How to choose the steel structure plan to reduce the amount of steel used?

Choice of building length and width

Generally speaking, the principle of length greater than width should be followed when arranging column nets, which can reduce the amount of steel used in the rigid frame and at the same time reduce the wind load of the support between the columns, thereby reducing the amount of steel used in the support system. For example, if the size of the building is 60×50m, when arranging the factory building, 60m should be taken as the length direction and 50m as the span direction, that is, 60(L)×50(W) instead of 50(L)×60(W) .

Choice of column distance

The technical and economic comparison shows that the most economical column distance under the standard load is 8-9m. When it exceeds 9m, the steel consumption of the roof purlin and wall frame system increases too much, thus the comprehensive cost is not economical. The standard load here refers to: roof live load 0.3KN/m2, basic wind pressure 0.5KN/m2. When the load is greater, the economic column distance should be reduced accordingly. For a workshop with a crane of more than 10 tons, the economic column distance should be 6~7m.

When arranging the column distance, if unequal column distance is required, then the end span should be arranged as small as possible than the middle span. This is because the wind load of the end span is larger than that of the middle span. In addition, when the continuous purlin design is adopted, the deflection of the span and the mid-span bending distance are always greater than those of other spans. The use of a smaller end span can make the roof purlin design more convenient and economical.

Example 1: Building length = 70m

The economic column distance is desirable: 1*7m + 7*8m + 1*7m or 1*8m+ 6*9m + 1*8m

Example 2: Building length = 130m, with a 10 tons crane

The economic column distance can be: 1*5.5m + 17*7m +1*5.5m or 20*6.5m

Determination of reasonable span

Different production processes and functions determine the span of the plant to a large extent. Some owners even require light steel manufacturers to determine a more economical span based on their own functions. In order to meet the production process and functions as much as possible, a reasonable span should be determined according to the height of the house. Under normal circumstances, when the column is high and the load is constant, the increase in steel consumption of the rigid frame is not obvious if the span is appropriately increased, but the space is saved, the foundation cost is low, and the overall benefits are considerable.

Through a large number of calculations, it is found that when the eaves height is 6m, the column distance is 7.5m, and the load conditions are exactly the same, the unit steel consumption (Q345-B) of the rigid frame with a span between 18-30m is 10-15kg/m2. The unit steel consumption of the rigid frame with a span between 21-48m is 12-24kg/m2. When the eaves height is 12m and the span exceeds 48m, a multi-span rigid frame (swinging column will be added in the middle) should be used. Then the steel consumption could save more than 40% that of a single-span rigid frame. So when designing the portal frame, choose a more economical span according to the specific requirements, and it is not advisable to blindly pursue a large span.

Choice of the roof slope

The roof slope needs to be determined according to comprehensive factors such as the structure of the roof panel, the length of the drainage slope and the height of the column structure, and it is 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.

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 larger the slope, the more saving the amount of steel used. However, the situation is different for multi-span frames. A large slope will increase the amount of steel used in the multi-span frame. This is because a large slope will increase the length of the inner column. When the span of the building is large, the increase in the roof slope can reduce the deflection of the roof steel beam. In fact, the choice of roof slope is also related to whether the building has a parapet or not. Increasing the slope will lead to an increase in the cost of the parapet.

Choice of eave height

1.The height of the eave has a greater impact on the cost, mainly in the following aspects.

a.The increase in the height of the eave will increase the area of the wall panel and the wall purlin, and the steel consumption of the column will be increased.

b.If the steel column has no lateral support (such as the center column or the side column cannot be provided with corner bracing), the height of the eave will have a more prominent influence on the weight of the frame.

c.An increase in the height of the eave will result in an increase in the wind load acting on the frame. If the cornice height/building width>0.8, in order to control the lateral displacement, sometimes it is even necessary to change the column foot from hinge to rigid connection.

2.The height of the eave is determined by the following factors：

a.The clear height requirement at the eave.

b.When there is a mezzanine, the clear height requirements of the mezzanine and the height of the mezzanine beam.

c.When there is a crane, the height of the crane beam and crane hook.

Temperature zone

According to light steel regulations, the maximum longitudinal length should not be greater than 300m, and the transverse length should not be greater than 150m. The expansion joints of the longitudinal temperature section can be arranged in a double-column arrangement (figure TZ -a), or the purlin can be connected to a single-column expansion joint with an elliptical hole (figure TZ -b).

Figure TZ -a  the practice of double frame expansion joint

Figure TZ -b  the practice of single frame expansion joint

What needs to be explained here is that the maximum length stipulated by the regulations is not the length that all buildings allow. For different buildings, the maximum temperature zone should be calculated according to the use conditions of the building itself and the natural conditions of the location.

Support arrangement

1.Function of support

Between each temperature section of the portal frame column network, a complete support system should be arranged to form a complete space structure system. The lateral stability of the light portal frame in the width direction is ensured by the rigidity of the frame to resist the lateral load it bears. Since the longitudinal structural rigidity in the length direction is relatively weak, it is necessary to provide supports along the longitudinal direction to ensure its longitudinal stability.

The forces on the supports are mainly longitudinal wind loads, crane braking forces, seismic effects, and temperature effects. When calculating the internal force of the support, it is generally assumed that the nodes are hinged, and the effect of eccentricity is ignored, and the general support is considered by tie rods. Therefore, it is generally suitable for a two-way layout.

2.Common types of support

Figure 1 shows the general layout of the layer support and the transmission path of the wind load acting on the gable wall. The type of support between columns commonly used in light portal frames is shown in Figure 2. Due to building function and appearance requirements, or process equipment layout, when the above supports are not allowed to be used, longitudinal frames can be considered. At this time, the bending stiffness of the weak shaft of the column needs to be used.

Figure 1  Roof support load transfer path

Figure 2  Common types of column support

3.Basic principles of support arrangement

a.Although some roof panels have a certain skin effect, this effect is currently difficult to quantify, so this skin effect cannot be considered when designing support.

b.The column support should be located on the same span as the roof support as much as possible. When the wall cannot be installed due to the door opening on the wall, the column support can be set on the adjacent span.

c.Generally speaking, the spacing of supports should generally not exceed 5 spans. It should be 30~45m if there is no crane, and when there is a crane, the spacing should generally not be greater than 60m.

e.The roof support needs to be disconnected at the ridge. (see figure 1)

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