Gravity Force Resisting Systems CE 636 - Design of Multi-Story Structures T. B. Quimby UAA School of Engineering Gravity Force Resisting Systems

Basic Elements • 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.

Joists, Girders, Purlins 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.

Typical Floor Plan

Some Typical Sections

Steel Deck 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

Steel Joists Use Manufacturer's Span Tables Industry standards have been produced by the Steel Joist Institute. Most manufacturers build their joist to SJI specifications

One-Way Slabs 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.

One-Way Slab and Beam Behavior Reference #1, pg. 164

Two-Way Slabs 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.

Approximate Flat Slab Behavior Reference #1, pg. 15

May use Stiff Beams for support Reference #1, pg. 161

Waffle Slabs 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.

Concrete Waffle Slab Reference #1, pg. 192

Waffle Slab Behavior Reference #1, pg. 168

Flat Plates 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.

Flat Slab Supported on Square Column Grid Reference #1, pg. 163

Punching Shear Resistance Reference #1, pg. 169

Space Trusses 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.

Roof for a High School Gym Reference #2, pg. 54

Alternative Truss Layouts Reference #1, pg. 198

References 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.