1.1 Characteristics of Steel Structures
With the advancement of human science and technology,construction materials andconstruction technologies have made qualitative leaps.Building materials have also evolvedfrom traditional timber and earth-rock to modern materials such as steel and reinforcedconcrete.For example,the world's first furnace steel was born in 1856.Nowadays,steelhas been widely used in the following fields:high-rise buildings,super high-rise build-ings,large-span bridges,spatial structures,modern single-story factory buildings,heavy-duty workshops,pressure vessel equipment,and pipelines in the petrochemical industry,marine drilling platforms,and other civil constructions.
1.1.1 Advantages
Why can steel structures be widely used in the types of buildings and structures men-tioned above?Because steel has many advantages compared to other construction materi-als,such as timber and reinforced concrete.
(1)Steel has high strength and low weight.There is a critical factor a which is theratio of material density to its strength.The lower a means the lower mass of the structureconstructed under the same load.The a of steel is about 1.7-3.7,the a of timber is about5.4,and the a of concrete is about 18,the unit is 10/m,
(2)Steel is a uniform material,and its properties have better predictability.In addi-tion,steel is an isotropic material,and some basic formulas of material mechanics can beapplied directly.

(3)Steel has good plasticity and toughness,which mcans sleel structures can be useddirctly under dynamic load and have good seismic performances under earthquakes.
(4)Stccl is good airtight and water-tight material.Therelore,after the steel struc-ture is connccted by welding,it can mect the airtight and watcr-tight requirements of somehigh-pressure vcsscls and pipclincs.
(5)Stccl structurcs are casy 1o manufacturc and construct.Stccl struclures are con-venient for prefabrication in the factory.Steel structures are easy to install on the con-struction sic.So,stccl structures arc natural prefabricated structures and can help to re-duce the construction cost.
(6)The seismic performance of the steel structure is good.Under appropriate designconditions,the steel structure is the least damaged in previous world-renowned earth-quakcs.As a result,it has been recognized as a structure suitable for use in earthquake-re-sistant areas.