Steel structure mezzanine construction

The steel structure mezzanine is a secondary fabrication and installation of structure . The steel structure mezzanine is to lay the wood and bamboo board etc on the steel structure frame. The wood board is easy to be deformed when it is wet; the wood board is not environmentally friendly, and the sound insulation is poor; when walking on it, there is a bit squeaky; the selection of these wooden boards has great defects.

A preferred method of steel structural mezzanine is to construct according to the mezzanine standard of steel structure.The recommend standard is that: The main beam structure of the steel structure mezzanine adopt Q235B H-type steel,while the secondary beam adopts Q235B rectangular tube and channel steel,etc. The steel plate should be placed on the main structure body and then the steel rebar,and finally pouring with 120-200mm thickness concrete.

1. Technical specifications and requirements for construction of steel structure mezzanine:

1.1 Firstly, measure and review the size;

1.2 Release the line after confirming that the size is correct;

1.3 Pre-embedded steel connecting plate and node drawing wall surface lofting;

1.4 Lofting and positioning of the main beam structure and the secondary beam structure;

1.5 Factory welding, processing, drilling

Technical details: It is strictly forbidden to use the gas cutting hole for the pre-embedded steel connecting plate, and the drilling machine must be used for drilling. Technical specification requirements: The diameter of the drilled pre-embedded steel connecting plate is allowed to be greater than or equal to 0.5 mm of the diameter of the rod. It is strictly forbidden to drill large holes in order to facilitate construction. The main beam structure of the steel structure mainly relies on the function of the pre-embedded gusset plate at both ends of the steel beam. If the anchoring (anchoring is a building structure reinforcement technology, it is strong professional,and its technicality is stronger) when connecting the steel plate to open a large hole, The tightening force of the bolt nuts and the steel plate changes, and the force of the rigid plate at this time is greatly reduced, and the required load of the structure cannot be satisfied;

1.6 The cutting and drilling of the main beam H-type steel: the wall pre-embedded part is made of steel plate,which is acted as the connection of steel beam (the anchor bolt is used to anchor the steel plate),and then welded wholly.The main beam is made of 200-125 or 250* 125mm H-type steel ,and the horizontally arranged span is welded by 80*120*5 rectangular tube or 14#-16# channel steel, and pre-stressing is applied to form a frame structure.

1.7 The steel structure frame is laid with galvanized steel slabs, and then tied two layers of steel mesh and finally pouring the concrete.

1.8 The designed use load is around 2000kg.

1.9 The lifetime should be more than 50 years.

1.10 The renovation can be carried on 3 weeks after the completion of the steel structure mezzanine. The Floor tiles, wooden floors, etc can be laid.

2. Type of steel structure

According to different angles, the steel structure can be classified as below:

2.1 Classified by application area

Steel structures are applied in many engineering construction areas and can be classified as:

(1) Construction engineering: support skeleton for industrial, agricultural, civil, public housing, and memorial buildings;

(2) Bridge engineering: railways, highway bridges, urban crossings, overpasses, etc.;

(3) Hydraulic engineering: hydraulic gates, pressure steel pipes, construction trestles, etc.;

(4) Ocean engineering: offshore oil platforms, facilities, submarine oil pipelines, etc.;

(5) Special projects: transmission and transmission towers, liquid and gas storage tanks and their pipelines, large cranes, etc.

Steel structures applied in different fields are often referred to as steel structures in the field, such as building steel structures, bridge steel structures, and marine steel structures. Due to difference of the application field, the environment and the requirements of use, various steel structures are also affected by the natural environment and the human environment. Although its design, construction and use are different, its basic attributes and characteristics and its overall design philosophy, principles, methods and theoretical basis are the same.

2.2 Classified according to the used steel specifications

The processing, manufacturing, working performance and failure mode of the steel structure are not only related to the type and nature of the steel structure, but also related to the thickness and specifications of the steel material. Therefore, It can also be classified as the thickness and specification (mode and size)of the composed main steel:

(1) Light steel structure: steel structure composed of 1.5~6mm steel plate and small round steel and section steel.

(2) Ordinary steel structure: steel structure composed of 8~40mm thickness steel plate and ordinary section steel.

