On the difference and advantages and disadvantages between steel truss floor bearing plate and ordinary floor bearing plate

Predecessor generation non-combined profiled steel plate (open plate).

Background: At the end of the 20th century, with the continuous development of steel structure buildings, the construction speed of traditional cast-in-place plates obviously could not keep up with the construction speed of steel columns and steel beams. Affects the progress of the entire structure. At the same time, because the construction of cast-in-place board still requires a large number of formwork and scaffolding, this has a huge deviation from the requirements of on-site construction management of steel structure, resulting in a mismatch in the entire construction process. On this basis, engineers have developed a profiled steel plate opening plate that can provide a construction platform without supporting and removing molds, and its role is a long-term formwork for floor slabs. It is only used as a formwork in the construction stage, which belongs to a long-term formwork, only calculates the force in the construction stage, and does not participate in the structural force in the use stage. Fire prevention measures may not be considered, but the overall thickness of the floor slab must meet the requirements of the specification (fire protection requirements), that is, the thickness of the concrete in the upper part of the rib of the profiled steel plate must reach 80mm. Non-composite floor slabs can be calculated by PKPM software, but there are only open slab slab types inside, which can be taken out separately for floor slabs to calculate, and technical standards are available for some large profiled steel plate manufacturers.

Its advantages are early domestic application, national standards, rich construction experience, fast production and low cost.

Disadvantages: high ribs, large thickness of floor structure layer, reduce the net height of the building, directly lead to an increase in the overall cost of the building. The lower surface of the floor is wavy, the bottom of the floor is uneven and unsightly, the two-way rigidity of the floor is inconsistent, the seismic performance is poor, and the ceiling must be made for hotels, residences and other projects. The steel bar binding is cumbersome, the spacing of the steel bar is not easy to guarantee, the lower stressed steel bar needs to be manually welded on site short steel bar, the efficiency is slow, and the thickness of the protective layer is not easy to control. The unidirectional plate design can only meet the force in the construction stage of the large-span floor slab by increasing the thickness of the overall steel plate, resulting in material waste; The construction of two-way slabs is inconvenient, and concrete and slab thickness below rib height must be sacrificed.

When laying pipeline facilities, vertical and ribbed laying must be placed in the upper part of the rib, which will affect the upper steel construction space for floor slabs with small plate thickness. Although the price of veneer is low, due to its cumbersome construction, the amount of labor required is large, the comprehensive cost is high, and the economic efficiency is low.

The second generation of combined profiled steel plate (closed plate).

Background: The early and late 90s of the 20th century was the peak period of multi-high-rise steel structure construction, and it was also the embodiment of material technology and construction technology entering the high-tech stage, and the emergence of ordinary open panels solved the supporting construction with steel structures. However, due to its high material price, the role of galvanized steel sheet instead of stressed steel bar is not considered in the use stage. The amount of concrete and rebar material used in the floor slab is basically the same or slightly higher than that of cast-in-place slab. Therefore, the overall price of ordinary open slab floor slab is basically more than 1.6 times that of cast-in-place floor slab.

Considering the above factors, the engineer improved the ordinary open plate, added a shear groove to the galvanized steel plate, and formed a joint effect of the two through the gripping effect of concrete. Vertical stiffness is provided by the rib, and horizontal shear bearing capacity is provided through the shape of the rib and the shear groove, and the floor bearing plate and concrete must be closely connected, and the rigidity of the floor bearing plate is adjusted by adjusting the thickness, so that the profiled steel plate participates in the force of the floor slab during use, replacing the lower stressed steel bar, thereby saving the amount of steel bar in concrete.

Material, plate type, effective width,

unfolded width, steel plate utilization rate, small thickness, anti-corrosion age economical, convenient, safe and reliable shrinkage plate

　　≥110mm。

Antiseptic years

According to British standards, the general hot-dip galvanized sheet needs protection for 22.5 years under double-sided measurement of 275g/m2, indoor, dry conditions.

Force characteristics and selection calculation

Force characteristics: as a combined profiled steel plate, as a template in the construction stage, bear the load of the construction stage, and in the use stage, the steel plate

Instead of part of the bottom stressed steel bar, together with the on-site configuration of the steel bar, bear the self-weight and use load of the floor.

