Study on Improved Large Span Propping System for Reinforced Concrete Composite Slabs with Lattice Girders

To further improve the installation efficiency of commonly used reinforced concrete composite slabs with lattice girders in China, an Improved Large Span Propping System is proposed. This article investigates the mechanical properties of the new propping system through in-situ monitoring. The monitoring results show that the displacement and stress of the prefabricated layer can meet the requirements of current Chinese standards, which indicates the feasibility of the new support system. Compared with the original Large Span Propping System, the new propping system can further utilize the bending stiffness of the prefabricated layer and reduce the installation amount of the lower propping frame.


Introduction
With the large-scale application of prefabricated buildings in recent years, the use of reinforced concrete composite slabs has increased rapidly.The most commonly used prefabricated slab is reinforced concrete composite slab with lattice girders (RCSLG) [1][2][3][4][5].The prefabricated layer of this kind of slab is prefabricated in factory and installed in on site, which can support its self-weight and a certain construction load, eliminating a large number of scaffolding, and has higher integrity than the fully prefabricated slab (Figure 1).
The production, application and research of RCSLGs in China was started relatively later than the United States, Japan and Germany and other developed countries [6,7].At present, the thickness of the prefabricated layer of RCSLGs commonly used in China is generally 60mm~100mm, which is far thinner than the thickness commonly used in European and American countries.The regulations related to the allowable propping span during construction stage are only specified in atlas 15G366-1 'reinforced concrete composite slab with lattice girders (60mm-thick prefabricated slab)' [8].In this atlas, the maximum allowable value of propping span is 1.8m.For the RCSLGs with prefabricated layer thickness larger than 60mm, most construction enterprises still use the propping systems of cast-in-situ slabs, resulting in insufficient utilization of the stiffness of prefabricated layer and excessive support [9].In the meanwhile, the research of foreign scholars and enterprises on the propping span during the construction stage of RCSLGs has shown that the prefabricated layer of RCSLGs shares large bending stiffness.The allowable propping span can reach 3.0m -6.26m with reasonable setting of lattice griders, which is much larger than the span common used in China [10][11][12][13][14].
In summary, the allowable propping span during construction of RCSLGs in China is often based on empirical values from cast-in-situ slabs with excessive support and small propping span.The progress of construction technology research and standard establishing for RCSLGs is severely lagging, which to some extent restricts the application of this type of slab, and fails to fully utilize the advantages of prefabricated construction.
In order to improve the construction quality and efficiency of typical specifications of RCSLGs in China, based on the concept of reasonable utilization of the bending stiffness of prefabricated layer and the strength of props, a new type of propping system called Large Span Propping System was proposed in literature [15], the feasibility of which was verified through in-situ tests and finite element analysis.The composition of this propping system is shown in figure 2a.In order to further improve the performance of this system and reduce the amount of lower propping frame, an Improved Large Span Propping System for RCSLGs is proposed in this article, as shown in figure 2b.In this new system, the propping frame at the end is replaced by steel brackets temporarily fixed on the beams.The advantages of this improvement measure are as follows: (1) Taking prefabricated beams and columns as part of the propping system to bear the load during the construction stage; (2) Improve the overall stability of the propping system; (3) Further reduce the amount of lower propping frame while retaining large propping span.
To verify the feasibility and reliability of Improved Large Span Propping System for RCSLGs, this article investigates the mechanical properties of the new propping system through in-situ monitoring .

Introduction of test
In order to compare with the original Large Span Propping System proposed in literature [15], in-situ monitoring of Improved Large Span Propping System was also carried out in the office building of Xuzhou Green Building Industrial Park Project in different floor with the same slab layout and specifications (except layout of propping system).The standard slab layout of this floor is shown in figure 3. The height of each floor is 3.9m.
The room intersecting 5-6 axis and A-B axis was selected for in-situ monitoring, the slab of which was composed by three close fitting RCSLGs (70mm-thick prefabricated layer and 130mm-thick cast-in-situ layer).The materials of concrete and reinforcements of the slab were C40 and HRB400 respectively.The detailed dimensions of prefabricated layer is shown in figure 4.

Layout of Propping System
The layout of Improved Large Span Propping System of the monitoring area is shown in figure 5.The prefabricated layers of the slab were set as three span continuous beam (2.4m+2.5m+2.4m).Two layers of bidirectional horizontal tubes were set at 1.7m and 3.0m away from the ground respectively on adjustable telescopic steel props.Tripods were also set to increase the stability of the propping system and reduce the calculation length of the adjustable telescopic steel props.The material of the props was Q235.The specifications of the inner tubes and outer tubes were ϕ48x3.6mmand ϕ60x3.6mmrespectively.The specification of the horizontal tube were ϕ60x3.2mm.

Measurement scheme
In order to better understand the feasibility of the Improved Large Span Propping System, the axial force of props, vertical displacement of prefabricated layer, strain of reinforcements of the slabs were measured, from concrete pouring to 7 days after pouring of the cast-in-situ layer.The displacement of the slab was measured by percent meters (Figure 6a).The force sensors were fixed at both top and bottom of props and on the steel brackets (Figure 6b).The strain measuring points of the reinforcements were set at upper chord of lattice griders and distribution bars in cast-in-situ layer (Figure 6c, Figure 6d).

