In view of the characteristics of a high floor and the heavy load of logistics buildings, a partially prefabricated partially encased assembled composite beam (PPEC) is proposed in order to achieve the low cost construction of such buildings. In this research, the mechanical properties of PPEC beams were studied experimentally. The effects of the concrete strength grade, steel content, shear span ratio, and fabrication methods on the mechanical properties of the PPEC beams were analyzed. The results showed that the proposed structural form of the PPEC beams was generally feasible. Based on the test results, a practical shear formula for PPEC beams was proposed, and the calculated results were in good agreement with the test results.
In China, e-commerce is gradually changing peoples’ lifestyles. With the prosperity of e-commerce, the logistics industry has achieved leapfrog development. The rise of the logistics industry has promoted the concept of logistics buildings. Logistics parks, which are flourishing all over China, which are composed of typical logistics buildings. A remarkable feature of a logistics building is its heavy load. However, under the heavy load of a traditional cast-in-place structure, the design of the horizontal members is often controlled by deflection rather than strength, which will lead to large cross-sections, high reinforcement ratios, and poor economic benefits. In view of this situation, the use of steel-concrete composite beams with high rigidity and high strength is a sensible choice for reducing construction costs. Another characteristic of a logistics building is a high floor height. The traditional cast-in-place structure popular in China needs a large amount of scaffolding and formwork, especially for a relatively high scaffolding project, which is very costly. Therefore, an unsupported construction method is a reasonable way to reduce the cost of construction. In conclusion, an assembled steel-concrete composite structure system is more suitable for the construction of logistics buildings.
Some scholars have carried out a series of studies on the assembled steel-concrete composite structure system. The SR-PC (a frame structure composed of steel reinforced precast concrete members) method proposed by the Japanese Prefabricated Building Association [
Before introducing PPEC beams, PEC (partially encased composite) beams and the research for these beams are briefly introduced. Partially encased beams are elements in which the web of the steel section is encased with reinforced concrete [
Prefabricated partial configuration.
Complete beam-slab system.
The PPEC beam proposed in this paper can be designed as unsupported construction due to the existence of I-steel, and the integral pouring of beam-slab concrete can increase the integrity. This beam has great potential advantages for simplifying construction technology and reducing project cost, and it has good prospects for promotion. Therefore, it is necessary to study the mechanical properties of this new composite beam and provide a reference for its engineering applications.
A total of sixteen specimens were designed, including six flexural specimens, eight shear specimens, and two cast-in-place contrast specimens. T-shaped beams with flanges were used in all the specimens. The web width of each T-shaped composite beam was 150 mm, the web height of each flexural specimen was 170 mm, and the web height of each shear specimen was 150 mm. The width and the thickness of the concrete flange slab for each specimen were 650 mm and 80 mm, respectively. Each I-steel beam was welded by Chinese steel (brand Q235), each upper flange width was 100 mm, and each lower flange width was 150 mm. In all the specimens, the distance between the upper flange of the I-beam and the slab surface of the T-shaped composite beam was 50 mm, and all specimens were reinforced with HRB335 steel bars. The diameter of the longitudinal rebar was 12 mm and that of the slab rebar was 8 mm. The diameters of the stirrups in all specimens were 8 mm except for one shear specimen (PPEC-7-S) with 10 mm stirrups. The concrete strength grades of the test beams were C20, C30, and C40. The prefabricated part of the PPEC beam was composed of prefabricated
Design and test results of the specimens.
