This research paper is about validation and optimization of dimensions, sizes, number, and details for the most commonly used vertical and horizontal bands for construction in Nepal. The research seeks to optimize the specific elements within the earthquake resistant construction by improving the overall strength and stability of the horizontal bands and vertical reinforcement. Through testing laboratory, the team will determine the amount of seismic force that these types of constructions can withstand.
Between 2007 and 2012, the Smart Shelter Foundation constructed 15 schools that are earthquake resistant in the mountainous region of Kaski, Nepal. Some of the structures were constructed with hollow concrete blocks while others with rubble stone masonry. The horizontal concrete bands were incorporated into the walls at numerous levels to improve their overall strength and stability, and stones were incorporated into the walls for better bonding. Good construction and design meant by the Smart Shelter Foundation survived the heavy earthquakes that struck the area in 2015 April without any substantial destruction, while numerous houses surrounding including the whole of the villages were flattened.
It is normally accepted that horizontal bands have a positive impacts on the seismic resistance of the structure. Assessment after the earthquakes indicates that structures with horizontal ties can sustain the seismic forces much better compared to the buildings without these specifications. It has been noted that block and brick walls need vertical reinforcement so as to strengthen the critical connections and to prevent shear cracks.
This research seeks to fully cover the collaboration and combination of vertical and horizontal steel bars and provide clear recommendations for their use in cement block masonry, brick and block, and rubble stone masonry. However, this report will not cover the process of lab testing concerning the amount of seismic force that these types of constructions can sustain. The report just presents the outcome of the experimental setup with recommendations for the optimized details, dimensions, sizes, and numbers of the horizontal and vertical bands for constructions in Nepal.
This task seeks to provide description of the performance of horizontal bands, describe things that have been accomplished, and difficulties encountered. There is analysis, testing and recommendations of diverse types of horizontal reinforcement and their details such as rods, lacing, and bands. The types and solutions of horizontal reinforcements are also considered together with their seismic behaviour. This last task for this section is the development of a matrix for optimum dimensions of the beam, optimum steel reinforcement, and correct details for different types of constructions and for different levels of seismic hazards.
The horizontal band is a method of reinforcing masonry construction through the provision of higher tensile strength. This is effective in regions where two elements of the structure of a building meet, hence a connection is formed all together and they would behave a single unit. Horizontal bands can also be referred to as seismic bands which entail reinforced concrete running flat in the whole of the internal and external masonry wall elements. The horizontal bands can be applied in the following levels; at the ceiling levels, at the levels of lintels such as windows and doors, and at the plinth of the construction (Aguilar, 2011).
The requirement of horizontal bands in the level of the roof is not necessary in case the roofs are reinforced masonry slab or reinforced concrete units provided that they possess a depth of 2/3rd thickness of the wall. There are four types of horizontal bands based of the region where the horizontal band is provided, these include plinth band, lintel band, roof band, and gable band. An illustration of horizontal bands is a masonry building is as shown in the figure below:
Gable Band: This is applied in the constructions that have the roof that is sloped such as truss construction where the gable bands are essential. The gable band is only applicable when the roof construction is by the use of truss and not when the construction has a flat roof.
Roof band: These types of bands are majorly implemented in constructions where the roofs are made of CGI sheets or flat timber. In case the roof of the building is made of brick roofs or reinforced concrete slabs, then the roof band is not necessary since reinforced slabs itself behave as a horizontal band (Aoyama, 2011).
Lintel band: These are horizontal bands applied at the lintel level and is used in almost all constructions. Under the action of ground motion as a result of the earthquake, the lintel band is exposed to continuous bending and also pulling
Plinth band: This category of horizontal bands is crucial in those regions where the soil on which the construction is to be erected is not strong. A weak soil will be very soft and unevenly distributed. This band may not be necessary in case of the stronger substructure and soil. However, in regions where there are seismic activities, this band must be used (Barda, 2013).
The recommendations on the types of horizontal reinforcement and their details are specified by the Code of the practice of IS: 4226:1996. These recommendations are applied to the constructions of concrete block or brick walls as well as those with reinforced flat slab roofs. The dimensions of the reinforced detail and horizontal bands depend on the length of the walls which is amongst the perpendicular cross walls (Breyer, 2014). The details of the reinforced details and dimensions of the band with respect to the thickness of the wall for the construction with diverse functionality is shown in the table below:
The bands used can be either reinforced concrete or wooden materials. A minimum of 50mm by 30mm is used for spacers and 75mm by 38mm is used for runners in the construction of a seismic resistance construction as shown.
In case of the use of reinforced concrete bars, a minimum thickness of 75mm is to be provided. A minimum of two bars of the diameter of 8mm is required that is joined across with the assistance of links of steel. The links of steel are incorporated as shown in the figure above with a minimum of the diameter of 6mm and spacing of 150mm distance from one centre to another (Duggal, 2011).