(3) Heavy-duty steel structure: steel structure composed of steel plates with thickness ranging from 40mm to 100mm and extra-large steel material.

A steel structure made of a steel plate with a thickness less than 1.5 mm may also be called an ultra-light steel structure; a steel structure composed of a steel plate with a thickness greater than 100 mm and a special type steel may also be called as an ultra-heavy steel structure. The stability and defect sensitivity of light and ultra-light steel structures are more prominent; As for the heavy steel,the welding residual stress, lamellar tearing of steel plates and fatigue and brittle fracture have a great influence on their normal work. In addition to meeting the use requirements, all types of steel structures must have sufficient reliability, namely safety, suitability, durability and good social and economic benefits.

2.3 classified according to the steel structure working system

(1) A beam structure, a structure consisting of a beam that is bent to work;

(2) Rigid frame structure, frame-shaped structure composed of straight beams and straight columns subjected to compression (pulling) and bending;

(3) Arch structure, a planar structure composed of one-way curved members; its cross-section is pressed, bent and sheared;

(4) Truss structure, mainly composed of pulled or pressed bars;

(5) Grid structure, a planar mesh structure mainly composed of pulled or pressed rods;

(6) Reticulated shell structure, a curved mesh structure mainly composed of pulled or pressed rods;

(7) Cable-strut structure: a structure composed of a tension cable (or a chain rod) and a pressure member;

(8) Cable structure, a structure mainly composed of tension cables;

(9) A mixed structure, a structure composed of two or more structures in (1) to (7);

(10) Hybrid structure, a new-type structure composed of two or more structural members in (1) to (7);

(11) Prestressed structure, a steel structure apply a pre-stress to (1) to (10).

The above-mentioned structural system usually has a lighter weight in order under the same space and environmental conditions, especially for the supporting structure of a building with large space and under the complex environmental conditions, its self-weight reduction degree is even more so. This is because the section stress of the bent or pressed-bent member is not uniform, and the section determined by the maximum edge stress cannot fully utilize the material strength property; the section stress of the axial compression member is uniform, and the material strength performance can be fully utilized.But when the slenderness ratio is large, the stability problem needs to be considered; the axial tension member not only has uniform cross-section stress, but also does not destabilize, and can make full use of its material strength performance.

3. The nature, shape and preventive measures of steel structure failure

3.1 Two types properties failure of steel members

3.1.1 Plastic failure with plastic deformation that is significantly irreversible;

3.1.2 Brittle failure of plastic deformation without significant irreversibility.

The destructive nature of structural steel depends on the nature of the steel itself and the environmental conditions and stress conditions. As for general structural steels, the damage is often brittle at room temperature, unidirectional, one time, slow and non-uniform stretching. Ordinary high-strength steels have poor plasticity and toughness, and their damage is often brittle. High-quality high-strength steel treated by special process has good plasticity and toughness. Under normal circumstances, its damage is often plastic; however, under special circumstances, the damage is often brittle under the brittle transition low temperature, two-way (or three-way), multiple times rapid speed and non-uniform stretching.

3.2 Destruction form of steel members and preventive measures

The steel component is manufactured and connected by section steel and steel plate. The damage can occur in the shaft or joint. The failure property and shape are related to the steel, the connection method and the stress condition. The reason may be due to the geometrical characteristics of the section couldn’t meet the force requirements, the plate or joint is out of services subjected to force exceeding its ultimate bearing capacity, causing damage to the steel components and connections. The failure modes and prevention measures are mainly:

3.2.1 Strength failure: The force or stress exceeds the maximum ultimate load carrying capacity of the components and the joints or the strength of the steel.

Main preventive measures: increase the cross-sectional area of the plate, strengthen the connection, select the high-strength steel, so that the conversion stress σeq is not greater than the design value of the steel ƒy/γR, (γR is the resistance partial coefficient) ie: σeq≤[σ2x+ Σ2y+σ2z-(σxσy+σyσz+σzσx)+3(τxy+τyzτzx)]1/2≤ƒy/γR(3.1) and make the connection have sufficient bearing capacity.

