Building Resistant to Earthquake through Hidden/Strip Beam at Footings

Professor S. V. Deodhar, Ex Director SSVPS College of Engineering Dhulia (Maharashtra), Professor of Civil Engineering Rishiraj Institute of Technology Indore (M.P.)

Studies on hidden beams have been carried by various researchers. Its use for high rise buildings with raft foundation is of very useful. However, its use for low rise buildings may not be effective to a considerable extent. In this paper, methods have been suggested to improve lateral load resisting capacity of low rise buildings during earthquakes.

Introduction
Earthquake causes strong random ground movement and ground shaking. These ground accelerations cause building to vibrate and thus induce inertia forces in them. Earth movement causes horizontal forces on foundation causing foundations to move along with soil. These cause deferential settlement causing building structures to develop severe cracks and consequently collapsing. In case of low rise structures of height about 12 to 14 meter having 4 to 6 floors including soft story, isolated footings are provided as foundation. These structures have less resistant to earthquake forces as having a soft story, mostly fails to resist earthquake forces. These five to six floors consists of ground floor as parking floor, called as soft story and three to four floors for residential purpose and fifth or sixth floor as terrace house or pent house, which, sometime, is an illegal construction.

For buildings beyond this, raft foundation or other type of foundation is used. As far as raft is concerned, it has good resistant to earthquake forces. The raft provides resistant to soil movement and therefore shear developed is resisted by huge concrete mass in the form of raft. However, in case of individual footings, soil movement is comparatively is easy and this earth movement causes tilting of building due to differential settlement of the soil-foundation system causing building to collapse, mainly due to unequal settlement. If this earth movement and earth/soil shaking could be resisted by some means, probably earth may move or vibrate but the soil-foundation system may resist these forces, minimizing possibility of collapse of structure all of sudden.

Hidden beams have been suggested by Deodhar and Dube. More studies have been carried by Deodhar. He has also given various locations where these hidden beams can be provided. However, they are effective in case of soft story. Further they resist movement of soft and other stories and minimizes possibility of sudden collapse of the structure. However, in case of soil movement, during earthquakes of small magnitude / intensity but of large duration or of large magnitude / intensity but of small time duration, these hidden beams may not be effective to that extent. Further, these hidden beams may be effective for high-rise / tall buildings where Raft / Mat foundations is provided. But in case of low-rise buildings of height 12 to 14 or 15 meter having 5 to 6 story construction with a soft story for parking purpose, having isolated or combined footing type of foundation, these hidden beam may not be effective to a considerable extent.