This paper introduces the design points and construction techniques of cast-in-place reinforced concrete truss slabs in steel structure buildings through a large-scale engineering example, which can serve as a reference for similar projects. The museum has one basement level and five above-ground levels, with a total building area of 66,927 square meters and a building height of 44.75 meters. The main structure adopts an internal steel pipe reinforced concrete shear wall-steel truss cantilevered suspension structure.

Because this project adopts a large-span suspended structure system, the research results of the floor vibration reduction system of the Department of Civil Engineering of Tsinghua University show that if a composite floor slab of profiled steel sheet and lightweight concrete is used, a large number of dampers need to be installed in order to reduce the floor vibration caused by pedestrian loads, which is technically difficult.

In addition, the height of the formwork installation work surface for each floor above the third floor is more than 5m, with a maximum of 23m, which is considered ultra-high-altitude work. Therefore, the floor slabs above the third floor use a combination of self-reinforcing steel truss profiled steel sheets with good rigidity that can withstand bidirectional stress and have good rigidity with ordinary concrete.

I. Construction and Characteristics of Reinforced Truss Steel Deck

1.Structural Construction

A steel truss steel deck is a concrete slab that integrates steel reinforcement with formwork to form a load-bearing component that can withstand the self-weight of wet concrete and construction loads during construction and serves as lateral support for steel beams. During service, the steel truss works together with the concrete to bear the service load.

2.Materials used

(1) Reinforcing steel: The upper and lower chords are made of hot-rolled steel bars HPB235, HRB400 or cold-rolled ribbed steel bars grade 550 supplied in coils; the web members are made of cold-rolled plain round steel bars grade 550 or 650 supplied in coils; the truss support reinforcement is made of hot-rolled steel bars HPB235 or HRB335.

(2) Galvanized steel sheet: The bottom mold is made of 0.5mm thick galvanized steel sheet, with a double-sided galvanizing amount of 120~180gm/㎡.

(3) Studs: To effectively connect the concrete and steel beams into a whole, studs are installed on the steel beams, and a special stud machine is used for construction. The cross-section of the steel truss steel slab is shown in the figure.

Cross-sectional diagram of reinforced steel truss steel deck

3.Construction characteristics

Unlike traditional construction methods, this method allows for the direct laying of steel truss steel slabs onto steel beams on-site, followed by simple reinforcement work before concrete pouring. This formwork eliminates the need for wooden formwork and scaffolding; the bottom galvanized profiled steel sheet serves only as formwork and does not replace the reinforcing steel. Therefore, fireproofing and corrosion protection are unnecessary, resulting in faster construction and reducing on-site reinforcement tying work by approximately 70%, shortening the construction period and saving costs. Furthermore, the uniform arrangement of the reinforcement, ensuring adequate spacing between upper and lower layers and sufficient concrete cover thickness, creates favorable conditions for improving the quality of the floor slab construction.

II. Stress Characteristics of Steel Truss Steel Deck

1.Floor slab deflection

Ordinary cast-in-place reinforced concrete floor slabs exhibit minimal deflection during construction due to the underlying formwork. However, once the concrete reaches a certain strength and the formwork is removed, the slab deflects under its own weight, causing tensile stress and even cracks in the bottom concrete. Reinforced truss concrete floor slabs, on the other hand, exhibit different deflections depending on the temporary support arrangements, as follows:

(1) Temporary supports are installed in the same manner as ordinary cast-in-place reinforced concrete floor slabs.

(2) No temporary support: Before the concrete solidifies, the self-weight of the formwork, the weight of the concrete and the construction load are all borne by the steel truss. The concrete solidifies under the deformation of the steel truss steel slab, and the self-weight of the floor slab will not cause tension on the concrete at the bottom of the slab.

2.Floor slab load-bearing capacity

During the service phase, the upper and lower chord steel bars of the steel truss work together with the concrete. This floor slab has the same stress performance as a reinforced concrete composite floor slab. Although the tensile stress of the steel bars is ahead, its bearing capacity is the same as that of an ordinary reinforced concrete floor slab.

III. Design Considerations for Steel Truss Steel Deck

The stress on the floor slab varies significantly from the time the concrete is poured until it reaches its design strength. Therefore, calculations should be performed for both the service and construction phases.

1.Usage Phase

This includes calculations of the floor slab's cross-sectional bearing capacity, stress control verification of the bottom reinforcement of the floor slab, support crack control verification, and deflection verification. The floor slab's cross-sectional bearing capacity is calculated according to the relevant provisions of GB50010-2002 "Code for Design of Concrete Structures" and JGJ95-2003 "Technical Specification for Cold-Rolled Ribbed Reinforced Concrete Structures".

2.Construction Phase

A truss model was used, including strength verification of the upper and lower chords, stability verification of the compression chords and web members, and truss deflection verification.

