In general, the principle of length greater than width should be followed when arranging the column net, which can reduce the amount of steel for the rigid frame, and also reduce the wind load supported by the column, thereby reducing the amount of steel in the support system.
Example 1: When the building size is 60×50m, 60m should be used as the length direction and 50m as the span direction, that is: 60(L)×50(W) instead of 50(L)×60(W).
The comparison of technical economy shows that the most economic column under standard load is 8~9m. When it exceeds 9m, the amount of steel for roofing purlins and wall frame system increases too much, and the comprehensive cost is not economic. The standard load here means: the roof live load is 0.3KN/m2, and the basic wind pressure is 0.5KN/m2. When the load is larger, the economic column distance should be reduced accordingly. For plants with more than 10 tons crane, the economic distance should be 6~7m.
When arranging the column spacing, if unequal column spacing is required, the end span should be arranged as small as possible than the middle span, because the end span wind load is larger than the middle span, and when the continuous purlin design is used, the end span and the mid-bend bend are always larger than the other spans. A smaller end span makes the roofing design more easier.
Different production processes and usage functions largely determine the span of the plant. Some owners even require light steel manufacturers to determine a more economic span according to their own functions. In order to meet the production process and function as much as possible, a reasonable span should be determined according to the height of the house. Under normal circumstances, when the column height and load are constant, the span is appropriately increased. The steel consumption of the rigid frame is not obvious, but the space is saved, the basic cost is low, and the comprehensive benefits are considerable. Through a large number of calculations, it is found that when the height is 6m and the column distance is 7.5m, the load condition is completely the same. The steel capacity (Q345-B) of the rigid frame with a span of 18-30m is 10-15kg/m2. The steel for the rigid frame unit between 21-48m is 12-24kg/m2. When the height is 12m and the span is more than 48m, multi-span rigid frame (intermediate design swing column) should be used. The frame saves more than 40%. Therefore, when designing the portal frame, the economical span is selected according to the specific requirements, and it is not appropriate to blindly pursue the large span.
The slope of the roof should be determined according to the comprehensive factors such as the structure of the roof panel and the length of the drainage slope and the height of the column structure, generally 1/10~1/30. Studies have shown that different roof slopes have a greater impact on the amount of steel used in the rigid frame.
Below we calculate and analyze the steel consumption under different roof fractures with a single span of 42m and a cornice height of 6m:
When the roof fracture is 0.5:10, the frame weight is: 3682 Kg
When the roof breakage is 1.0:10, the frame weight is: 3466 Kg
When the roof fracture is 1.5:10, the frame weight is: 3328 Kg
When the roof fracture is 2.0:10, the frame weight is: 3240 Kg
It can be seen that for a single-span rigid frame, a better way to reduce the weight of the rigid frame is to increase the slope of the roof. The greater the slope, the more steel is saved.
However, for multi-span frames, the situation is different. A large slope will increase the amount of steel used in the frame, because large breaks will result in an increase in the length of the inner column. When the span of the building is large, the increase in the degree of breakage can reduce the deflection of the roofing steel.
The economical slope is calculated by research:
Multi-span building: 1:20
Single span, span less than 45m: 0.5:10
Single span, span less than 60m: 1.5:10
Single span, span greater than 60m: 2.0:10
In fact, the selection of the slope of the roof is also related to the presence or absence of the daughter wall, and the increase in the slope will lead to an increase in the cost of the daughter wall.
The height of the eave has a great influence on the cost, mainly in the following aspects.
1. The increase in the height of the eave will lead to an increase in the area of the wall panel, an increase in the wall purlins, and an increase in the amount of steel used in the column;
2. If the steel column has no lateral support (such as the middle column, or the side column can not be set up), the height of the eave will have more influence on the weight of the frame;
3. An increase in the height of the eave will result in an increase in the wind load acting on the frame. If the height of the building width is >0.8, in order to control the lateral displacement, sometimes it is even necessary to change the column foot from the hinge to the just joint.
The height of the eave is determined by the following factors:
1. The net height requirement at the mouth of the mouth;
2. The net height requirement of the interlayer and the height of the sandwich beam when there is a sandwich;
3. When there is a crane, the height of the crane beam and the crane hook.