Earthquake Resistant Building Construction - part 3

Posted By: Usha Rama, 03/13/2019 18:01

Arches and Domes

The behavior of arches has been found very unsatisfactory during the earthquake. However, domes perform very satisfactorily due to symmetrical in nature. Arches during the earthquake have a tendency to separate out and collapse. however, mild steel ties if provided at the ends, their resistance can be increased to a considerable extent.

Staircases

These are the worst affected part of any building during an earthquake. Studies reveal that this is mainly due to a differential displacement of connected floors. This can be avoided by providing open joints on each floor at the stair way to eliminate the bracing effect.

Beam Column Joints

Inframed structures the monolithic beam column connections are attractive so as to accommodate reversible deformations. The maximum moments occur at the beam-column junction. Therefore most of the ductility requirements should be provided at the ends. Therefore spacing of ties in the column is limited to 100mm center and in a case of beam strips and rings should be closely spaced near the joints. The spacing should be restricted to 100mm centre to centre only near the supports. In a case of columns, vertical ties are provided; performance of columns to earthquake forces can be amplified to a considerable extent. Steel columns for all buildings ie buildings more than 8 storey height should be provided as their performance is better than concrete column due to ductility behavior of the material.

Masonry Building

Mortar plays an important role in masonry construction. Mortar possessing adequate strength should only be used. Studies reveal that a cement sand ratio of 1:5 or 1:6 is quite strong compared to economical. If reinforcing bars are put after 8 to 10 bricklayers, their performance to the earthquake is still better. Other studies have revealed that masonry in fill should not be considered as the non-structural element. It has been seen that in case of column bars are provided with joints at particular level about 600-700 mm above floor level at all storey should be spread out. It may be working as a weak zone at complete floor level in that storey.

As such if few measures are adopted during stages of design and construction of building their resistance to earthquake forces can be improved considerably. Though buildings cannot be made 100% earthquake proof their resistance to seismic forces can be improved to minimize loss of property and human life during the tremors.

Earthquake Resistant Building Construction With Reinforced Hollow Concrete Block

Reinforced hollow concrete blocks are designed both as load behavior walls for gravity loads and also as shear walls for lateral seismic loads, to safely withstand the earthquakes. This structural system of construction is known as the shear wall-diaphragm concept, which gives three- dimensional structural integrity or the buildings.

Structural Features

Each masonry element is vertically reinforced with steel bars and concrete grouts fill, at regular intervals, throughout the continuous vertical cavities of hollow blocks.
Likewise, each masonry element is horizontally reinforced with steel bars and concrete grout fills at plinth, sill, lintel and roof levels, as continuous RC bands using U-shaped concrete blocks in the masonry course, at recurring levels.
A grid of reinforcement can be build into each masonry element without the obligation of any extra shuttering and it reduces the scope of corrosion of the reinforcement.
As the reinforcement bars in both vertical and horizontal directions can be continued into the roof slab and lateral walls respectively, the structural integrity in all three dimensions is achieved.

Structural Advantages

In this construction system, structurally, each wall and slab behaves as a shear wall and diaphragm respectively, reducing the vulnerability of disastrous damage to the structure during natural hazards.

Due to the uniform distribution of reinforcement in both vertical and horizontal directions, through each masonry element, increased tensile resistance and ductile behavior of elements could be achieved. Hence the construction system can safely resist lateral or cyclic loading, when compared to other masonry construction systems. This construction system has also been proved to offer better resistance under dynamic loading, when compared to the other conventional systems of construction.

Constructional Advantages

No extra formwork or any special construction machinery is necessary for reinforcing the hollow block masonry. Only semi-skilled labor is required for this type of construction.

It is faster and easier construction system, when compared to the other conservative construction systems. It is also cost-effective.

Architectural and other advantages

This type of constructional system provides better audio and thermal insulation for the building.
This system is durable and maintenance free.
Studies on the comparative cost economics of RHCBM

There is a general apprehension that the RHCBM would be a costlier system, as it advocates reinforcing and use of concrete grout in the hollow spaces within the masonry. To dispel the apprehension, there elative cost economics of RHCBM structures are worked out in comparison with conventional construction systems.

Mid-Level Isolation

This includes mid-level isolation system installed while the buildings are still being used. This new method entails improving and classifying the columns on intermediate floors of an existing building into flexible columns that incorporate rubber bearings base isolation systems and rigid columns which have been wrapped in steel plates to add to their toughness.

This is the first method of improving earthquake resistance in Japan that classifies the columns on the same floor as flexible columns and rigid columns, and it is the first casein west Japan the Kansai region of attaching rubber bearings by cutting columns on the intermediate floors an existing building. This method involves improving earthquake resistance while the buildings are still being used as normal operations.

There are three types of base isolation systems, depending on the location where rubber bearings are incorporated:

Pile-head isolation
Foundation isolation
Mid-level isolation

Earthquake Resistance Using Slurry Infiltrated Mat Concrete - SIMCON

Following the devastating earthquakes in Turkey this summer that killed as many as 20,000 people and injured another 27,000, images of survivors trapped beneath the rubble of collapsed buildings appeared daily in news reports worldwide. However, a North Carolin a State University engineer is developing a new type of concrete to help prevent such scenes from happening again. Because it's reinforced with mats made of thousands of stainless steel fibers injected with special concrete slurry, the new material, called Slurry Infiltrated Mat Concrete (SIMCON), can sustain much higher stress loads and deformations than traditional concrete. Tests how that concrete buildings or bridges reinforced with SIMCON are far more earthquake- resistant and less likely to break apart in large chunks that falloff and cause injury to people below.

If extreme stresses cause SIMCON to fail, its mass of fibers and concrete doesn't collapse in the same way traditional concrete does. Instead of large chunks breaking and falling from a structure, the material crumbles into small, harmless flakes. This controlled form of failure is a key advantage of SIMCON. Because failure is inevitable in all structures, engineers must design buildings and bridges to fail in the safest way. In conventional concrete structures, this is achieved through the use of steel reinforcing bars--rebars--that give the concrete tensile strength it would otherwise lack. For safety and design reasons, the concrete is designed so that the rebars will fail before the concrete does. Unfortunately, many structures have not been designed to sustain the powerful stresses caused by earthquakes. When such extreme stresses occur, the concrete can crack, explode and break away from the rebars, causing the structure to collapse.

Concrete foundations for greater stability

Wooden columns treated with tar or pitch to protect against humidity, concreted into the ground with nails embedded in the wood at the base to give extra anchorage.
Using concrete wall bases to prevent humidity affecting the wood and the canes in the walls.
Careful jointing between columns and beams to improve structural integrity.
Canes woven in a vertical fashion to provide greater stability.
Lightweight metal sheet roofing to reduce danger of falling tiles.
Nailing roofing material to roof beams; tying of beams and columns with roof wires.
Incorporating roof eaves of sufficient width to ensure protection of walls from heavy rains.

Conclusion

However the builders and constructors should adopt the coal provisions in all the future construction, as prevention is better than cure. On the light of avoiding the risk, this may not be an impossible task as earthquake resistant measures in building involves only 2% - 6% additional cost depending on the type of building. Using construction techniques like Simon and RHCBM can not only mitigate earthquake effects but also are cost effective.