Earthquake Resistant Construction
I. S. GILL (Structural Consultant)
When a building experiences earthquake vibrations its foundation will move back and forth with the ground. These vibrations can be quite intense, creating stresses and deformation throughout the structure.
For the lay man the experience for the building is like travelling in a bus where you are standing in it and a rash driver is at the steering who keeps on doing sudden pickups fast turns and sudden application of brakes with a quick pickup immediately after that. In the above conditions your feet are in contact with the flooring of the vehicle and thus start moving with it while the upper part of your body is still stationery giving you the feeling of falling backwards. When brakes are applied your total body is by then in motion at the speed of the bus in forward direction. Since your feet were in contact with the bus they stopped in the same instance as the bus but the upper part of your body kept moving forward thus making your body bend forward.
During a earthquake a building experiences similar forces and swings accordingly but in a more complex manner.
Our body has flexibility which stops it from cracking or breaking, and it has muscles which take the stresses and try to stabilise it. Thus a building also needs to be slightly flexible and also have components which can withstand or counter the stresses caused in various parts of the building due to these horizontal movements.
During construction of a building some simple precautions may be taken.
1. Providing a Separation Section: Bureau Of Indian Standards clearly gives in its codes that a Separation Section is to be provided which is defined as "A gap of specified width between adjacent buildings or parts of the same building, to permit movement, in order to avoid hammering due to earthquake ".
Thus it is advised to provide adequate gap between two buildings so that they have enough space to vibrate independently.
This is discussed in detail in the article " Gaps between buildings ".
2.Precautions to be taken while deciding the way Electrical conduits are to be placed so as not to create planes of weaknesses in the slabs and walls.
This is discussed in detail in the article "Electrical conduits".
3. Precautions to be taken while doing Sanitary works and plumbing so as not to puncture structural components and weaken walls by chasing.
This is discussed in detail in the article "Sanitory Systems ".
In various countries around the world where earthquakes have taken place a lot of research work has been done to find procedures to be adopted for reducing the effect of earthquakes on buildings. One of the methods being implemented in most of the new constructions is Base Isolation with the use of Seismic Isolation Bearings this is detailed below.
Base Isolation: This technology involves the installation of lead-rubber bearings in a structure, or at or near the interface of a structure and the ground, effectively uncoupling it from destructive horizontal and vertical ground motions. The result: Seismic energies are isolated and dissipated by these massive "shock absorbers," significantly buffering the damaging resonance transmitted up through the protected structure. Instead, isolation bearings allow a structure to slide back and forth fluidly on its foundation. Thus, as the ground accelerates in one direction, the counterbalance moves in the other and the forces acting on the structure are dampened or cancelled out.
The bearings are made by vulcanisation bonding of sheets of rubber to thin steel reinforcing plates. Because the bearings are very stiff in the vertical direction and very flexible in the horizontal direction, under seismic loading the bearing layer isolates the building from the horizontal components of the ground movement while the vertical components are transmitted to the structure relatively unchanged. Although vertical accelerations do not affect most buildings, the bearings also isolate the building from unwanted high-frequency vertical vibrations produced by underground railways and local traffic. Rubber bearings are suitable for stiff buildings up to seven stories in height. For this type of building, uplift on the bearings will not occur and wind load will be unimportant.
A) Steel Frame Structures
B) R.C.C Frame Structures
Lets first discuss Steel Framed Structures.
Foundations: Here a suitable foundation is designed taking into consideration the test reports of the soil conditions, various forces and loading coming on it from the building is calculated. On this rubber bearings are placed. These rubber bearings are designed to take the vertical load of the building above and the calculated horizontal displacements due to earthquake. Above this the frame of the superstructure rests thus isolating it from the base that is the foundation. Thus even though the building foundation may experience horizontal movements caused by say an earthquake having an intensity of 8 on the Richter scale the building above it would experience very low impact as if that caused by an earthquake of negligible intensity.
Frames: Frames of the superstructure are generally made of Steel Sections and is designed for the stresses acting on the structure with cross bracing and shear walls where required.
Slabs: The slabs used are again supported by steel sections and made of wooden sections, this system provides nominal flexibility and thus safety, since in an extreme condition a falling plank from the roof has less chances of giving a fatal injury than a falling concrete slab.All connections and joints are designed to withstand shear forces and where required flexible joints are provided so that there are minimum chances of slab failure.The safety of the human life during the worst scenario should always be of the highest consideration during the selection of structural components and the design of the building.
Walls: The walls are again made of wood, toughened glass with flexible anchorage systems or of any other material taking precaution that in case of extreme condition of breakage they have minimum chances causing the total failure of the structure system. The walls are also designed to be tough enough to protect you from the impacts of nature such as cyclones, storms and hurricanes and of course from people trying to break into your home.
While designing the structure precaution also needs to be taken to ensure that in case any one structural component buckles it does not trigger a chain reaction of failures of other components and thus collapse of the total building structure. Thus structural isolation zones should be created for additional safety and additional supporting systems be created where considered critical.
In the case of steel structures in bridges many failures have been seen due to the failure of welding due to rupture this should also be considered in buildings. Different stiffening techniques for strengthening under a constant compressive axial load and cyclic lateral loads should be investigated. Research has shown that some of the new techniques for the strengthening of rectangular steel piers in bridges with inside angle and outside corner plates or inside angle plates will increase ductility as well as prevent cracking at corners after local buckling has begun in both rectangular and circular steel columns. For circular steel columns, an additional thin layer of steel confinement should be provided at the bottom of the pier to restrain the progress of out-of-plane deformation after local buckling.
In the case of R.C.C. structures a major change in approach to design has to be undertaken. The design should be done using a ductility approach under an assumed magnitude 8 earthquake, where force levels of up to 2.0 g are applied to the columns of the structure. Gal is the same as centimetres per second squared (cm/s2), and 980 gal equals 1 g (gravity force). This has now been made mandatory in Japan after revision of there building codes in the wake of Kobe earthquake in 1995.
In Japan during the recent earthquakes in Kobe the maximum velocity and maximum displacement of the earthquake motion recorded on solid ground near the JMA Kobe station were 90 cm/s and 21 cm, respectively. The same measurements for the soft ground near the Higashi Kobe Bridge were 91cm/s and 49 cm, respectively. It was observed that coloumns failed at the point where the main vertical longitudinal bars were reduced in the coloumn above the base or in between floors. Thus in Japan now the termination of longitudinal reinforcement in columns at mid-height is not permitted in columns and spacing of shear reinforcement stirrups in columns should not exceed 15 cm.
While designing the building with seismic isolation bearings the total building has to be isolated from the surrounding ground thus a gap is provided all around the building. This gap should be 1.5 times the expected movement of the ground and building relative to each other and this has been recorded to be up to 49 cm. The gap should also be sized with respect to maintenance and easy accessibility for repairs and maintenance of the sub structure. This gap should be covered from safety point of view with slab or steel/aluminium grill cantilevered from the building.
All the approaches for accessing the building such as stairs and ramps are to be cantilevered from the building and should not come in contact with surrounding ground. All services such as water supply, sanitary and drainage pipes have to be connected from the municipal connection to the building with a flexible connection so that they do not break due to the movement of the ground under the building.
To summarise it is essential that along with an architect you should also take the services of experienced structural designer's with experience in earthquake resistant construction, and having awareness w.r.t the provisions given in I.S. Codes and the latest procedures being adopted around the world.