3.2.2 Loss of stability: The force of the component reaches or exceeds the critical force of the component, or the pressure of partial plates exceeds its critical force and loses its stable and then exit, resulting in the loss of the ability of the component to continue to bear.

The main countermeasures are: adjusting the geometrical characteristics of the section and the boundary support conditions, reducing the width-to-thickness ratio and the slenderness ratio of the plate, so that the nominal stress σnomin is smaller than the critical stress design value σcr/γR, ie: σnomin≤σcr/γR

3.2.3 Fatigue failure: Due to the repeated action of the load, the crack initiates, expands, and thus the instability and expansion cause the member to lose its bearing capacity.

Main preventive measures: use steel with good ductility and toughness to reduce defects, reduce stress concentration, reduce stress amplitude Δσ, and meet the requirements of formula (3.3):

Δσ=σmax-σmin≤[Δσ]=(C/N)1/β(3.3)

Among them: σmax, σmin-maximum, minimum acting stress; β, C-fatigue characteristic parameters; N-acting stress cycle number.

3.2.4 Brittle fracture failure: As for the components with cracked (or crack-like) defects, the crack is unstable and the damage occurs under the action of crack open tensile stress or composite stress.

Main preventive measures: Select steel with good ductility and toughness, reduce crack defects, reduce stress levels, and make the stress intensity factor K1 not greater than the static or dynamic fracture toughness of steel (K1C or K1d):

K1=Yσa1/2≤K1C (static) or K1d (dynamic)

Where: σ—crack field stress; a—crack length; Y—parameter-related cracks.

3.2.5 Excessive plastic deformation: Due to the large unrecoverable deformation of the component, the component may cause secondary damage of the above-mentioned four types.

Main preventive measures: improve the boundary and stress conditions of the components; increase the stiffness of the components and select high-strength steel.

The failure mode of the steel components listed above is also related to the selected structural steel. The failure properties may be plastic or brittle, depending on the strength grade, quality grade, quality condition of the used steel; machining deviation, defects, welding, the size and condition of the residual stress; the nature of the force is static, dynamic, or impact,one time or repeated more times; the high or low of the acting stress; simple stress or complex stress; good or bad environment. When the steel is in high strength,poor grade and secondary quality, severe crack-like defects and residual stress,the dynamic or impact load is applied,high stress level, complicated condition and under low temperature and severe environment, the damage is brittle. On the contrary, in general, its damage is often plastic.

As for design of steel structure, firstly ensure structural safety and prevent sudden brittle damage.

3.3 Destruction form of steel structure

Steel structure is an engineering structure composed of processed, connected and installed steel materials . It is an engineering structural system in which many steel components are combined. Its destruction form has the following characteristics:

3.3.1 Destruction site: may at a local connection, a node, a component,or start from those local damage and gradually expand to cause partial or total collapse of the structure;

3.3.2 Destruction form: its type is basically similar to that of steel members, but its destruction has more characteristic of more structural system.That is, it may be local, integral, or its mutual influence develops into a generality;

3.3.3 Destructive properties: may be plastic or brittle;

3.3.4 Destruction path: diverse;

3.3.5 Destruction reason: poor performance and quality of steel; improper selection of structure, system, and layout; unreasonable geometrical structure, joint structure, and connection; complex force conditions, false structural calculation diagrams and force analysis; serious deviation defects of manufacturing and installation. It is in a separate or combined effect of complex factors;

3.3.6 Destruction process: may be gradually expansive or sudden;

3.3.7 Destructive consequences: often severe.

Steel structure design, first of all, to ensure structural safety and prevent sudden brittle damage.

3.4 Dominant destruction form of various types of steel structure

In general, various destruction forms of various types of steel structures may occur, but steel structures of different types and environments have different dominant destruction forms, such as:

(1) Light weight, ultra-light steel structure: Strength failure; partial, overall, or total instability.

(2) Ordinary steel structure: strength failure; local, overall, or total instability; fatigue damage; excessive plastic deformation damage.

(3) Heavy-duty steel structure: strength failure; overall, or total instability; fracture failure; fatigue failure.

