs 3. Building the Form-work for Concrete Stairs 4. Steel Reinforcement for Concrete Stairs 5. Pouring of Concrete for Concrete Stairs 6. Removal of Formwork  7. Finishing of Concrete Stairs Steps for Construction of Concrete Stairs   The followings are the steps involved in construction of concrete stairs, 1. Designing of Conc...
Read more