(1) When reliable temporary supports are provided during the construction phase, no construction phase verification is required during the design phase.

(2) When no temporary supports are provided during the construction phase, the internal forces of the truss members and the deflection of the formwork in the steel truss steel slab are calculated using a truss model. The loads during this phase include the self-weight of the steel truss steel slab, the weight of the wet concrete, and the construction loads. The construction load is the more unfavorable of the uniformly distributed load of 1.5 kN/m² and the concentrated load at mid-span along the width of the slab of 2.5 kN/m, and the simultaneous action of the two is not considered.

c. Truss deflection

During the construction phase, the maximum deflection of the steel truss steel slab should be calculated according to the standard combination of loads. The ratio of deflection to span should not exceed 1/180, and the maximum deflection should not exceed 20mm.

3.Calculation Example

Taking the three-story mezzanine slab of this project as an example, the calculation results of the steel truss steel deck are shown in Table 1.

Calculation results of reinforced steel truss steel deck

IV. Construction Technology of Reinforced Steel Truss Steel Deck

1.Process Flow

Marking lines → Cleaning the slab → Hoisting → Laying out the slab → Cutting → Pressing → Side welding and end welding → Leaving holes → Sealing → Acceptance → Stud nailing → Block construction → Reinforcing bar placement → Embedded parts → Concrete pouring and curing.

2.Panel connection method

(1) The steel truss steel deck is laid and connected to the steel beam. The ends of the deck are spot-welded to the steel beam, and the middle is welded to the steel beam through with studs. The steel truss steel decks are connected by clamping holes with special clamping pliers. The ends are spot-welded to the steel truss steel deck or steel beam with special galvanized edge mold steel plates. A typical connection method is shown in the figure.

Support connection diagram

(2) When connecting the steel truss steel deck to the concrete wall, angle steel should be added and fixed to the concrete wall with expansion bolts, as shown in the figure.

Connection between floor slab and concrete wall (column)

3.Methods for adjusting floor elevations

(1) When the floor level varies, Z-shaped supports and additional reinforcing bars are welded to create a stepped transition in the horizontal structure, as shown in Figure a;

Node diagram at elevation difference of slab surface

(2) When lowering the elevation, weld Z-shaped brackets and additional reinforcing bars to the web of the I-beam, as shown in Figure b.

4.Treatment of openings in the floor surface

For pre-reserved holes in concrete floor slabs, no reinforcing bars are required if the diameter or width of the hole is less than 100mm. The hole should be cut after the concrete is poured and reaches the design strength. Reinforcing bars should be added if the diameter or width of the hole is 100—1000mm.

5.Construction of temporary supports for steel trusses

Temporary supports need to be set in the middle of some steel truss formwork. These can be constructed using various gantry frames of different heights depending on the floor height. The gantry frames are spaced 18m apart and covered with 100×100 timber, as shown in the figure.

Schematic diagram of temporary support for steel truss

6.Placement of lightweight aerated concrete blocks

Based on the structural characteristics, in order to reduce the self-weight of the concrete floor slab, lightweight aerated concrete blocks with a density greater than 8kN/m³ and a strength grade not lower than MU5 need to be placed in the slab with a thickness h≥140mm. During construction, 26 supporting steel bars need to be added under each block, as shown in the figure.

Plan and elevation views of aerated concrete block installation

7.Construction Precautions

(1) When pouring concrete, the material should not be concentrated too much, and a plate vibrator should be used to distribute and vibrate it in a timely manner.

(2) When pouring concrete, a pressure and flow meter should be added at the pump switch to control the flow and prevent large impact forces and excessive concrete from affecting the steel deck structure. Construction personnel and equipment should be concentrated at the main and secondary beams to avoid direct stacking on the steel plate and dragging, which would cause the steel truss steel deck to twist and deform.

(3) During construction, the concrete slump should be controlled to be 12—15cm. Because the steel truss steel slab has poor water permeability and air permeability, the water vapor on the slab surface evaporates quickly during the concrete setting process. After pouring, the shrinkage deformation of the concrete slab surface is greater than that of the bottom of the slab. Therefore, strengthening water curing can prevent and reduce shrinkage cracks on the slab surface and ensure the strength of the concrete.

(4) Welding shall be performed by penetration spot welding. Before welding, scrap materials shall be prepared for arc ignition and test welding, and the welding current shall be adjusted.

(5) Special soft slings should be used when hoisting steel truss steel decks. Each time the slings are hoisted, they should be checked for tearing or cutting. When the steel truss steel decks are placed on steel beams, care should be taken to prevent them from protruding. When laying the materials, the operators should wear safety belts.

steel truss steel decking has the advantages of reasonable stress distribution, convenient construction, safety and reliability, good fire resistance and corrosion resistance, and significant overall cost advantage, and has a very good prospect for development and application.