The practical study of steel-wood composite column in light and low-carbon building

Steel improves the safety performance of buildings, but also greatly increases the carbon emissions of buildings, which is not conducive to environmental development. Based on the consideration of reducing building carbon emissions, this study proposes to replace the pure steel load-bearing column in the low-rise building with a new load-bearing column in the form of steel-wood combination, and establishes a theoretical and numerical model to explore its reasonable combination size to verify the safety of the structure in practical applications. And the carbon emission assessment and analysis of the whole life cycle. It is found that the steel and wood structure can replace the pure steel structure of the corresponding size in the low-rise building, which greatly reduces the carbon emissions of the building while achieving structural safety.


Introduction
As an emerging building structure system, steel-wood structure has the advantage of not only making up for the defects of traditional steel structures, but also meeting the requirements of environmental protection and sustainable development [1] .The aim of this study is to explore the sustainability of applying steel-wood composite column buildings by replacing the pure steel load-bearing columns in buildings with new load-bearing columns in the form of steel-wood combinations [2] .With the help of theoretical and numerical models to find the suitable combination size, and the latest method for assessing the carbon emission of the whole life cycle of the building is used to evaluate the two forms of building models separately and to compare and analyze the environmental value of the new structure.

Introduction to Architectural Models
This research model is built with reference to existing examples of environmentally friendly buildings, and the main body of the building is restored 1:1, as shown in Figure 1.The modeling process strives to accurately restore the actual situation of the research object, so as to better carry out subsequent theoretical calculations and numerical simulations.At the same time, the selection of the building in the eco-friendly villa area as the research object is also in line with the goal of exploring the sustainability of the building.

Theoretical load demand and dimensional analysis
The predicted building mass data is the key to calculate the load-bearing column loads.This study analyzes the most unfavorable working condition for the first floor, so only the mass prediction for the second floor is used.Considering the actual living conditions and the requirements of the industry safety index, the final design load mass of the first floor building model is 55.97 tons.

Figure 1. Illustration of architectural model
In this study, the steel pipe is made of Q235, and the external dimensions are all set to 90mm*90mm, the initial square steel pipe columns of the building have a wall thickness of 12mm, and both load-bearing columns are bolted and welded through the steel pipe to the steel beam frame.From the load equalization theory, the initial load on each load-bearing column is 39978.57N,and the sectional compression is 7711.92kPa [3].The load carrying capacity of the initial steel columns is verified in the following by applying Euler's method: where Pcr is the load carrying capacity value (N); E is the modulus of elasticity of steel; I is the moment of inertia of the cross-section; K is the effective length coefficient; l is the length (mm); r is the equivalent cross-sectional radius (mm), and r = ; A is the cross-sectional area (mm 2 ); T is the interface concentration coefficient.The calculation results show that the theoretical yield limit of the initial load-bearing column is 81.32kN and the critical pressure it can withstand is 18.16MPa, which meets the architectural design requirements.

Figure 2. Static analysis diagram of bearing column
Preliminary calculations of the combined dimensions of single columns are required to meet the structural requirements after replacement with steel and wood structural columns, and the calculation method of equivalent modulus of elasticity is introduced according to the study of Li Wei et al. on the bearing capacity test of L-shaped wood and steel columns [4] .In the performance study of hollow glued laminated timber columns by Chen Link et al. the formula for calculating the bearing capacity of axially compressed specimens subjected to bending moment along the main axis direction of one section is given, and the working conditions of this model are consistent, so it can be used as a reference [5] .Its ultimate bearing capacity can be calculated according to equations ( 2) to (3) as follows:

= +
(2) where EA (Es) is the modulus of elasticity of the combined member (fir); μ is the cross-sectional anisotropy coefficient; C (Cs) is the steel (wood) content rate, i.e., the percentage of steel (wood) in the cross-section to the total cross-sectional area; δt is the yield strength of steel; N is the design value of axial pressure (N); An is the net cross-sectional area (mm 2 ), An=A in the non-porous state; fc is the smooth grain compressive strength of fir (N/mm 2 ); e0 is the equivalent eccentric moment of the load-bearing column under force (mm); Wn is the net section resistance moment (mm 3 ); fn is the design value of section bending strength (N/mm 2 ).The sketch of the static analysis of the load-bearing column is shown in Figure 2. Using the load-bearing data of the original design pure steel column, the optimal value of the cedar filling size b is 82mm, corresponding to the steel pipe wall thickness of 4mm, based on the iterative calculation of the ant colony algorithm in MATLAB software.This optimization method based on ant colony algorithm has better global search performance and robustness, which can effectively improve the efficiency and accuracy of infill structure design.

Model setting and parameter adjustment
The objects of investigation in this study include pure steel tube columns, steel and wood composite columns and the corresponding first floor support system, which are subjected to displacement and load analysis by means of surface loading or point loading, respectively, with the compression direction kept vertical down and parallel to the axis of the column.All structural models are meshed using the hexahedral scanning method, and the neutral axis algorithm is applied [6] .The meshing can effectively reflect the geometry and material properties of the structural model and improve the calculation accuracy.The schematic diagram of the meshing and model is shown in Figure 3.

