Steel design

Advantages and disadvantages of steel structures

Advantages

High strength (~200–500 MPa) $\implies$ weight is small
Elasticity (Hooke's law) E=210 GPa
Permanence (under some conditions no painting needed)
Ductility (large deformations without a failure)
Toughness: have both strength and ductility
Variety: ability to have many sizes/shapes; ability to be fastened together

Disadvantages

Corrosion (air, water)
Fireproofing costs
Suspectibility to buckling
Fatigue (repeated reversals with tension)
Brittle fracture (that fits to some kinds of steels and to winter temperatures)

Iron & Steel

Iron was in use thousands of years ago already. There is a story of Battle of Marathon (490 BC): greatly outnumbered Athenians killed 6400 Persians and lost only 192 of their own men. The victors wore 25 kg of iron armor.

Steel containts a small amount of carbon, usually < 1 %. The mass production of steel had started in USA in 1870—1890.

Low carbon content steel	High carbon content steel
Plastic strain is usually 10—15 times larger as the elastic strain; Failure might be at strain 100—150 times of elastic strain.	High strength but not ductile; difficulties with welding.

Low carbon content steel

High carbon content steel

Plastic strain is usually 10—15 times larger as the elastic strain;
Failure might be at strain 100—150 times of elastic strain.

High strength but

not ductile;
difficulties with welding.

1—Low carbon steel is ductile, it can sustain large deformations without a failure, it behaves under the Hooke's law until yield is being achieved (elasticity);
2—High strength steel is brittle, the yield stress is not readily available from the diagram and is defined as stress that corresponds to 0.002 (0.2 %) of permanent strain when unloaded. The steel with more components added might have other special quality as corrosion resistance.

Temperature

Lowering the temperature can strengthen the steel but steel becomes brittle. For illustration, increasing the temperature to

600 °C → the strength is roughly at 50 %,
750 °C → the strength is less than 20 %.

Common shapes

Variety of sections is available and sections can be combined to make a new built-up member. The structure should not contain many cross-sections. Such structure will be complicated to assemble, suspectible to mistakes during construction. As a result it can be more expensive than the "less efficient" case.

1 (top)—Most common sections; 2 (bottom)—built-up sections

Design philosophies

Required strength	≤	Available strength
(from a load)		(property of material/section)

(might be a force, a moment, ...)

	or

Maximum applied stress	≤	Allowable stress
(from a load)		(property of material)

Load is usually increased (exceptionally decreased) by a load factor (uncertainty, combinations of loads)		(strength is usually decreased by a safety factor)

The factors depend on a norm/code and national dialects. The allowable strength (stress) is decreased more when dimensioning on tensile strength $f_u$ (ultimate strength). Eurocode works with both above approaches.

Characteristic loads are loads which have an acceptably small probability of not being exceeded during the lifetime of the structure. The characteristic strength of a material is the specified strength below which not more than a small percentage (typically 5%) of the results of tests may be expected to fall. Partial safety factors $(\gamma_M)$ are the factors applied to the characteristic loads, strengths, and properties of materials to take account of probability.

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