Review of basic principles of steel structure

Basic principle of steel structure Chapter one The introduction 1、 Advantages of steel structure ： （1） High strength 、 Light weight ; （2） Good material properties , High reliability ; （3） High degree of industrialization , Short construction period ; （4） Good sealing ; （5） Good seismic performance ; （6） Good heat resistance ; 2、 Disadvantages of steel structure ： （1） Steel is relatively expensive ; （2） Poor corrosion resistance ; （3） Poor fire resistance ; Chapter two Steel structure materials 1、 Requirements of steel structure for steel ： （1） Have high strength ; （2） Good plasticity ; （3） Good impact toughness ; （4） Good cold working performance ; （5） Good solderability ; （6） Good durability ; 2、 Yield point and flow amplitude are two important mechanical properties of steel , The former refers to the strength index of steel , The latter represents the plastic deformation index of steel . 3、 Working performance of steel ： （1） In the design, the yield point is taken as the maximum stress that the steel can achieve ; （2） The steel conforms to the ideal elastoplastic constitutive ; （3） The plastic deformation of steel before failure is very large , Almost equal to elastic deformation 200 times ; （4） Yield ratio can be regarded as a coefficient to measure the strength reserve of steel , The lower the yield ratio, the greater the safety reserve of steel . 4、 Elongation cannot represent the maximum plastic deformation capacity of steel , However, it is easy to produce large errors when measuring the reduction of area . 5、 The plasticity index of steel is more important than the strength index , Plasticity can adjust the initial defect . 6、 Aging phenomenon ： Under intermittent repeated load , The yield point of steel is increased , Reduced toughness , And the ultimate strength is also slightly improved . 7、 fatigue ： After repeated loading , The strength of steel decreases . 8、 The fourth strength theory （ Reduced stress ）：

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9、 The shear yield point of steel is of the tensile yield point 0.58 times . 10、 toughness ： The ability of steel to absorb energy during plastic deformation and fracture , It is an index to measure the impact resistance of steel . 11、 Cold bending performance ： When plastic deformation occurs in steel during cold processing , Resistance to cracks , It is a comprehensive index to measure the mechanical properties of steel .

12、

13、 Good weldability means that the weld is safe 、 reliable 、 No welding cracks , Impact toughness of welded joints and welds and elongation of heat affected zone （ plasticity ） And mechanical properties are not lower than the base metal .

14、 Carbon equivalent is a measure of the comprehensive performance of the carbonization reaction of the base metal after butt welding of various elements in ordinary low alloy steel .

15、 Plastic failure ： Also known as ductile failure .

16、 Brittle failure ： There is no obvious deformation before destruction , The average stress is small .

17、 The shear stress first exceeds the shear capacity of the grain , Plastic failure will occur ; The tensile stress first exceeds the tensile capacity of the grain , Brittle failure will occur .

18、 The direct factor affecting the brittle fracture of steel is the crack size 、 Force and toughness of materials .

19、 The main measures to improve the brittle fracture resistance of steel ：

（1） Strengthen the management of welding process ;

（2） The weld should not be too concentrated , During welding, it is not advisable to restrict too strongly , Avoid excessive residual stress ;

（3） Carry out reasonable detailed structural design , Avoid stress concentration ;

（4） Choose reasonable steel ;

20、 Fatigue failure ： Steel under continuous repeated load , Although the stress is still lower than the ultimate strength , Even below the yield point , There will also be damage . It belongs to brittle failure under repeated load .

21、 Fatigue limit ： The stress cycle is repeated infinitely many times , The test piece will still not be damaged .

22、 Damage ： Plastic damage 、 fatigue damage 、 Material change 、 Creep damage, etc .

23、 Chemical composition affects steel properties ：

（1） carbon ： Yield point and tensile strength increase ; But plasticity and toughness , Especially the impact toughness at low temperature decreases , Solderability 、 Corrosion resistance 、 Fatigue strength and cold bending performance decreased significantly .

（2） silicon ： Used to make high quality killed steel . An appropriate amount of silicon can greatly improve the strength , For plasticity 、 Impact toughness 、 Cold bending performance and weldability have no obvious influence ; Too much reduces performance .

（3） manganese ： Increase the strength moderately , Eliminate hot brittleness , Improve the tendency of cold brittleness ; Excess makes steel brittle , Reduce rust resistance and weldability .