Selection calculation: When the design and selection of the combined profiled steel plate, the relevant bending, deformation and bearing capacity calculation must be carried out, which can refer to the professional factory

The selection table provided by the home needs to be queried according to the span, thickness and allowable use load except for the self-weight of the floor; The site also needs to be attached

The added steel bar has negative bending moment steel bar, the stressed steel bar at the bottom of the plate

that must be configured when the bearing capacity provided by the steel plate itself is insufficient, the fire-resistant steel bar for fire prevention, and the distributed steel bar network considered for preventing cracking of the plate surface. For example, negative bending moment steel bars and additional plate bottom stressed steel bars need to be designed according to concrete design specifications, and fire-resistant steel bars and distributed steel reinforcement networks need to be configured according to structure and experience.

Pros and cons

Advantages: the pressive steel plate has a special cogging or indentation, which improves the adhesion with concrete, can replace the stressed steel bar at the bottom of the plate, and saves the construction period and labor cost to a certain extent. For closed plates, the bottom of the plate is flatter than the open plate, which is beautiful and the rigidity in both directions is improved. Domestic application is earlier than the third-generation steel truss formwork, and the national specification is included.

Disadvantages: the plate type plate rib height, limited the plate thickness and building net height, not suitable for use in high-rise and super high-rise buildings, and the bottom of the plate is wavy, two-way stiffness is inconsistent, poor seismic performance, easy to cause floor cracking. When the pressed steel plate design replaces the stressed steel bar at the bottom of the plate, there is a great fire safety hazard, and the stressed steel plate is on the fire surface, directly in contact with the flame, and it is easy to form high temperature. First, the strength of the high-temperature steel plate is weakened or disappeared; Second, the crystalline water in the concrete forms steam and high pressure forces the steel plate to bulge, the steel plate and the concrete will slip and detach, the steel plate and the concrete can no longer work together, the role of the steel plate instead of the stressed steel bar will disappear, and the floor will be destroyed under the condition of force. The second generation of combined profiled steel plate cannot be repaired after fire: first, after the steel plate is subjected to high temperature, the material itself changes, and the strength and ductility change; Second, after the steel plate is separated from the concrete, the bonding contact surface is destroyed, and the steel plate cannot be bonded and embedded with the solidified concrete here; Third, the anti-corrosion coating of the steel plate is damaged by high temperature oxidation, and it is necessary to re-brush the anti-corrosion coating, which is difficult to construct. In particular, the ribs of the closed plate are almost closed structures and cannot be repaired. On-site steel bar binding is cumbersome, the spacing of steel bars and the thickness of the concrete protective layer are not easy to control, and short steel bars need to be welded on site to control the protection

layer, the construction quality is difficult to guarantee when using pads. The unidirectional plate design can only meet the force in the construction stage of the large-span floor slab by increasing the thickness of the overall steel plate, resulting in material waste; The construction of two-way slabs is inconvenient, and concrete and slab thickness below rib height must be sacrificed. When laying pipeline facilities, vertical and ribbed laying must be placed in the upper part of the rib, which will affect the upper steel construction space for floor slabs with small plate thickness. The steel plate has anti-corrosion age requirements, and the thickness of the galvanized layer is high, but it will increase the material cost. During construction, the end needs to be treated with zinc removal, otherwise the quality of bolt welding will be affected. After the bolt penetrates the steel plate and beam welding, the steel plate and galvanized layer at least 8mm around the welding joint will be damaged by high temperature, and the end steel plate is gradually corroded with the point belt surface, and the bolt loses the role of fixing the pressing steel plate, which will affect the anchoring of the profiled steel plate in the use stage and the slippage of the contact surface with concrete, thus affecting the actual bearing capacity of the floor.

Due to the consideration of its fire resistance and anti-corrosion defects, designers generally consider adding fire-resistant steel bars (or temperature steel bars, as a reserve of force reinforcement in the event of a fire) to the bottom

of the plate , plus the distribution of steel bars,

support negative bars, etc., in the construction site also need to tie about 50~70% of the on-site steel bars, the construction is cumbersome, the amount of labor required is large, and the comprehensive cost is high.

The third generation of steel truss floor bearing plate

With the rapid development of multi-high-rise steel structure, higher requirements are put forward for the construction period of the project, and the industrialized production of steel structure component factories has greatly shortened the project construction period, and the construction method of the floor slab has become an important factor affecting the construction period. The previous generation and the second generation of profiled steel plate have higher ribs, which reduces the net height of the building, the uneven surface of the floor slab, the difficulty of two-way plate design and construction, cumbersome steel bar binding, poor control of steel bar spacing and concrete protective layer thickness, and serious fire resistance and anti-corrosion defects, which must be solved. In addition to the various advantages of the first two generations of steel truss floor bearing plate and cast-in-place plate, the third generation of steel truss floor bearing plate also has its own characteristics, and the skills give full play to the advantages of short construction period of steel structure and easy control of construction quality, which has been highly recognized and praised by the market.