A diaphragm can be defined as structural members which distribute seismic forces to the horizontal force resisting system. The role of the diaphragm is of a dissimilar nature from that of other basic members of earthquake load resisting assemblies such as the wall, columns, and beams. The fundamental function of a roof diaphragm is to shelter a structure from the external weather conditions, it also assists in distributing and resisting lateral loads. Majority of the roofs in Nepal are constructed with roof trusses especially low-rise wood frame buildings (Gülhan, 2011).
The top chord of the roof truss assists as the framing member of the roof chord. Insufficient ties between the roof and wall have proven to be a serious cause of damage to structures of wood during seismic activities. It has been found that the use of stone gables with gable bands are more resistive to seismic forces that the wooden roof. The combination of the stone gables with gable bands and continuous wooden roof with trusses on top of the end walls are currently recommended for stronger seismic forces (Housner, 2011). The gable band is used in enclosing the triangular section of the building walls, the part that is horizontal will be continuous with the eave level band on the wall that is longitudinal. The ridged band is used on the top walls of the building forming ridges running longitudinally inside the construction from gable to gable (Institute, 2014).
From the results above, the matrix of the horizontal band can be developed by considering the different seismic hazard levels, different construction types, optimum steel reinforcements, and the optimum beam dimensions that can withstand the strong seismic forces experienced in Nepal (Japan, 2010).
|
Seismic hazard levels |
Construction type |
Optimum steel reinforcement |
Optimum beam dimension |
|
MSK intensity VII |
Wood-laced masonry structures |
2 bars of steel |
10 cm by wall width with the diameter of 10mm |
|
MSK intensity of VII |
Brick and block structures |
4 bars of steel |
15cm by wall width with the diameter of 12mm |
|
MSK intensity IX |
Reinforced concrete |
4 bars of steel |
15cm by wall width with the diameter of 12mm |
This research seeks to evaluate the optimization of the vertical reinforcements by determining the best position of vertical steel bars, steel diameter and spacing optimization, and spacing for vertical reinforcement in block and brick masonry. The seismic behaviour and role of the vertical reinforcement elements such as grouting of steel bars and bamboo and wood poles protection. There is also need of developing a matric of optimum spacing of steel spacing, anchoring details, and proper bending of vertical steel reinforcement.
Vertical reinforcement is placed after the stacking and completion of the wall. Masonry has been used for many years due to its low cost as compared to other construction materials. Unreinforced masonry has been proved to be very vulnerable when seismic forces are subjected to it and has led to huge losses of property and lives. During the earthquakes, the surface of the ground moves in every direction. The damages of the construction are as a result of the lateral movement which is opposite to the lateral loads being applied (Correia, 2012).
The bars for vertical reinforcement are slid into position from the top and weaved into horizontal reinforcement and safeguarded into correct location depending on the specs and plans of the project. The bars for reinforcement have been embedded in brick masonry at the corners of every room and the section of the openings of doors for earthquake safety zone V. Windows opening in the dimensions larger than 60 cm will also require bars for reinforcement (Kuniyoshi, 2015). The bar diameter that is used in the reinforcement of the vertical sections depends on the number of storeys in the construction.
The provision of the vertical bars in the concrete blocks and brickwork needs exceptional methods which could be learnt easily. These vertical bars have to be initiated from the concrete of foundation, then pass through all seismic bands where they are attached to the reinforcement bands by the use of wire for binding and then lastly embedded to the ceiling roof or band slab (Parducci, 2014). The figure below shows the details of vertical steel bar in a mason made of brick:
Some of the vertical reinforcement elements that need to be protected and maintained include bamboo poles, wood, and steel bars. The proposed methods for corrosion protection of the reinforced steel do not usurp or replace the benefits of good quality concrete as the basic source of protection from barrier against the attack of corrosion of the steel reinforcement (Tiwari, 2010). There is numerous method of protection and maintenance of the steel reinforcement, one of the methods is the application of a coating to the steel itself. The coating that is recommended is the use of epoxy coating and specifically zinc in the form of hot dip galvanizing.
The use of membrane-type coatings applied to the surface of the structures of concrete is also another method of maintenance of the steel reinforcement since it prevents the corrosion of the steel surface by covering the whole of steel from access to oxygen. Other methods of maintenance and protection of steel reinforcement include catholic protection of the reinforcement, addition of corrosion inhibitors to concrete, and painting the outer surface of the concrete (Russell, 2013). The application of the coating which is sometimes known as penetrating pore-liners may be used to minimize the content of moisture of the reinforced bar and hence suppressing the reaction leading to corrosion.