No. | Shear span length (mm) | Shear span ratio | Steel ratio (%) | Concrete strength | Stirrup | Ultimate load (kN) | Bearing capacity | |
---|---|---|---|---|---|---|---|---|
Mu | Vu | |||||||
PPEC-1-M | 650 | 2.6 | 7.01 | C30 | B8@150 | 402.75 | 130.89 | — |
PPEC-2-M | 650 | 2.6 | 8.28 | C30 | B8@150 | 463.50 | 150.64 | — |
PPEC-3-M | 650 | 2.6 | 9.55 | C30 | B8@150 | 468.13 | 152.14 | — |
PPEC-4-M | 650 | 2.6 | 8.28 | C20 | B8@150 | 421.55 | 137.00 | — |
PPEC-5-M | 650 | 2.6 | 8.28 | C40 | B8@150 | 453.50 | 147.39 | — |
PPEC-6-M | 650 | 2.6 | 8.28 | C30 | B8@150 | 382.40 | 124.28 | — |
PEC-7-M | 650 | 2.6 | 8.28 | C30 | B8@150 | 488.75 | 158.84 | — |
PPEC-1-S | 400 | 2.0 | 6.84 | C30 | B8@200 | 428.25 | — | 214.13 |
PPEC-2-S | 400 | 2.0 | 7.76 | C30 | B8@200 | 527.88 | — | 263.94 |
PPEC-3-S | 500 | 2.5 | 6.84 | C30 | B8@200 | 399.38 | — | 199.69 |
PPEC-4-S | 300 | 1.5 | 6.84 | C30 | B8@200 | 464.13 | — | 232.07 |
PPEC-5-S | 400 | 2.0 | 6.84 | C40 | B8@200 | 513.75 | — | 256.88 |
PPEC-6-S | 400 | 2.0 | 6.84 | C20 | B8@200 | 407.25 | — | 203.63 |
PPEC-7-S | 400 | 2.0 | 6.84 | C30 | B10@200 | 452.63 | — | 226.32 |
PPEC-8-S | 400 | 2.0 | 6.84 | C30 | B8@100 | 476.88 | — | 238.44 |
PEC-9-S | 400 | 2.0 | 6.84 | C30 | B8@200 | 478.75 | — | 239.38 |
Measured value of material mechanical performance index.
Mechanical properties of steel | Mechanical properties of concrete | |||||
---|---|---|---|---|---|---|
Steel form | Specifications (mm) |
|
|
Concrete strength (MPa) |
|
|
Steel plate | 4 | 276.5 | 453 | C20 | 21.0 | 20.6 |
6 | 287.0 | 442.0 | ||||
8 | 279.2 | 421.0 | C30 | 23.9 | 22.6 | |
10 | 308.0 | 410.0 | ||||
Rebar | 8 | 192.0 | 273.0 | C40 | 36.3 | 35.0 |
10 | 200.8 | 302.6 | ||||
12 | 194.0 | 234.5 |
Cross-sectional feature information of the PPEC beam.
First, the unequal flange I-beam was forged, and then the polystyrene board with the corresponding thickness was adhered to the upper part of the web and so that it clung to the top flange. Second, the prepared semiclosed stirrups were put into the I-steel one at a time. Third, the longitudinal bars are placed in the stirrup hooks along the length of the I-steel and fixed in place. Fourth, the template was supported and the concrete was poured to complete the precast parts. Fifth, when the precast part reached the predetermined strength, the cast-in-place part was made. The supporting template, the binding plate reinforcement, and the pouring concrete were completed in turn. This process is illustrated in Figure
Construction process of the specimens. (a) Preparation of prefabricated parts. (b) Completed prefabricated parts. (c) Preparation of cast-in-place parts. (d) Completed cast-in-place parts.
The test was completed on a 500
Test set-up and measuring point layout of beam specimens.
Under the influence of the shear span ratio, the sixteen beam specimens usually presented bending and shear failure modes. The shear span ratio caused the shear failure to be mainly divided into shear-compression failure and cable-stayed failure. The bending failure was a typical bending failure mode. The failure modes of the specimens are shown in Figure
Typical failure modes. (a) Bending failure. (b) Shear-compression failure.
For beams subjected to bending failure, there was no significant difference in the failure modes between the PPEC specimens and the PEC specimens. Figure
Moment-rotation curves at the midspans of the specimens.
Shear force-rotation curves of the specimens.
From Figures
The experimental results showed that the typical failure modes of the PPEC beam members could be divided into bending failure and shear failure. The performances of the PPEC beam members under the two failure modes were quite different. The mechanical properties of the beam members were related to the shear span ratio, concrete strength grade, and steel ratio because they determined the interaction between the I-steel and the concrete to some degree and then they affected the failure mode and the bearing capacity of the members. The concrete strength and the steel ratio had a significant impact on the shear resistance and the bending performance of the beam. The shear properties of the beams were more sensitive to the shear span ratio and the stirrup ratio, and the flexural properties of the beams were more sensitive to the stirrup mode.
The influence of concrete strength on the bearing capacity of the PPEC beams is shown in Figure
Effect of
With special instructions, it was shown that the change of steel ratio referred to the change of the flange thickness of the I-steel for the flexural specimens and to the change of the web thickness of the I-steel for the shear specimens. Figure
Effect of the steel ratio on the specimen capacity.
