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

2. 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.

3. 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.

4. Typical Floor Plan

5. Some Typical Sections

6. 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

7. 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

8. 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.

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

10. 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.

11. Approximate Flat Slab Behavior
Reference #1, pg. 15

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

13. 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.

14. Concrete Waffle Slab
Reference #1, pg. 192

15. Waffle Slab Behavior
Reference #1, pg. 168

16. 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.

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

18. Punching Shear Resistance
Reference #1, pg. 169

19. 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.

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

21. Alternative Truss Layouts
Reference #1, pg. 198

22. 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.