What is a Load Bearing Structural System?
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It is a structural system where loads of buildings i.e. weight of building itself plus the live loads get transferred to the ground through walls. Walls bear the load of roofs, floor and of course self weight. The most constructive use of load bearing is seen in load bearing structural system wherein it performs a range of functions from supporting of loads, subdividing the space (creating room), providing thermal and acoustic insulation to structure, as well as fire and weather protection, which normally in a framed building has to be accounted for separately by means of walls. However as loads coming on the structure are taken care by walls of structure, the thickness of walls at bottom increases to a considerable extent. Hence masonry structures are found to be very uneconomical beyond 3 to 4 stories.
In Asian countries there is not much progress in the construction of tall load bearing masonry structures mainly because of poor quality of workmanship for masonry and clay bricks which even today have only 3.5 to 10 MPa strength. However recently mechanized brick plants are producing brick units of strength 17.5 to 25 MPa and therefore it is possible to construct 5 to 6 storey load bearing structures at a cost less than those of RCC framed structural system.
Reinforcement in masonry can further improve its load carrying capacity and improve flexure and shear behavior under earthquake loads. Now a days masonry units are being manufactured in such shapes and sizes that make reinforcement embedding in masonry less cumbersome.
There are different building codes for masonry structures in world and they are as follows:
- IS (Indian Standard) 1905-1987
- ACI (American Concrete Institute) 530-02
- ASCE (American Society of Civil Engineers) 5-02
- TMS (The Masonry Society) 402-02
- ICC (International Building Code) 2000
- NZS (New Zealand Standard) 4230: Part 1:1990
- Euro 6
From all the above codes, Indian Code provides a semi empirical approach to the design of unreinforced masonry, especially for stresses arising from vertical and moderate lateral loads, such as wind. The permissible stress values are not directly linked to prism test values and do not address the strength and ductility of masonry members under large lateral loads due to earthquakes.
Further the use of reinforcement is necessary to improve its flexural resistance and ductility required for seismic loads. The load bearing system needs many elements like plinth band, lintel band, roof band, corner reinforcement, reinforcement at openings etc to make it safer against earthquake loads. This needs slightly skilled and experienced manpower.