（4） vanadium ： Improve strength and rust resistance , No significant reduction in plasticity , Sometimes it has hardening effect .

（5） sulfur ： Greatly reduce plasticity 、 Impact toughness 、 Fatigue strength and rust resistance , High temperature embrittlement produces cracks —— Hot crisp .

（6） phosphorus ： Improve strength and rust resistance , But the plasticity is seriously reduced 、 Impact toughness 、 Cold bending performance , Cold and crisp .

（7） Oxygen and nitrogen ： Oxygen hot crisp , Nitrogen is cold and crisp .

24、 Boiling steel ： Manganese as deoxidizer , plasticity 、 toughness 、 Poor weldability , Easy to age and become brittle .

25、 Killed steel ： Silicon as deoxidizer , Low yield , The high cost , High yield point 、 Good impact toughness 、 Cold bending performance 、 Solderability 、 Good rust resistance , Less time sensitive .

26、 Hot rolled steel , Due to uneven cooling, residual stress will occur , Generally, tensile stress is generated at the place with slow cooling , Compressive stress occurs in the place where cooling is early . Residual stress self balance .

27、 Sclerosis ： Under repeated load , The elastic limit of steel has been improved .

28、 Blue brittleness ： stay 250℃ Left and right , The tensile strength of steel increases while the impact toughness decreases .

29、 Low temperature cold crisp ： When the temperature drops to a certain value , The impact toughness of steel suddenly drops sharply , The fracture of the specimen is brittle .

30、 In general , Due to the good plasticity of structural steel , When the internal force increases , The phenomenon of uneven stress distribution will gradually flatten .

31、 The principle of selecting steel ：

（1） Type and importance of structure ;

（2） The nature of the load ;

（3） Connection method ;

（4） The working temperature of the structure ;

（5） The mechanical properties of the structure ;

（6） Structural form and steel thickness ;

The third chapter Possible failure forms of steel structures

1、 Possible failure forms of steel structures ：

（1） The overall instability of the structure ;

（2） Local instability of structures and components ;

（3） Plastic failure of structure ;

（4） Brittle fracture of structure ;

（5） Fatigue failure of structure ;

（6） Cumulative damage of the structure ;

Chapter four Tension member and cable

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The fifth chapter Axial compression members

1、 In general, the instability form of axial compression members ：

a) Biaxial symmetry ： I-type 、H shape , Only bending instability occurs ;

b) Uniaxial symmetry ： Asymmetric I-shape 、T shape 、 Groove shape , Buckling around an asymmetric axis ; Bending torsional instability around the axis of symmetry ;

c) No axis of symmetry ： Unequal limbs L shape , Bending and torsion instability ;

d) Centrosymmetry ： Cross 、Z shape , Torsional instability .

2、 Strength of axial compression members ：

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3、 Overall stability ：

a) Euler critical force （ Ideal elastic axial compression bar ）：

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b) Ultimate bearing capacity ：

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c) The converted slenderness ratio of lattice members

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According to the formula

4、 Local stability ：

a) Stability coefficient k Take the corresponding value according to the bearing conditions

b) After buckling, the plate will have a great bearing capacity , Its post buckling strength comes from the transverse membrane tension in the plate .

c) The local instability of solid web members is controlled by the width thickness ratio of plates .

d) In order to ensure that the stability of single limb of lattice column is not lower than that of the whole , Its slenderness ratio

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a) The rolled section does not need to be checked for local stability , Automatically meet ;

Chapter six Flexural member

1、 The main failure form of flexural members ：

a) Section strength failure ： After the edge yields, the bearing capacity is judged according to the size of the elastic core ; The fabrication site is damaged by shear ;

b) Overall instability ： In the action plane of bending moment 、 The bending moment acts out of the plane ;

c) Local instability ： It is divided into elasticity and elastoplasticity ; Elastic local instability means that the bending member is locally damaged , The bearing performance began to deteriorate , But it is not necessarily used as the criterion for judging the damage of the whole component .

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6、 Shear center ： Bending center 、 Torsion center , The center of rotation where the section is twisted ; In design, the action line of transverse force should be close to the shear center .