Analysis process and results
According to GB 50011-2010, the allowable range of deformation of load-bearing columns in civil buildings should be between 0 and 1/500, so the allowable deformation of load-bearing columns in this building model should not exceed 5.4 mm [7] .During the actual loading, the response of the load-bearing column is observed through the FEA cloud diagram.The FEA cloud in Figure 4 shows the analysis results of four types of load-bearing columns, including a steel column with a wall thickness of 12 mm, a steel column with a wall thickness of 4 mm, a wooden column with a side length of 82 mm, and a steel-wood combination column, corresponding to Ⅰ, Ⅱ, Ⅲ, and Ⅳ in Figure 4. Figure 6a shows the corresponding load displacement curves.When loaded with axial displacement, the pure steel load-bearing column with a wall thickness of 12 mm showed localized scaling shrinkage, but no lateral buckling occurred.The same loading test was carried out on a pure steel column with a wall thickness of 4 mm, and the displacement maps showed that the column showed scaling shrinkage and a certain degree of lateral bending, which proved that it could not meet the load-bearing requirements of the building.Wooden columns with a side length of 82 mm are also unsuitable as load-bearing columns for this building.Although wood has high compressive strength, it will deform when subjected to axial pressure beyond its load-bearing capacity, which may increase the risks and hazards of the building.4-IV gives the external and longitudinal cross-section cloud diagrams of the combined structural columns of steel columns with a wall thickness of 4 mm and wood columns with a side length of 82 mm.The maximum stress point of the combined structural column appeared on the four longitudinal edges of the outer square steel tube, showing a wavy distribution pattern, with a maximum stress value of 249.9 MPa.The stress distribution of the internal filler square wood was more even, with no odd deformation, while the outer 4 mm steel tube appeared to have a tiny collapse or protrusion in the center of the external façade.From Fig. 6a, it can be seen that the maximum load capacity of the combined column is 69.14 kN, corresponding to a displacement of 2.18 mm, which satisfies the requirement of coping with the actual 39.97 kN load as a load-bearing column in this building.The maximum stress value of the combined column was 144.3MPa and the longitudinal displacement was 1.42mm when loaded with live force alone, and there was no obvious deformation on the external facade and internal part of the column, which proved that the combined column could meet the design requirements of this building.In order to verify the practical applicability of the steel-wood combination columns, the first floor building model was subjected to live load analysis, and the corresponding displacement and stress cloud diagrams are shown in Fig. 5a and b, and the load displacement curves are displayed in Fig. 6b.14 load-bearing columns were uniformly loaded and did not show any strange deformation, and the great stress points appeared in the corners at the top and bottom parts of each column.When the load of each column reaches 39978.57N, the displacement and deformation scales of the 14 load-bearing columns are between 1.347 and 1.418mm, with an error rate of 5.27%, and the performance of each column's pressure-bearing performance is basically the same, and the whole also meets the requirements of the building code.From the test results, there is an error rate of 9.64% between the maximum bearing capacity of the steel column with 12mm wall thickness and the theory, and the error rate of the combined column is 14.97%, which may be caused by the theoretical model not fully considering the connection, material anisotropy and other factors.The load-bearing performance of the replaced combined column is about 22.45% lower than before, but its load-bearing capacity, deformation scale and other aspects have met the design requirements and comply with the code, and have been fully satisfied in actual use, and the initial design has essentially caused partial waste of resources.Therefore, the test shows that the steel and wood structural load-bearing columns of this size have the value of safety performance for practical application.

Evaluation Program and Content
In this study, Rhino software is used to build the corresponding BIM model for building material inventory statistics, and the whole life cycle carbon emission assessment is carried out under the guidance of the energy consumption and energy efficiency evaluation system method.The system boundary of building carbon emission assessment is shown in Figure 7.A bidirectional adjustment factor is added to the original evaluation system for some evaluation modules during the carbon emission assessment in order to highlight the differences between the two types of buildings and identify the root causes of the problems [8] .

Analysis of carbon emission assessment results
The results of the optimized building carbon emissions based on a 50-year building lifetime are shown in Table 1 The results of the assessment show that the replacement of load-bearing columns more than doubles the carbon reduction of the original building.Only in the maintenance process of the building the latter exceeds the original building, because the steel and wood columns are more complex than the pure steel column structure, not only need to pay attention to the use of rust prevention but also need to prevent pests, but the carbon emissions of this process only accounts for the whole life cycle carbon emissions of 1.82% to 3.09%, which is not enough to change the results [9] .Steel and wood bearing columns reduce the use of steel and use wood instead, saving transportation costs and facilitating the separation of steel and wood for recasting or direct reuse at the end of the building life cycle, avoiding the waste of resources caused by being discarded as construction waste.In general, steel and wood columns have higher environmental value than pure steel columns.

Conclusion
This paper proposes a steel and wood structural load-bearing column for lightweight construction, and analyzes its comparative safety and environmental performance.The results of the study found that the steel-wood structural column combines the advantages of both steel and wood, which not only has a bright mechanical performance, but also can greatly reduce the impact on the environment, which is an environmentally friendly building material.This study will continue to follow the development trend of steel and wood construction materials, and strive to provide more theoretical support for the promotion of green building.

Figure 3 .
Figure 3. Construction drawings and load-bearing column marking

Figure 4 .Figure 5 .Figure 6 .
Figure 4.The first layer support system model and grid division

Figure 7 .
Figure 7. Building carbon emission assessment boundary identification diagram

Table 1 .
. Building carbon emissions data