In order to ensure the bearing capacity of load-bearing structure and prevent brittle failure under certain conditions, appropriate steel grade and material properties should be selected according to the importance of structure, load characteristics, structural form, stress state, connection method, steel thickness and working environment.

Q235 steel, Q345 steel, Q390 steel and Q420 steel should be used for bearing structure, and their quality should meet the requirements of current national standards carbon structural steel GB / T 700 and low alloy high strength structural steel GB / T 1591 respectively.When other grades of steel are used, they shall meet the requirements of relevant standards.For Q235 steel, killed steel or semi killed steel should be selected.

The steel for load-bearing structure shall be qualified for tensile strength, elongation, yield strength and sulfur and phosphorus content, and carbon content for welding structure.

The steel of welded load-bearing structure and important non welded load-bearing structure should also have the qualification guarantee of cold bending test.

For the welding structure steel which needs to check fatigue, it should have the qualified guarantee of impact toughness at room temperature.When the working temperature of the structure is equal to or lower than 0 ℃ but higher than – 20 ℃, Q235 steel and Q345 steel should have qualified guarantee of 0 ℃ C impact toughness; Q390 steel and Q420 steel should have qualified assurance of impact toughness at – 20 ℃.When the working temperature of the structure is equal to or lower than – 20 ℃, Q235 steel and Q345 steel should be qualified with – 20 ℃ impact toughness; Q390 steel and Q420 steel should be qualified with – 40 ℃ impact toughness assurance.

For the non welded structure steel which needs to check the fatigue, it should also have the qualified guarantee of normal temperature impact toughness. When the working temperature of the structure is equal to or lower than – 20 ℃, the qualified guarantee of 0 ℃ impact toughness should be provided for Q235 steel and Q345 steel, and – 20 ℃ impact toughness guarantee should be provided for Q390 steel and Q420 steel.

When Z-direction steel is used for welding load-bearing structure to prevent lamellar tearing of steel, the material shall comply with the current national standard GB / T 5313.

The design value of steel strength (standard value of material strength divided by partial coefficient of resistance) shall be adopted according to table 2-77 according to steel thickness or diameter.The strength design value of steel castings shall be adopted according to table 2-78.The strength design value of the connection shall be adopted according to table 2-79 to table 2-81.

 Design value of steel strength (n / mm 2)

Note: the thickness in the table refers to the thickness of steel at the calculation point, and the thickness of the thicker plate in the section for the axial load-bearing member.

 Strength design value of steel casting (n / mm 2)

 Design value of weld strength (n / mm 2)

Note: 1. The welding wire and flux used for automatic welding and semi-automatic welding shall ensure that the mechanical properties of the deposited metal are not lower than the relevant provisions in the current national standards “flux for submerged arc welding of carbon steel” (GB / T 5293) and “flux for submerged arc welding of low alloy steel” (GB / T 12470);

2.The weld quality grade shall meet the requirements of the current national standard code for acceptance of construction quality of steel structures (GB 50205).It is not suitable to use ultrasonic testing to determine the quality grade of butt weld of steel with thickness less than 8mm;

3.The strength design value of bending compression zone of butt weld is FCW, and that of bending tensile zone is FTW.

 Strength design value of bolted connection (n / mm 2)

Note: 1. Grade a bolts are used for bolts with D ≤ 24mm and l ≤ 10d or l ≤ 150mm (according to the smaller value); grade B bolts are used for bolts with d > 24mm or l > 10d or l > 150mm (according to the smaller value).D is the nominal diameter and l is the nominal length of the screw;

2The accuracy and surface roughness of Grade A and B bolt holes, the allowable deviation of grade C bolt holes and the surface roughness of hole wall shall meet the requirements of the current national standard code for acceptance of construction quality of steel structures (GB 50205).

 Strength design value of rivet connection (n / mm 2)

Note: 1. Class I holes are classified as follows:

1）The hole drilled according to the designed hole diameter on the assembled component;

2）The holes drilled on a single part and component by drilling die according to the designed hole diameter;

3）A small hole diameter is drilled or punched on a single part, and then expanded to the designed hole diameter on the assembled component.

2The hole with designed hole diameter punched or not drilled on a single part at one time belongs to class II hole.