Propping frame
Figure 7a presents the force distribution of force sensors after initial setting of the concrete of cast-in-situ layer.About 80% of the self-weight of the slab (including cast-in-situ layer) acts on the propping system.During the concrete pouring stage, the measured maximum force of single force sensor was 26.36kN (Figure 7b, including live load), which is basically consistent with the measurement value using the original propping system proposed in literature [15].During the test, the propping system kept good overall and local stability, and no flexural or instability of the propping system was observed.
Figure 7c presents the variation process of the measured values of typical force measurement points over the entire monitor process.It is shown that with the development of concrete hardening, the overall force on the props shows an obvious downward trend.Figure 7d shows the force distribution after 7 days after pouring of cast-in-situ layer.About 47% load of the tested slab was supported by the lower props, while the rest assigned to the beams.During the entire monitor process, the measured strain of some steel bars in the upper chord of the lattice griders is shown in figure 8.During the concrete pouring stage, affected by the construction loads, the strain of the bars varies significantly.From initial setting of concrete to 3 days after pouring, the strain developed slowly and the amplitude of change was relatively small.From concrete pouring to 3 days after pouring, the strain of upper chord of lattice griders was mainly in tension stage near the support and in compression stage at the mid span.
(a) Strain of measuring points at different stages (b) Whole process strain curves of some points S0-before pouring, S1-concrete pouring, S2-initial setting, S3-1 day after pouring, S4-3 days after pouring, S5-7 days after pouring, S6-removing of propping system, S7-1 day after removing propping The measured strain of some distribution reinforcements in cast-in-situ layer is shown in figure 9. From concrete pouring to 3 days after pouring, affected by the construction loads, the reinforcements were mainly in tension stage and the stain of which varied significantly.To 7 days after pouring, most of the measuring points gradually transited to tension stage.After removing of the propping system, the strain decreased slightly.Disregarding the data with significant fluctuations, the strain of all measured steel bars was in elastic stage.
(a) Strain of some points at different stages (b) Whole process strain curve of B4 S0-before pouring, S1-concrete pouring, S2-initial setting, S3-1 day after pouring, S4-3 days after pouring, S5-7 days after pouring, S6-removing of propping system, S7-1 day after removing propping Compared with the monitoring proposed in literature [15], during the concrete pouring stage, the maximum strain of the bars was about 680με (larger than [15]), indicating that the new system can make better use of bending stiffness of the prefabricated layer.
3.2.2.Displacement of the prefabricated layers.Take the status before concrete pouring of the slab as zero point of displacement, Figure 10a presents the measured displacement of the middle part of the slab.Figure 10b is the displacement distribution of the tested area from concrete pouring to initial setting.The displacement of the slab near the support beams was 0.7mm -3.0mm.And the displacement of the points at the mid-span of the propping system was 1.3mm -4.0mm.During the concrete pouring stage, the displacement of the slab was mainly caused by the vertical displacement of the propping system and the displacement of the composite slab itself.After concrete pouring, with the development of concrete hardening and enhanced constraint effect of surrounding beams, the displacement of the slab decreased gradually.To 7 days after pouring, the maximum displacement was 5.69mm, about 1/1300 of the slab span.After removing of the propping system, the displacement of the slab decreased gradually.To 7 days after pouring, the maximum displacement was 6.54mm, about 1/1131 of the slab span.Overall, during the construction stage, the vertical displacement of the slab is small, which indicats that the Improved Large Span Propping System can meet the displacement requirements during the construction stage.
Compared with the monitoring proposed in literature [15], during the concrete pouring stage, the maximum displacement of the slab was about 4mm (1.6 times of literature [15]), indicating that the new system can make better use of the strength of props.

Conclusions
In order to further improve the performance of Large Span Propping System for RCSLGs and reduce its lower propping frame, an improved propping system was proposed in this article.And an compared in-situ monitoring was conducted to verify its feasibility.The main conclusions are as follows: (1) The new type of propping system can further reduce the amount of lower propping frame than the original Large Span Propping System and improve the construction efficiency.
(2) The in-situ monitoring illustrates that the stress and displacement of the slab can meet the requirements of the current Chinese codes, indicating the feasibility of the improved system.
(3) Compared with the original system, the new system maintains the advantages of fully utilizing the bending stiffness of prefabricated layer and the strength of props.

Figure 1 .
Figure 1.Structure of reinforced concrete composite slab with lattice girders.

Figure 2 .
Figure 2. Schematic of two types of propping system for RCSLGs.

Figure 3 .
Figure 3. Standard slab layout of the tested floor.

Figure 7 .
Figure 7. Load distribution of force sensors.

Figure 8 .
Figure 8. Curves of strain of upper chord of the lattice griders.

Figure 9 .
Figure 9. Curves of strain of the distribution reinforcements in cast-in-situ layer.

Figure 10 .
Figure 10.Distribution of displacement of the whole slab from concrete pouring to initial setting.