CONCRETE STRUCTURES
CONCRETE STRUCTURES 
Concrete: ======= high in compression low in tension (tensile strength of concrete will be neglected in calculations) Steel Reinforcement: ================ resists all tension place steel where bending occurs (gravity loaded beam - bottom typically) Design Methods: ============ Concrete Design is based on empirically derived equations! Working Stress Design = Elastic Design = Alternate Design ACI Code 1963 last used Application of the working stress method consists of designing members to WORK in an adequate manner (without exceeding established stress limits) under actual service load conditions outdated - assumes that concrete behaves (cracks) like a linear-elastic material Strength Design The basic procedure in strength design is to design members to FAIL Thus the ultimate strength of the member at failure (called its design strength) is the only resistance considered used under current ACI code Safety Factors: =========== Working Stress safety in working stress method is provided by limiting stresses Strength Design safety in strength design is provided by factoring design loads (called required strength U) that is greater than the service load U = 1.4 D + 1.7 L U = required strength D = design load effect L = Live Load effect Strength Reduction Factor (phi /0) ========================== An additional margin of safety is provided by the stress reduction factor (phi /0) /0 = 0.90 for flexure axial tension and combinations of flexure and tension /0 = 0.75 for columns with spirals /0 = 0.70 for columns with ties /0 = 0.85 for shear and torsion /0 = 0.70 for compressive bearing /0 = 0.65 for flexure in plain concrete (not reinforced) Types of Beams: ============ simple beam rests on a support on each end the ends of the beam being free to rotate ---------------------- x x cantilever beam is supported at one end only --------------------- x overhanging beam is a beam whose end or ends project beyond its supports ------------------------ x x continuous beam rests on more than two supports ---------------------- x x x restrained beam has one or both ends restrained or fixed against rotation x--------------------x