Figure
Effect of
Figure
Effect of the stirrup ratio on the shear capacity.
The test results showed that the stirrups had a significant effect on the flexural capacity of the PPEC beams, and the flexural capacity of the semiclosed stirrups was approximately 21.2% higher than that of tie-rod PPEC beams. The details are shown in Figure
Effect of the stirrup form on the moment capacity.
Figure
Effect of the construction technology on the specimen capacity.
Based on the plasticity theory [
The theoretical values versus the test values of the flexural capacity.
No. | Theoretical value (kN·m) | Test value (kN·m) | Theoretical value/test value |
---|---|---|---|
PECB-1-M | 117.10 | 130.90 | 0.90 |
PPEC-2-M | 136.72 | 150.64 | 0.91 |
PPEC-3-M | 152.20 | 152.14 | 1.00 |
PPEC-4-M | 135.94 | 137.00 | 0.99 |
PPEC-5-M | 141.73 | 147.39 | 0.96 |
Average value | — | — | 0.95 |
Variation coefficients | — | — | 0.05 |
The calculation model of the shear capacity of the PPEC beams is shown in Figure In the cross section of the PPEC composite beam, the precast concrete section occupied a considerable proportion. The calculation based on the small cast-in-place concrete strength grade was too conservative, and the shearing effect of the cast-in-place and precast two-part concrete needed to be considered reasonably. Using the area as the weight, the weighted average method was used to obtain the converted concrete strength, as follows: where In the shear specimens, the concrete flange was in a state of shear-compression force. The concrete flange in the compression zone contributed to the shear capacity of the inclined section. This has been proven in paper [ where
Referring to the expression of the shear capacity of a JGJ-138 composite beam, combined with the analysis of the factors affecting the shear capacity described in the previous section, the shear capacity of a PPEC beam can be expressed as
The regression analysis was carried out by the shear capacity values obtained by the test, and the values of
The shear capacity of the PPEC beam obtained according to the above method is shown in Table
The theoretical values versus the test values of the shear capacity.
No. | Theoretical value (kN·m) | Test value (kN·m) | Theoretical value/test value |
---|---|---|---|
PECB-1-S | 208.93 | 214.13 | 0.98 |
PPEC-2-S | 275.80 | 263.94 | 1.04 |
PPEC-3-S | 195.64 | 199.69 | 0.98 |
PPEC-4-S | 230.44 | 232.07 | 0.99 |
PPEC-5-S | 252.91 | 256.88 | 0.98 |
PPEC-6-S | 222.51 | 203.63 | 1.09 |
PPEC-7-S | 221.26 | 226.32 | 0.98 |
PPEC-8-S | 228.13 | 238.44 | 0.96 |
Average value | — | — | 1.00 |
Variation coefficients | — | — | 0.045 |
In summary, the mechanical properties of the PPEC beams proposed in this paper are excellent. Based on the experimental analysis, the corresponding flexural and shear capacity design methods were proposed. The results showed that the calculated results were in good agreement with the experimental results, which had certain guiding significance for the practical application and popularization of this kind of composite beam.
Through an experimental study of the mechanical properties of fourteen T-shaped PPEC beams and two T-shaped PEC beams, the following conclusions were drawn: The improved PPEC beam structural form proposed in this paper is generally feasible. The I-steel, cast-in-place concrete, and precast concrete worked well together without obvious slips. Compared with the cast-in-place PEC beams, the PPEC beams had the same failure mode and a similar bearing capacity. Prefabrication and casting-in-place had little effect on the mechanical properties of the specimens, especially in the elastic stage. With the increase of the concrete strength, the flexural and shear capacities of the PPEC beams were improved except for the flexural specimen of C40. With the increase of the steel ratio, the flexural and shear capacities of the PPEC beams increased. With the increase of the shear span ratio, the shear capacity of the PPEC beams decreased gradually. With the increase of the stirrup ratio, the shear capacity of the PPEC beams increased gradually. Based on the test results, the design method of the shear capacity of the PPEC beams was presented. The results show that the test results were in good agreement with the calculation results, which can be used for reference in practical engineering design.
The data used to support the findings of this study are included within the article
The authors declare that they have no conflicts of interest.
The research described in this paper was financially supported by the Natural Science Foundation of Jiangxi Province of China (no. 20171BAB206053). The authors thank LetPub (