An already damaged steel reinforcement can be repaired through reactive repair strategies such as patch repair where the concrete cover is removed to easy access to the corroded steel and then the corroded parts are cleaned by the use of wire brush or sandblasting (Paulay, 2011). The other vertical reinforcement elements like bamboo and wood should also be maintained and protected by carrying out some basic rules such as avoiding direct contact with soil, not using nails on the bamboo or wood, and handling the elements with proper care. The elements should be coated with especial bitumen paint or tar which provides protection from rotting. Nails can make the bamboo poles to split because of the anatomical structure of the fibres of bamboo which run in the similar longitudinal direction. Screws are recommended in case a slightly smaller pilot hole is pre-drilled in the pole (Parducci, 2014).
Plinth beam is a beam in a structure that is framed delivered above or at ground level that supports the weight of the wall constructed on top of it. Majority of the beams are subjected to loads from the slab and also loads from walls. The plinth beams also serve another purpose like reducing the column lengths hence minimizing their effective thinness and length. It is also known as level of the ground floor and it is the region where the column start rising. The plinth beams are used to avoid difficulties in wall construction, connecting all columns in case foundation depth is high, and avoiding differential settlement.
Plinth beams are normally situated where the foundations are little deeper and hence perform like a tying or bracing element. Plinth beam should be constructed initially as soon as the foundation is erected. The plinth beam is usually expected to be strong enough for bearing effectively the superimposing brick walls tying the foundation and the column. The depth of the plinth is 20 cm while the width being equivalent to the final foundation course (Parducci, 2014).
From the results above, the matrix of the vertical steel reinforcement can be developed by considering optimum anchoring details, proper bending, diameters, and steel spacing that can withstand the strong seismic forces experienced in Nepal.
|
Optimum Steel spacing |
Diameters |
Proper bending |
Anchoring details |
|
65mm |
4 bar diameters |
180oC bend |
Bar anchor |
|
60mm |
12 bar diameters |
90oC bend |
Bar anchorage |
|
40mm |
90oC bend |
Tie anchorage |
|
|
25mm |
12 bar diameter |
135oC bend |
Tie anchorage |
|
75mm |
6 bar diameter |
135oC bend |
Tie anchorage |
The manner in which the communities earning the lowest income reacts to the earthquakes reflects their cultural identity. These communities can come up with designs for houses and schools with the best solution and practices which have the ability to resist seismic forces. The approaches that can be adopted to ensure that a house of a school is earthquake free building is the rigidity approach and the flexible approach. The rigidity approach depends on the use of techniques and materials that have the ability to resist by themselves to the impacts of seismic forces subjected to an earthquake (Parducci, 2014).
The structural connections and elements, as well as materials, should be rigid enough to counter the motion through their capability to absorb the deformation caused by the tremors. For this research, the proposed seismic resistant construction technique that is affordable to the low-income communities is the implementation of a timber frame construction. This system is composed of flexible and resistant structure that entails diagonal, horizontal, vertical timber members filed with light masonry and used inside the construction (Jigyasu, 2014).
Wall typology: The house and school design should be made of stone masonry walls embedded timber beams. The beams of timber are positioned perpendicular, linking both beams and other timber beams positioned in parallel to walls on both sides hence creating an arrangement like a ladder. This technique of timber lacing ties the wall together, keeping them from out-of-plane overturning and spreading, and also improves the system’s ductility. The details of the horizontal bands that would be used in the design of the school and house in the region with seismic hazards of MSK intensity VII will be optimum steel reinforcement of 4 bars of steel and the beam dimensions of 10 cm by wall width and diameter of 10mm. The vertical reinforcement details for this design will be steel spacing of 40mm, diameter of 6 bar diameter, bending of 90oC, and anchoring design of Tie anchor.
Floor typology: The floor of this design should be made of wooden floors aimed at connecting and coupling the surrounding walls and also a horizontal element that is flexible to absorb shock from the seismic force by incorporating the perimeter wall movement.
Connections: The connections between elements of the structure are the major aspects of efficient construction that are seismic resistant. This design uses quoins so as to improve the connections between corners from walls. This will improve the connection between wall-to-roof, wall-to-floor, and wall-to-wall. The watt to roof and wall to the floor is improved through piercing the wall of masonry with roof rafters and floor beams (Brzev, 2015).
Joints: Flexible housed wedges and joints are used in this design of school and house while enabling the tightening of the joints, they act as pin joints and permit motion within the joints.
Foundation: The width of the foundation should be determined by the bearing capacity load coming on the foundation. A school and house will have a different loading capacity hence both should have different foundation width (Naeim, 2014).
Conclusion
This research paper validates and optimizes details, dimensions, sizes, and numbers for the most commonly used vertical and horizontal bands for the constructions in Nepal. Unreinforced masonry has been proved to be very vulnerable when seismic forces are subjected to it and has led to huge losses of property and lives. During the earthquakes, the surface of the ground moves in every direction. In case of the confined concrete masonry wall system, both of the horizontal and vertical reinforcement in the wall of masonry play a critical role for expecting higher ultimate ductility and lateral strength.
