Message from Chairman PEC Engr. Dr. Muhammad Akram Sheikh

ENGINEER PEC September 2007Lateral displacementAfter the earthquake or wind the building move which isknown as drift. In tall building the design is controlled bydrift design and structural stability is dependent on it. Theratio of maximum lateral displacement to the height of thebuilding is called drift ratio. A story drift is relativedisplacement of a floor to the story height at that level. Driftlimitation by seismic codes serves as guide to safe guardthe structural integrity. The increase in lateral stiffness, thecritical load of building increases, reducing failures.However, the gravity loads play a key factor against lateralstability and rotation at the base structure.DiaphragmsThe diaphragms support the entire gravity load andtransfer these to columns and walls. Further, thesetransmit the wind and seismic forces to frames andstructural walls. The building acts as a unit andredundancy and strength is increased. In design, the floorstiffness below and above diaphragms can causeproblem. In buildings having many wings such as L-shape,H-shape etc. junctions are to be dealt carefully as stress atjoints of wings may increase. The diaphragms aredesigned according to codes and deflection of diaphragmsshall not exceed the permissible deflection of attachedelements. Diaphragms designs can be further simplifiedfor three-storey building or lower according to codes. Thecontinuous ties are to be provided between the supportedwalls and the diaphragms in order to distribute anchorageforces. In wooden diaphragms the ends should beperfectly embedded in walls and projected wings properlydesigned. The floor slab and topping should not be lessthan two inches.Miscellaneous design techniquesMasonry wallsTwo procedure are adopted, one for slender wallsdesigned for out of plan forces (perpendicular to wall), thesecond one for in plan forces (parallel to wall length)stFlexible 1 floorChange of stiffnessDiscontinuousShear Wallis like horizontal continuous beam supported by vertical requiring steel at ends for support to resist shear, momentslateral load resisting elements. (Floor deck as web and and over turning. Walls having height, 30 times ofperiphery beam as flanges of continuous beam).thickness are called slender walls. The block masonrywhether hollow or solid may be used in panel walls subjectThe shear and bending moments are worked out and three to condition that the minimum thickness should be sixdimension analyses are needed for distribution of forces inches. The ratio of unsupported height may not exceedon vertical elements. For few stories, a simple analysis is 30-time thickness. Minimum steel for temperaturesufficient to determine the distribution of lateral forces./shrinkage (.003 each direction) and maximum notexceeding, 3% of cell area be provided. No splicing of theThe diaphragm rigidity plays a major role in the design, as main steel in the wall be allowed and units continuouslythe diaphragms can be rigid, flexible and semi-rigid. The poured and joined to each other.definitions of rigid means that lateral forces are transferredas per relative stiffness. In flexible diaphragms, horizontal Existing structuresforces to the vertical load resisting elements areindependent of their stiffness. The diaphragms are semi- The existing structures should be made earthquakerigid, when deflection of diaphragms and vertical elements resistant to a level desired by the requirements. Thecan not be assured rigid or flexible. The diaphragms strength, materials, detailing and quality affect theproblems are more in low-rise building and in stiffer column performance. Other considerations are the age of/ shear walls, in taller buildings. Abrupt changes in wall structure, economy, occupancy, losses, history, site9