Design of a Residential Soft-Story Structure
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Ahmad Husam Zarour
Bashar Emad Abdulhadi
Muath Naser Abo Al-Zait
An earthquake is without a shadow of a doubt one of the most challenging events that any structure can be subjected to. In the event of an earthquake, a given structure’s behavior during its brief occurrence can vary widely depending on many different factors, such as, the lateral stiffness of the structure’s stories in the direction of the earthquake occurring, the structure’s geometry, orientation and height. The growth in population worldwide and the scarcity of land nowadays, forces most building owners to facilitate parking space below the residential floors, with it either being under the building, or on the building’s ground floor. Due to the nature of parking spaces and the need for maximum space for cars to maneuver freely, relatively small columns and limited shear walls are usually used in such buildings. These factors combine to form a story which has a lateral stiffness considerably smaller than that of the stories above it, such a story is called a “soft story”. Buildings that have such a story perform extremely poorly during earthquake occurrences and usually collapse through their soft story. It should be noted that the soft story problem is not exclusive to buildings that have parking spaces within them, but also to buildings that have relatively large storage areas within them and even to some building where the architectural engineer imposes certain aesthetic limitations that prevent the use of large column sections or shear walls. A study of a singular case of a soft story building and its behavior under earthquake conditions is conducted in this project. The building in question is a six-story residential building in the Palestinian city of Nablus that is to be built on a sloping ground. This building has parking spaces allocated in an underground story, which uses minimal shear walls, relatively small columns, and thus, is a soft story candidate. Moreover, the ground floor in this building is also a soft story candidate, due to the fact that it is considerably taller in height compared to the other floors. During this study, an attempt was be made to find a solution to this problem. After that, the structure was designed and full structural details were provided. This was done using the help of the computer program “ETABS”, where the building was modeled. The model was then analyzed by the mentioned computer program to determine its viability by finding a set of variables, such as, the story drift, the base shear, the deflection values and other structural parameters. It should be noted that all analysis results and design details, were manually tested to make sure that all final results were accurate and within an acceptable range of error. A general consensus on the most practical and viable solution to this problem, locally, was agreed to be the addition of shear walls. For this project, the placement of two 25-centimeter shear walls on both sides of the building was sufficient to solve the soft-story problem.