The horizontal bands can be applied in the following levels; at the ceiling levels, at the levels of lintels such as windows and doors, and at the plinth of the construction. The requirement of horizontal bands in the level of the roof is not necessary in case the roofs are reinforced masonry slab or reinforced concrete units provided that they possess a depth of 2/3rd thickness of the wall. There are four types of horizontal bands based of the region where the horizontal band is provided, these include plinth band, lintel band, roof band, and gable band. The bands used can be either reinforced concrete or wooden materials. A minimum of 50mm by 30mm is used for spacers and 75mm by 38mm is used for runners in the construction of a seismic resistance construction.
Vertical reinforcement is placed after the stacking and completion of the wall. Plinth beam is a beam in a structure that is framed delivered above or at ground level that supports the weight of the wall constructed on top of it. The proposed seismic resistant construction technique that is affordable to the low-income communities is the implementation of a timber frame construction. The details of the horizontal bands that would be used in the design of the school and house in the region with seismic hazards of MSK intensity VII will be optimum steel reinforcement of 4 bars of steel and the beam dimensions of 10 cm by wall width and diameter of 10mm. The vertical reinforcement details for this design will be steel spacing of 40mm, diameter of 6 bar diameter, bending of 90oC, and anchoring design of Tie anchor.
From the project above, after implementation of the horizontal bands in the development of beam dimensions, optimum steel reinforcement and also the vertical reinforcement in the development of the steel spacing, diameters, proper bending, and anchoring details, the resultant structure will have the ability to resist heavy earthquakes without any substantial destruction. These proposed horizontal bands dimensions and vertical reinforcement dimensions have positive effects of the seismic resistance of the construction. .
An additional information regarding the horizontal bands and vertical reinforcement can be acquire from Government of Nepal under the Ministry of urban Development and Building Construction. Other information were acquired from the references listed below.
Aguilar, G., 2011. Effect of Horizontal Reinforcement on the Behavior of Confined Masonry Walls under Lateral Loads. Kanpur: Bureau of Indian Standards.
Aoyama, H., 2011. The design philosophy for shear in earthquake resistance in Japan. Hiroshima: International Workshop on Concrete in Earthquake.
Arnold, C., 2013. Building configuration and seismic design. New York: John Wiley and Sons.
Barda, F., 2013. Reinforced Concrete Structures in Seismic Zones. Santiago: ACI SP.
Breyer, D., 2014. Guidelines for the Design of Horizontal Wood Diaphragms. Tacoma: McGraw-Hill.
Brzev, S., 2015. Earthquake resistant confined masonry. Colorado: National information centre for earthquake engineering.
Chopra, A., 2014. Dynamics of Structures. Nepal: Prentice Hall,
Duggal, S., 2011. Earthquake Resistant Design, Moscow: Oxford University Press.
Gülhan, D., 2011. The Behaviour of Traditional Building Systems Against Earthquake and its Comparison to Reinforced Concrete Systems. London: IEEE.
Housner, G., 2011. The behaviour of Structures during Earthquakes. New York: Engineering mechanics division.
Institute, A. C., 2014. Standard Specifications for Tolerances for Concrete Construction and Materials and Commentary. Farmington Hills: American Concrete Institute.
Japan, A. I. o., 2010. Guidelines for Performance Evaluation of Earthquake Resistant Reinforced Concrete Buildings. Hiroshima: Architectural Institute of Japan.
Kokusho, S., 2012. Ductile shear walls in earthquake-resistant multistory buildings. Houston: third world Conference on Earthquake Engineering.
Kuniyoshi, G., 2015. An Evaluation Method for Restoring Force Characteristics of Reinforced Concrete Columns and Beams. Nepal: Structural and Construction Engineering.
Money, T, 2012. Ancient Buildings and Earthquakes. Colorado: IEEE.
Naeim, L., 2014. Design of Seismic Isolated Structure. Chichester: John Wiley & Sons.
Parducci, A., 2014. Seismic Isolation and architectural configurations. Melbourne: Laterconsult.
Paulay, T., 2011. Coupling beams of reinforced concrete shear walls. Wairakei: J. Struct. Eng.
Raynol, J., 2014. Reducing Disaster Vulnerability through Local Knowledge and Capacity. Norway: Norwegian University of Science and Technology.
Russell, J., 2013. Design of Wood Structures. Perth: The Engineered Wood Association.
Tiwari, R., 2010. Responsiveness to earthquakes through empiricism. Colorado: IEEE.
Wood, S., 2010. Shear strength of low-rise reinforced concrete walls. Mexico: Eleventh World Conference on Earthquake Engineering.
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