7、 Free twist ： The section stress is the shear stress caused by torsion ; The torsion angle per unit length is equal everywhere

a) A pair of torques with equivalent response on the section ;

b) The longitudinal fibers of the end section of the member are not restrained .

8、 Restraint torsion ： There are equivalent reverse double moments on the upper and lower flanges .

9、 warping ： When restraining torsion , The member section is no longer maintained as a plane .

10、 Overall instability , The main factors affecting the critical moment ：

factors	Critical bending moment
Lateral bending stiffness 、 Torsional stiffness 、 Anti warping stiffness	+
Member span	-
Section asymmetry	+
Load action mode coefficient	+
Position of transverse load	+
The degree of constraint of support on displacement	+

11、 It is not necessary to calculate the overall stability ：

a) The slab with sufficient rigidity covers the compression flange of the bending member and is firmly connected with it , It can effectively prevent the lateral deformation of the hospital under pressure ;

b) The free length of the bending member is less than a certain critical value .

12、 Local stability design criteria ：

a) The critical stress of local instability of the plate is not less than the yield strength of the material ;

b) The critical stress of local instability of the plate is not less than the critical stress of overall stability of the member ;

c) The critical stress of local instability of the plate is not less than the actual working stress .（ Most economical ）

13、 Truss mechanism ： After shear local instability of simply supported beams with transverse stiffeners , The principal compressive stress remains unchanged , The principal tensile stress increases . The upper and lower flanges of the beam are like upper and lower chords , The transverse stiffener is like a compression vertical bar , The diagonal tension belt in the unstable section acts as a tension inclined rod .

14、 Repeated load 、 In plastic design , The use of post buckling strength is not considered .

Chapter vii. Bending members

1、 The form of destruction

Strength failure	Overall instability	Local instability
Similar to flexural members	In plane instability 、 Out of plane instability	Similar to the axial force bearing member

2、 Strength

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3、 Overall stability

a) Overall stability in plane

i. The section composed of plates with considerable width thickness ratio （ Cold formed thin-walled members ）, It is less likely to develop plasticity in the full section , Generally, the edge yield criterion is used as the design criterion .

ii. Two ways to determine in-plane stability ： Test data statistics 、 Numerical analysis of mechanical model

iii. Calculation method of in-plane stability bearing capacity ：

name	The formula	characteristic
Single expression		Values are affected by many factors , Inconvenient to use .
The relevant formula		Partially developed plasticity

b) Overall stability out of plane

i. The overall out of plane instability presents a bending and twisting state .

ii. Calculation formula ：

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c) Lattice member

One limb is stable	For lacing , The stability of a single limb under the axial force of each rod can be calculated according to the axial force bearing member . For battens , Local bending moment caused by shear force shall be considered for in-plane stability .
Batten stability	Similar to axial compression

4、 Local stability

a) Design criteria ：

i. Local instability is not allowed

ii. Consider using post buckling strength

Chapter viii. Connection of steel structure

1、 Welding method

Arc welding	The most commonly used , Welding of different steel grades , Welding rods suitable for low-strength steel should be used
Electroslag welding	A kind of arc welding , It is often used in the welding of internal diaphragm and column of box column in high-rise buildings and other steel structures
Resistance welding	It is applicable to the thickness of plate stack not exceeding 12mm
Gas welding	Sheet steel 、 Small structure

2、 Welding form

a) butt weld ： Straight seam 、 Oblique seam ;

b) Fillet welds ： Side seam 、 End seam .

3、 Advantages and disadvantages of welded structure

a) advantage

i. There is no need to drill holes , Save time and labor , The section is not weakened ;

ii. Unlimited shape 、 No auxiliary parts 、 Simple construction 、 The transmission route is short 、 It has a wide range of adaptability ;

iii. Air tightness 、 Good water tightness , The structure is rigid , Good integrity ;

b) shortcoming ：

i. The heat affected zone makes the steel brittle ;

ii. Residual stress is prone to brittle failure , Reduce the critical load of pressure bar stability , Affect size and shape ;

iii. Local cracks are easy to spread to the whole ;

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1、 Failure form of shear bolt ：

a) The screw is sheared and damaged ;

b) Steel plate hole wall crushing failure ;

c) Net section fracture ;

d) Edge punching failure ;

e) Bending failure ;

2、 Ordinary bolt calculation ：

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