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 Structural Design

Structural Design

Steel Structure detail shows the specification of the portal frame structure which commonly use building structure for Warehouse and Aircraft Hangar.

The steel structure may be a structure mainly made from steel and it’s one among the most sorts of building structures. The characteristics of steel are high strength, lightweight, excellent overall rigidity and powerful deformability. So it’s especially suitable for large-span, super-high and super-heavy buildings. The steel structure may be a steel beam, steel column, steel truss and other components made from section steel and plate or rivets connect each component or part.

Steel Structure Detail for Port Frame Structure:

The lightweight portal frame may be a single-story steel building which beams and columns rigidly connected. It’s the benefits of straight forward structure, lightweight and every one the components manufactured within the factory easy for assembling on site.

The portal frame steel structure widely used for Industrial, commercial, and agricultural buildings, such as steel warehouse, workshop building, Storage, Poultry building, and aircraft hangar.

The portal frame can divide into single-span (Figure a), double-span (Figure b), multi-span (Figure c) cantilever steel frame (Figure d) ) and steel frame with the adjacent frame (Figure e).

In the multi-span rigid frame, the connection between the column and the roof beam generally hinged, and the multi-span rigid frame single-slope roof (Figure f).

The multi-span rigid frame consisting of multiple double-slope roofs can also be used (Figure g). The beam-column cross-section can be equal or variable, and the base of columns hinged or rigid connected.

Length and Width of Steel Building :

Generally speaking, according to the principle that the long side is greater than the width, the amount of steel used in the rigid frame can be reduced, and the support between the columns can be reduced, thereby reducing the amount of metal used in the support system.

Example 1: The size of the building is 60x50m, 60m should use as the length and 50m as the width, that is: 60 (L) x50 (W), not 50 (L) x60 (W).

Column Distance

The most economical column distance under standard load is 7.5-9m. When it exceeds 9m, the steel consumption of roof purlin and wall girt will increase too much, and the overall cost is not economical. The standard load here refers to 0.3KN / m2 for live roof load and 0.5KN / m2 for essential wind pressure. When the loading is more significant, the economic column distance should reduce accordingly. As for workshop building with more than 10 tons crane, the financial column spacing should be 6-7m.

When arranging column spacing, if unequal column spacing is required, try to arrange the end span column spacing to be smaller than the center span. It is because the wind load at the end span is larger than the center span. Besides, when using continuous purlin design, The deflection of the end span and the mid-span bend are always more significant than other spans. Using smaller end spans can make roof purlin design more convenient and economical.

Example 1: Building length = 70m

Economical column distance is available: 1 @ 7 + 7 @ 8 + 1 @ 7 or 1 @ 8 + 6 @ 9 + 1 @ 8

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

Economical column distance is preferable: 1 @ 5.5 + 17 @ 7 + 1 @ 5.5 or 20 @ 6.5

Determination of a reasonable span

Different production processes and use functions largely determine the span of the metal building. Some owners even require steel building manufacturers to determine a more economical span based on their useful features. A reasonable span should decide according to the height of the steel building. In general, when the column height and load are constant, the span is appropriately increased. The increase in steel consumption of the rigid frame is not apparent, but it saves space, the foundation cost is low, and the comprehensive benefits are considerable.

 

Through a large number of calculations, it finds that when the eaves height is 6m, the column distance is 7.5m, and the load conditions are entirely consistent, the steel consumption of the rigid frame (For Q345B steel) width between 18-30m is 10-15kg / m2. The amount of metal used for rigid frame units between 21-48m is 12-24kg / m2. When the eave height is 12m, and the width exceeds 48m, a multi-span rigid frame (sway column set in the middle) should use. The frame saves more than 40%, so when designing the rigid portal structure, you should choose a more economical span according to specific requirements, and you should not pursue a large span.

Steel Structure Detail-Roof slope

 

The roof slope decides according to comprehensive factors including the structure of the roof, the length of the drainage slope, and the height of the columns. Generally, it is 1/10 ~ 1/30. Studies have shown that different roof slopes have a significant impact on the amount of steel used in rigid steel frames. The following is the result of the calculation and analysis of the steel consumption under different roof slopes with a single span of 42m and an eave height of 6m.

When the roof slope is 0.5: 10, the weight of a frame is 3682 Kg when the roof slope is 1: 10, the amount of a frame structure is 3466 Kg When the roof slope is 1.5: 10, the weight of a frame is: 3328 Kg When the roof slope is 2: 10, the amount of a frame structure is: 3240 Kg.

So for a single-span rigid frame, a better way to reduce the weight of the rigid frame is to increase the roof slope. The larger the hill, the less steel used. However, the situation is different for a multi-span frame. A large slope will increase the amount of metal used in the frame. It is because a large hill will increase the length of the inner column.

