Structural optimization design is not based on sacrificing structural safety and seismic performance to obtain economic benefits, but on the basis of structural theory, on the premise of engineering experience, under the guidance of understanding and flexible application of the essence of structural design norms, and with advanced structural analysis methods as means, the design is deeply adjusted, improved and enhanced, and the cost is reviewed and monitored. It is a process of further reprocessing of structural design. Structural cost control must run through the whole process of design and planning, including the geological investigation in the early demonstration and planning stage, the preliminary survey in the planning stage, the structural intervention in the scheme stage, the optimization of the structural scheme in the early expansion stage, the instilling cost awareness in the design institute in the construction drawing stage, the change of the construction drawing coordination stage, and the management of visa. Its technical key points are mainly reflected in the following aspects:
1. Consider the basement layout
The basement structure accounts for a large proportion in the structural cost, and the dispersion of the basement structure is relatively large, and the influence and correlation on other parts are not obvious, so the optimization design of the basement structure is very important to control the entire structural cost. First of all, we should pay attention to the full use of the area of public large basements, and do a good job of comparing the schemes of single-storey basements and multi-storey basements. Secondly, it is necessary to grasp the reduction of support cost and raise the elevation of the whole ±0.000 as far as possible, because this not only reduces the cost of support but also saves the excavation and external transportation of earth, reduces the influence of water pressure in the area rich in groundwater, and has a favorable impact on the design of basement floor and anti-pulling pile. For the structural cost control of the basement, it is also necessary to control the thickness of the soil cover at the top of the basement and the live load at the top. The thickness of the soil cover at the top of the basement is generally related to the landscape layout and the buried requirements of underground pipelines, which requires the landscape design and pipe network design to be involved in advance in the design and management process, and do a good job in fine design and professional cooperation. Strictly control the thickness of the soil covering the top of the basement. Finally, it is necessary to grasp the layout of the basement roof and bottom plate, to do a good job of comparing the cost of these structural layout schemes, to grasp the feasibility of the scheme in a full range, and to be careful about the choice of the scheme.
2. control the height
Under the premise of meeting the building facade and the net height of use, reducing the height of the height can not only reduce the length and volume of the vertical components, but also reduce the civil construction costs such as foundation and external installation, equipment and operating costs. For the general mid-range house, the cost can be reduced by 30 to 40 yuan /m for every 100mm reduction in height, and the basement will be higher. The height reduction can be achieved by controlling the beam height of the structure and the integrated wiring of each layer of the equipment. Some parts can also use a variable section beam or embedded in the beam and other measures to ensure the requirements of the floor height.
3, control the aspect ratio (that is, the ratio of the structure height and the effective width of the structure)
The larger the ratio of height to width of the building, the greater the anti-overturning moment of the main structure, and the longer the anti-lateral force components (shear walls) required for safety, which will increase the structural cost. Controlling aspect ratio has become an important part of structural optimization design.
4. Optimize shear wall setting
In order to meet the requirements of the code, the wall thickness of the shear wall must be increased more, and because it has become a short-limb shear wall, the reinforcement will be further increased. At this time, the wall thickness can be reduced by checking the stability of the overlimit wall. As a result, the wall thickness becomes smaller and the cost will be greatly reduced. The length and quantity of shear walls are mainly controlled by displacement index. The standard stipulates that the limit of interstory displacement ratio of pure shear wall structures is 1/1000. In order to give full play to the maximum role of shear walls, 1/1050~1/2000 can be used as the target limit of interstory displacement ratio in design, and unnecessary shear walls can be reduced under the premise of ensuring the safety and surplus of buildings. Thus saving the cost on the whole.
5. Optimize the arrangement and reinforcement of beams
The floor cost accounts for 9% to 13% of the total cost of the structure, and its weight accounts for about 22% of the weight of the entire house. The beam is a part of the floor, and its layout directly affects the stress and reinforcement of the plate. Reasonable layout of the beam has become the key to control the cost. For the common 8.1mx8.1m~9.0mx9.0m column network, parallel beams or cross beams with smaller load bearing floors such as standard floors are given priority, and their approximate comprehensive cost (calculating the number of steel bars, the number of concrete and the number of formwork) will be about 10% lower than that of well-shaped beams, while well-shaped beams should be selected for floors bearing larger loads such as basement roof. To meet the requirements of coordinated force and ensure the net height. At the same time, the trabeculars (such as kitchen, bathroom, etc.) set at the lower part of the small-span floor to support the upper shorter filled wall can be eliminated, because the floor itself is enough to carry those filled walls, and there is no need to set a separate beam, so that the beam is reduced, the cost is reduced, and the space is better. In the reinforcement of large-span frame beams, the upper thick steel bar should be avoided through the whole beam, and the small diameter steel bar configuration should be adopted as far as possible, which can effectively save the reinforcement amount of frame beams.
6. Control floor thickness
Floor thickness directly affects the structural load, 20mm thick floor weight accounts for about 3.3% of the total load of the standard layer, the increase in thickness, the increase in load, the beam, wall, foundation will increase, the cost will also increase. At the same time, for the smaller standard floor, the reinforcement is required according to the minimum reinforcement ratio, and the increase of the floor thickness will increase the reinforcement, so the floor thickness is also a control factor to reduce the cost in the actual engineering design.
7, the reasonable use of concrete labels
The unit price increases by about 5-8% for each grade of concrete marking; The influence on axial compression ratio of column and shear wall is obvious, so high grade concrete should be preferred. For the beam, the bearing capacity of the beam has little change, so the low grade concrete should be selected. For the plate, although the improvement of the label has improved the bearing capacity, the minimum reinforcement rate increases correspondingly after the improvement of the label, and the probability of cracking of the floor also increases, so the low label concrete should also be selected. In engineering design, the wall column concrete is usually one to two grades higher than the beam slab concrete, so as to maximize the bearing capacity of concrete.
8, the reasonable choice of steel bar materials
HRB400 steel bars are unmatched by other steels in terms of strength, ductility or cost performance. HRB335 steel bars are about 4% more expensive than HRB235 steel bars, and their strength is 43% higher. HRB400 grade rebar is about 2.9% more expensive than HRB335 grade rebar and 20% stronger. Therefore, the use of HRB400 grade steel bars instead of the traditional HRB235 grade steel bars and HRB335 grade steel bars as the bearing steel bars (except hooks) can effectively reduce the amount of steel bars in the project. Every project needs to bring together the wisdom and hard work of all the parties involved. In the early stage, the comparative analysis of multiple schemes should be carried out to find the balance point of safety, economy and aesthetics. In the process of optimization, measures are taken to reduce the section of the member, reduce the thickness of the sheet, adopt a shorter beam height, reasonably increase the net height of the floor, reduce the shear wall, etc. These practices can not only reduce the amount of concrete, reduce the weight of the structure, but also reduce the structural stiffness, reduce the seismic force. The purpose of structural optimization is to remove the ineffective structural cost while ensuring the quality of the project, achieve the minimum input-output ratio, and finally achieve the perfect unity of building safety, economy and beauty.
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