The above strength design value shall be multiplied by the corresponding reduction factor when calculating the structural member or connection under the following conditions:

1. single angle with single side connection

1）According to the axial force, the strength and connection are 0.85;

2）Stability calculation according to axial compression

Equilateral angle steel 0.6 + 0.0015 δ, but not greater than 1.0:

The unequal angle steel connected with short side is 0.5 + 0.0025 δ, but not greater than 1.0;

The angle steel with long sides connected is 0.70;

When δ is less than 20, take δ = 20;

2.The butt weld of single side welding without backing plate is 0.85;

3.The welding seam and rivet connection of high-altitude installation with poor construction conditions is 0.90;

4. countersunk and half countersunk rivet connection 0.80.

Note: when several conditions exist at the same time, the reduction coefficient should be multiplied.

 Physical properties of steel and steel castings

The deflections of crane beams, roof beams, roof beams, working platform beams and wall frame members should not exceed the allowable values listed in table 2-83.

 Allowable deflection value of flexural member

Note: 1. L is the span of bending member (for cantilever beam and cantilever beam, it is 2 times of cantilever length).

2. [ν t] is the allowable value of deflection (the camber should be subtracted if there is arch camber) generated by the standard value of all loads;

[ν q] is the allowable deflection value generated by the standard value of variable load.

Allowable value of horizontal displacement of frame structure: under the action of standard value of wind load, horizontal displacement of frame column top and relative displacement between floors should not exceed the following values.

1.The displacement of column top of single story frame without overhead crane H / 150

2.The displacement of column top of single story frame with overhead crane H / 400

3. column top displacement H / 500 of multi story frame

4.The inter story relative displacement of multi story frame H / 400

H is the total height from the top of the foundation to the top of the column; h is the floor height.

 Effective length of chord and web member l0

Note: 1. L is the geometric length of the member (distance between node centers); L1 is the distance between the lateral support points of truss chord.

2.The inclined plane refers to the plane which is oblique to the truss plane. It is suitable for single angle steel web member and double angle cross section web member whose two main axes of component section are not in the truss plane.

3.The effective length of web member without gusset plate is equal to geometric length in any plane (except steel pipe structure).

 Allowable slenderness ratio of tension member

Note: 1. In the structure bearing static load, the slenderness ratio of tension member in vertical plane can be calculated only.

2.In the structure bearing dynamic load directly or indirectly, the calculation method of slenderness ratio of single angle tension member is the same as that in Note 2 of table 2-86.

3.The slenderness ratio of the bottom chord of the middle and heavy duty crane truss should not exceed 200.

4.The slenderness ratio of the support (except item 2 in the table) should not exceed 300 in the workshop with hard hook crane such as clamp or rigid rake.

5.The slenderness ratio of tension member should not exceed 250 under the combined action of permanent load and wind load.

6.The slenderness ratio of tension chord and web member should not exceed 300 (bearing static load or indirectly bearing dynamic load) or 250 (directly bearing dynamic load) for truss with span equal to or greater than 60m.

 Allowable slenderness ratio of compression member

Note: 1. When the internal force of the compressed web member of truss (including space truss) is equal to or less than 50% of the bearing capacity, the allowable slenderness ratio can be taken as 200.

2.When calculating the slenderness ratio of single angle steel compression member, the minimum radius of rotation of angle steel should be used, but the radius of rotation parallel to the leg edge of angle steel can be used when calculating the slenderness ratio of cross member out of plane.

3.For the truss with span equal to or greater than 60m, the allowable slenderness ratio of compression chord and end compression member should be 100, and other compressed web members should be 150 (bearing static load or indirectly bearing dynamic load) or 120 (directly bearing dynamic load).

 Reduction factor of effective length of stepped column in single story workshop

Note: for open-air structures with beams (such as drop hammer workshop), the reduction factor can be 0.9.

 Anti sliding coefficient μ of friction surface

 Pretension of a high strength bolt P (KN)

 Maximum and minimum allowable distance of bolt or rivet

Note: 1. D0 is the hole diameter of bolt or rivet, t is the thickness of outer thin plate.

 2.The maximum spacing of bolts or rivets between steel plate edge and rigid components (such as angle steel, channel steel, etc.) can be adopted according to the value of middle row.