Material Types and Properties::Design::Knowledgebase::SAFAS

Various materials such as steel, aluminum, wood, concrete and plastics have been used for the manufacture of spatial structural components. Steel is the most common material used. Steel spatial structural members have been made from cold-formed sections, and hot-rolled shapes such as channels, angles, hollow round or square tubes, bolted or welded to connectors (nodes) or to each other. Hollow tubes or Hollow Structural Sections (HSS) have been the most common shape used for spatial structures. In the event of fire, the structural steel members can lose their strength greatly due to the elevated temperatures which may lead to the total failure of the spatial structure. Therefore, protective measures against fire should be considered. Since spatial structures are generally highly indeterminate systems, the loss of one or a few members may result in the redistribution of the internal forces and may not cause a total failure.

After steel, aluminum has also been commonly used for spatial structure components such as round or rectangular tubes, rods or connectors. Compared to steel, aluminum has the following advantages and disadvantages when used in spatial structures:

Strength to weight ratio: The allowable stress for aluminum in tension and compression (disregarding the member buckling) is about 19 ksi for 6061-T6 aluminum. For A-500 steel this is about 26 ksi. Aluminum unit weight and modulus of elasticity are about 1/3 that of steel. This light weight results in cost savings for shipping and erection of aluminum structures. Therefore, for long span spatial structures, using aluminum results in substantial cost savings as compared to steel.
Seismic zones application: Spatial structures made of steel or aluminum with a large number of connectors have inherent stiffness and ductility, which provide great earthquake-resistance. In addition, the joints can act as energy-absorbing components for the structure. Since aluminum is lighter than steel, the seismic loads are generally smaller if an aluminum spatial structure is used. Therefore, a spatial structure made of aluminum performs better than a steel structure when subjected to earthquake forces.
Corrosion resistance: Exposed steel has to be protected against corrosion, which can be achieved by hot-dip galvanization. The corrosion of aluminum depends on its alloys as there are sea-water resistant and sulfur-acid-resistant alloys. Aluminum has a better corrosion resistance than steel, which results in less maintenance costs.
Forming: Aluminum can be easily formed into any shape and wall thickness.
Extreme low temperatures: The ultimate tensile strength and ductility of aluminum increase significantly at low temperatures. This makes it an ideal material for cold weather environments.
Thermal behavior: The coefficient of thermal expansion of aluminum is larger than that of steel, therefore, provisions for the movement of the aluminum structures due to changes in temperature have to be made.
Welding: Welding of aluminum components is generally more difficult than steel members. Therefore, steel is more commonly used for structural Components of Spatial Structuress when welding is required.
Costs: Aluminum structures are generally more expensive than their steel counterparts.

The modulus of elasticity of steel (E) is 29,000-30,000 ksi, for aluminum it ranges between 10,000 - 10,500 ksi. The yield strength (Fy) for high-strength steel is about 75-270 ksi, and for mild steel it ranges between 36-50 ksi.