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Gravity Force Resisting SystemsPowerPoint Presentation

Gravity Force Resisting Systems

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### Gravity Force Resisting Systems

CE 636 - Design of Multi-Story Structures

T. B. Quimby

UAA School of Engineering

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.

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.

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