In addition, different steel structure stress systems will have large differences in their destruction forms, properties, paths and processes. Under normal working conditions, the failure of the beam structure is often gradually expanding, plastic, and partial; as for tension cable, cable net steel structure, prestressed steel structure, etc., If the steel is in high strength, low toughness steel and design under high stress state, its damage can often be sudden, brittle, or even overall or total.

4. Steel structure design and selection of structural steel material

4.1 Main physical and mechanical properties and steel structure design of structural steel material

Yield strength fy: the linear elasticity analysis of the structural design and the limit value of the strength and stability calculation.

Tensile strength fu: the limit of tensile damage. Strong bending ratio is an important indicator of strength reserve.

Elongation δ5 (δ10): The plastic properties of the uniaxial tension of the steel.

Cold bend angle α: The plastic properties of the steel when it is bent and extruded, and the revealing of the internal defects of the steel.

Section shrinkage ψ: plasticity of steel under complex stress conditions.

Impact toughness AK: A measure of the kinetic energy required to break a steel.

Fracture toughness K1C: A limit for the crack instability expanding of steel.

4.2 Steel type, mark, strength grade and quality grade of structural steel:

Steel type: carbon steel, low alloy steel; ordinary steel, high quality steel, special process steel (such as TMPS).

Mark:composed of Yield strength letterQ,Yield strength value,quality grade(A,B,C,D,E),Deoxygenation method symbol(F,B,Z,TZ).

Strength grade:Q235, Q345, Q390, Q420…,

Quality level: When the delivery is subjected to standard stipulates, the qualified performance should generally be as follows:

A grade— fu, fy,δ5 (δ10) ,α； S , P.

B grade — fu, fy,δ5 (δ10) ,α； C,S , P, AK(+200C).

C grade— fu, fy,δ5 (δ10) ,α； C,S , P, AK( 00C).

D grade— fu, fy,δ5 (δ10) ,α； C,S , P, AK(-200C).

E grade— fu, fy,δ5 (δ10) ,α； C,S , P, AK(-400C).

Other performance indicator requirements shall be proposed by the design and determined by agreement with the supplier. However, the higher level and grade required, the more additional conditions and demanding, the more expensive.

4.3 The basic principle of selecting the properties of structural steel: it is reliable enough to meet the needs (preventing brittle fracture due to poor material): good process performance is good and the low price.

4.4 Main factors need to be considered during the selection of the steel structure material: structural importance; structural type; connection method; node structure; stress state; environmental conditions.

4.5 Steel structure selection

4.5.1 Selection of quality grade

Generally, non-welded steel structures can be selected from grade A steel;

For welded steel structures, B grade steel should be used for static load effect;

For welded steel structures, when dynamic load is applied, C, D, E grade or special grade steel should be selected according to the ambient temperature of the structure. Make sure that the brittleness transition temperature of the steel is lower than the ambient temperature of the structure.

For thicker steel plates with structural parts with lamellar tearing forces, there should be a requirement for resistance to lamellar tearing.

As for heavy-duty welded steel structures with complex joint construction and stress conditions and harsh working environment conditions,the steel quality standards requirements shall be improved.

The selection of the quality grade of important structural steel should be improved.

4.5.2 Selection of strength level

The strength grade of ordinary steel structural steel is often selected as Q235 or Q345;

The strength grade of heavy-duty and super-heavy structural steel can be selected as Q345, Q390, Q420 or higher strength grade special steel;

Cold-formed thin-walled lightweight steel structure, Class A can be used for non-welding, Grade B steel for welding; generally Q235 or Q345 strength grade steel should be adopted; ultra-light steel structure and profiled color coated steel plates for roof and wall should adopt the steel material with the same strength grade as Q235 or Q345. When the lock seams are connected, they should have good cold bending performance. If necessary, the multi-layer roll flattening without damage test shall be added. As for press button connection, the steel with high-strength and good toughness should be selected.

Key to the steel structure mezzanine

The key to the steel structure mezzanine: the main beam calculation, if you want to do the mezzanine, firstly consider the floor load, and then calculate the main beam distribution. If you do not care about the size of the load, the size of the span, the steel structure mezzanine will be dangerous!