CE 636 - Design of Multi-Story Structures T. B. Quimby UAA School of Engineering • Horizontal Systems (Floors & Roofs) – Joists, purlins, girders – One-way slabs – Two-way slabs – Waffle Slabs – Flat plates – Space Trusses • Vertical Components – Columns – Load Bearing Walls • Horizontal systems are generally unaffected by lateral loads • • Vertical components often are affected by both gravity and lateral loads. • • • • • • • • • • • The system consists of: a horizontal slab or sheathed surface. a system of beams (i.e. joists, purlins, girders) supporting the bearing surface. columns or walls that support the beam system. Use manufacturers load tables. Used without concrete for roof decks Used with concrete for floor decks. Lots of different profiles and gage thicknesses. Design connection to supports for diaphragm shears Use Manufacturer's Span Tables Industry standards have been produced by the Steel Joist Institute. Most manufacturers build their joist to SJI specifications Slab spans between supports on two opposite sides Generally true for slabs with an aspect ratio of 3:1 or more. Analyzed and Designed as a beam without shear reinforcement. Reference #1, pg. 164 Support on all edges. May be “fixed” or “free” at supports. Moments and shears are somewhat less than seen in one-way slabs of comparable span. Precise analysis is somewhat complex. Reference #1, pg. 15 Reference #1, pg. 161 A grid consisting on beams running in two directions. Slab spans between joists. Easy to construct with reusable “pans”. Fairly common in areas that use lots of concrete. Reference #1, pg. 192 Reference #1, pg. 168 Slabs supported by columns only. Slab is divided into strips. Each strip is designed as a beam to carry a portion of the load. Punching shear at columns is often a critical problem. The problem becomes very complex if the columns are not laid out on a rectangular grid. Reference #1, pg. 163 Reference #1, pg. 169 Generally steel, sometimes wood or concrete. Used mostly for exposed for roof structures. Can be used to achieve many different shapes. Often used for long span roof structures. Reference #2, pg. 54 Reference #1, pg. 198 1. 2. 3. Lin, T.Y., and Stotesbury, S.D., Structural Concepts and Systems for Architects and Engineers, 2nd edition, Van Nostrand Reinhold, 1988. Ambrose, J., Building Structures, Wiley Interscience, 1988. Council on Tall Buildings & Urban Habitat, Advances in Tall Buildings, Van Nostrand Reinhold, 1986.