When the span of the building is large, increasing the incline can reduce the deflection of the roof steel beam. Through research and calculation, the more economical slope is: multi-span buildings: 1:20 single span, span less than 45 m: 0.5: 10 single span, span less than 60m: 1.5: 10 single span, more than 60 m span : 2: 10

The roof slope is also related to whether the building has a parapet wall, and the large hill will lead to an increase in the cost of the parapet wall.

Steel Structure detail-Eave Height

 

The eave height has a significant impact on the cost, which mainly manifested in the following aspects:

  1. Increasing the height prefab steel building will cause the wall cladding to grow, the wall girt to grow, and the amount of steel used for columns will be more.
  2. If the steel column has no lateral bracing (such as the center column or the side column without a brace), the influence of the eave height on the frame weight will be more prominent; An increase in eave height will result in an increase in wind load on the frame. If the height/building width is> 0.8, to control the lateral displacement, sometimes it is even necessary to change the column foot from hinged to rigid.

The following factors determine the height:

  1. The height requirements at the eave;
  2. When there is a mezzanine structure, the net height of the mezzanine and the height of the mezzanine beam;
  3. Height of crane beam and crane hook when the crane is available.

Temperature section

According to the code of steel structure building, the maximum length is not more than 300m, and the width is not more than 150m. The first temperature segmented expansion joint can be set with a double-column arrangement (Figure 2a) or a single-column expansion joint with slotted holes connected to the purlin (Figure 2b).

 

Steel Structure Detail-Bracing

(A) the use of the Bracing

In the longitudinal structure of the portal frame, a complete bracing system should arrange to form a whole spatial structural system. The lateral stability of the lightweight portal frame in the width direction is ensured by the rigidity of the frame to resist the lateral load.

Due to the weak stiffness of the longitudinal structure in the length direction, it is necessary to set bracing in the longitudinal direction to ensure its longitudinal stability. The forces of bracing are mainly longitudinal wind load, crane braking force, earthquake action, and temperature action. When calculating the internal strength of brace, the joints are generally assumed to be hinged, and the effect of eccentricity ignores. The general support considered according to the tie bean. Therefore, a two-way arrangement is suitable.

(B) Common types of bracing

Figure 3-3 shows the general arrangement of roof bracing and the force transmission path of wind loads acting on the gable wall. Figure 3-4 shows the common support types between columns for portal frames. Due to building function and appearance requirements, or process equipment layout, when the above supports are not allowed to use, consider using a longitudinal structure. At this time, the bending stiffness of the weak axis of the column needs to be used.

Steel Structure Detail-Basic principles of bracing settings

  1. The column bracing should be located on the same span as the roof bracing. When it cannot be installed due to the door opening on the wall, the column bracing can be set on the adjacent span;
  2. The distance between bracing should not exceed 5 spans; 30 ~ 45m should be taken when there is no crane, and the distance should not be greater than 60m when there is a crane;
  3. The roof bracing needs to divide into the ridge. (See Figure 3-3)

The following situations need to consider the installation of vertical-horizontal roof bracing

 

(1) When there is a removing column(In the column net, one or more columns are removed), such as a local removing column, only longitudinal bracing is required, as shown in Figure 3-5a;

(2) When the column distance is large, and the side column adopts the scheme of false wall frame column, see Figure 3-5b;

(3) The tonnage of the crane is greater than 15 tons.

When the building width is greater than 60m, it is appropriate to increase the column bracing. When cross bracing cannot be arranged, the bracing forms are shown in Figures 3-4b and 3-4c can be used. It is also possible to increase the section size of the roof bracing or the column bracing without increasing the internal inter-pillar support. At this time, the internal force calculation needs to be strictly performed to ensure the bracing is safe.

In the same column, different types of bracing should not be mixed, otherwise, the bracing with small stiffness will have less force and will not function as it should, and the bracing with high rigidity will be damaged due to overload work. The columnBracing should preferably choose to use cross braces.

In the following cases, the column bracing needs to be layered.

 

(1) When there is a high-low span (or with a large rain canopy), the upper and lower bracing between the columns need to be layered at the high-low span (or the large rain canopy) (see 3-6a);

(2) When the eave height is greater than 9m, double-layer column bracing can be set according to the included angle of the bracing. The angle between the cross-bracing and the horizontal plane is preferably 45 °, and it should not be greater than 55 °. The upper and lower bracing are set between the columns. The end-opening room may not be provided with a lower bracing to reduce the temperature stress of the crane beam (see 3-6b).

Round steel, angle steel, or round (square) pipe can be used for Column Cross Bracing.

When the internal force of column bracing is large or the lifting tonnage of the crane is more than 5 tons, round steel support is not suitable. At this time, it should use angle steel Or round (square) pipe as column bracing.

Steel Structure detail – use and setting of the flange bracing

The function of the flange bracing is mainly to prevent the instability of the lower flange of the beam and the inner flange of the column. The brace is connected to the lower flange of the beam on one side and the purlin on the other. Refer to Figure 3-7 for the practice of bracing.