Steel truss floor bearing plate belongs to the third generation of steel structure supporting floor bearing plate, and the ordinary non-combined profiled steel plate and the plate type of combined profiled steel plate is quite different, is the concrete floor slab in the factory to process the stressed steel bar into a steel truss, and then with the profiled steel plate resistance spot welding as one of the steel floor bearing products. The rebar truss adopts a combination of high-frequency resistance spot welding to form a triangular truss with stable structure

The ribs of the bottom profiled steel plate are significantly reduced, only 2 mm, almost equal to the flat plate.

Section view

of rebar truss floor deck

Force characteristics and calculation

Force characteristics: As a new generation of steel floor bearing plate, its force mode is more reasonable, and it no longer relies solely on steel plate to provide strength and stiffness in the construction stage. The strength and stiffness of the construction stage are provided by the steel trusses with more reasonable forces. In the use phase, the rebar truss and concrete work together. The galvanized floor is only used as a formwork in the construction stage, without considering the structural force, but under normal use conditions, the presence of the steel plate increases the rigidity of the floor slab and improves the stress performance of the concrete in the lower part of the floor.

Selection calculation

The reinforced truss concrete floor can be designed as a unidirectional slab

or a two-way slab according to the engineering situation, without having to comply with the length ratio relationship between the long side and the short side of the floor, but when designed as a unidirectional slab, a sufficient number of structural reinforcements should be arranged in the direction of the long side. In the process of concrete floor slab from pouring concrete to reaching the design strength, the calculation of two stages of construction and use is carried out. In the construction stage, the strength test of the upper and lower chords, the stability of the compression chord and the web rod, and the deflection of the steel truss are carried out in three parts.

The use stage must calculate four parts: the bearing capacity calculation of the positive section of the floor, the stress control test of the stressed steel bar at the bottom of the slab, and the control verification of the support crack.

In order to facilitate designers to quickly and easily select the appropriate steel truss floor bearing plate model in the design process, you can refer to the design selection table, the template model in the table includes the commonly used TD1~TD7

Series, all

models with truss height range of 70mm~270mm, corresponding floor structure thickness range of 100mm~300mm. When checking the table, according to the thickness and span of the floor, in addition to the additional use load of the slab's own weight, the corresponding model and the negative reinforcement that need to be added can be checked.

Pros and cons

Advantages: It makes up for the defects of the front and second generation profiled steel plates, and has its own characteristic advantages: reasonable force mode, superior overall performance of the floor, convenient construction, environmental protection, and guaranteed construction period; The third-generation rebar truss floor bearing plate adopts the mode of combining rebar truss and galvanized floor slab, which makes the overall stress performance of the floor slab equal to or even better than the traditional cast-in-place reinforced concrete floor slab. Compared with the traditional cast-in-place concrete floor slab, it eliminates cumbersome construction processes such as mold support, mold removal, and steel bar binding, which greatly improves the construction speed of the floor slab, especially for high-rise buildings, and provides a certain guarantee for the overall progress of the project. The bottom of the slab is flat, the net height is guaranteed, the two-way stiffness of the floor is consistent, and the seismic performance is good; The spacing of steel bars and the thickness of the concrete protective layer are guaranteed; The steel truss is automatically welded by imported equipment, and the node spacing between the upper and lower chords and the rebar steel bar is stable, and the thickness of the concrete protective layer can be effectively guaranteed, which brings convenience to the construction and provides guarantee for the quality of the floor slab. Traditional stressed structure, steel plate does not participate in force, no need for fireproof and anti-corrosion coatings, that is, safe and economical; The design and construction of two-way slabs are simple, suitable for long-span floor slabs; When designing a two-way plate, it is only necessary to configure the force and structural reinforcement in the direction of the vertical truss on site, and the lower steel bar can pass through the truss and be placed in the upper part of the lower chord steel bar. For floor slabs with large spans

When designed as a two-way slab, the thickness of the floor slab and the diameter of the truss rebar can be increased; A temporary support can also be set in the middle of the span, which can save the amount of steel bar and reduce the cost. The product types are diverse and the application areas are wide. So far, steel truss formwork has been applied in multi-storey workshops, multi-high-rise steel structure offices, hotels, residential buildings, super high-rise steel structure buildings, irregular floor (round, elliptical, other shape) steel structure buildings, falling plate structures, thick plate structures, reinforced concrete structures, reinforced concrete precast slabs and other projects, with mature technology and huge market capacity.