Primarily Occur as:
- Columns in buildings;
- Chord Members in trusses and diagonal members in end panels of trusses
- Stability is an important consideration in design and behavior of compression members
- Area is generally spread out to maximize Radius of Gyration
- Primarily loaded transverse to the longitudinal axis and resist loading by flexure
- X-sectional area is located as far from the neutral axis as is practical
- Commonly W shapes are used in most cases
- For deeper beams I-shaped sections made by welding plates are commonly used
- For smaller loads and spans open-web joists are commonly used
- Instability due to lateral Torsional Buckling is an important consideration
Open-Web Steel Joist
- Economical & popular for Roof & floor framing.
- Bar joist are supported by bearing walls or steel girders.
- Floor of thin concrete reinforced in both directions with rebars or welded wire fabric.
Classification Based on Shape
- Classical Skeleton framing
- Steel truss
- Rigid frames
- Cable supported Roofs
- Classical system supported by beams, girders and columns.
- Beams: W or S shapes, Channel shapes for roof purlins
- Columns: generally W shapes
- Triangular rigid structure
- Most common double pitched roof trusses
- Fink & Pratt.
- Most common flat trusses: Pratt & Warren
- No span limit
- Often prefabricated
- Used with wood or steel purlins to support the roof.
- Bracing: if resting on masonry walls
- Diagonal bracing in alternate bays
- Continuous struts(angles, channels
- For large unobstructed floor areas and ceiling heights.
- Spans generally 40' to 100'
- Members are connected by bolting and welding
- Members: W shapes or web plates fillet welded to flange plates
- Connection to foundation with a base plate(bolted)
Steel Arch Structures
- Used in field houses, exhibition halls,. with span over 300'
- Most common type: three hinged arch
- Lateral bracing;
- Diagonal bracing in curved surfaces between arches
- Lateral bracing of ribs with purlins or trussed purlins.
- Used for large circular areas: Assembly halls, gymnasium, field houses..
- Spans up to 400' in diameter
- Structural members:
- Perimeter (Tension ring)
- Domes and rings are supported by columns braced laterally or by bearing walls
- Purlins supports the roof deck and span between ribs
Cable Supported Roofs
Chicago O'Hare International Airport Restaurant
- Reinforced concrete compression ring 190'dia
- Ring is supported on 26 RCC columns 58ft above ground
- Tension ring W-shape 13' dia
- Sag of cables 10.5'
- Ends of cables are anchored to the two rings
- Roof deck: precast RCC slabs 3.5" thick which fit between cables with projecting ends of reinforcement hooking over the cables.
Examples of Famous Steel Structures
Eiffel Tower, Paris
- The World's tallest structure at times(990').
- Was originally built to last 20 years.
James R,Thompson Center, Chicago
- The building is enclosed by 17 story curtain walls.
- The diameter of rotunda is 160'
- The rotunda projects as a cylinder and its top resembles a drum without a dome slanting towards plaza.
- Office spaces are between rotunda's walls and outer skin
The Geodesic Dome at Walt Disney World
- Spaceship earth is a huge golf ball, standing 180 ft.
- The structure is designed to withstand wind speeds of 200 mph
- Structure: steel framing clad with faceted aluminum panels, and stands on 3 pairs of steel legs.
- Site was mostly swamp, filled with much, organic material with 95% water content.
- As a solution, 2.5 million cubic yards of soil was removed and replaced by clean material
Indoor Football Facility,University of Illinois, Urbana
- The roof structure is semi-parabolic dome
- A large single arched box truss spans the length of the field and supports 1/2 of roof load.
- 110 Stories Tall
- Total Height = 1725 ft
- Based on revolutionary Bundled Tube Design
- Rigid outer walls act as walls of hollow tube
- There are 9 tubes in all
- The number of tubes reduces with height
- Designed by late Fazl-ur-Rehma from Bangladesh
- Supported by 114 piles