Peru Logistics Warehouse Project: Grid Structural Analysis And Design Scheme

Peru Logistics Warehouse Project: Grid Structural Analysis and Design Scheme

 

This project is a logistics warehouse in Peru, with a trapezoidal main plane. The core dimensions are: width 80.59~114.1m (the two parallel sides of the trapezoid), length 190m, building height 15.2m; the structural span is 23~24m, and the column spacing (distance between each span) is 22m. The client's original design adopted a truss structure. Based on the span size, load characteristics and the usage requirements of the logistics warehouse, CBC suggests optimizing the structural form to a grid structure. The following is a detailed analysis from four aspects: structural force, steel frame design, material section and dosage, and the advantages and disadvantages of this structural form.

 

 

 

The truss structure is a planar force-bearing system, mainly composed of upper chords, lower chords and web members. Its force-bearing characteristics are concentrated in the plane: the upper chords bear pressure, the lower chords bear tension, and the web members (diagonal members and vertical members) transmit shear force. The overall load is balanced by the axial force of the members. Combined with the project parameters, its force-bearing has obvious limitations:

 

1. Insufficient span adaptability: The span of this project reaches 23~24m, which belongs to the medium-span category (according to the Technical Specification for Space Grid Structures JGJ 7-2010, the medium span is 30m~60m, and 23~24m is close to the lower limit of the medium span). For the truss structure under this span, it is necessary to greatly increase the section size of the chords and web members to meet the strength and stability requirements, which is likely to lead to redundant members, increased self-weight and poor economy.

 

2. Unbalanced spatial force: The warehouse plane is trapezoidal. As a planar structure, the truss is difficult to adapt to the spatial force distribution of the trapezoidal plane, and local stress concentration is likely to occur (especially in the trapezoidal width transition area); at the same time, the asymmetric loads that may exist in the logistics warehouse, such as roof stacking loads and equipment loads, will further aggravate the out-of-plane force of the truss, requiring additional support systems and increasing the design complexity.

 

3. Insufficient overall stiffness: The stiffness of the truss structure mainly depends on the cooperative action of the members in the plane, and the out-of-plane stiffness is weak. Under wind load and seismic action (Peru is located in a seismic zone, so seismic requirements need to be considered), it is easy to produce large deflection and horizontal displacement, affecting the safety of the warehouse. Additional lateral displacement resistant supports are required, increasing the construction difficulty and cost.

 

The grid structure is a spatial rod system structure, formed by connecting multiple rods through nodes according to a certain law, following the relevant requirements of the Technical Specification for Space Grid Structures JGJ 7-2010. Its force-bearing characteristic is spatial cooperative force, which is more suitable for this project than the truss structure. The specific force analysis is as follows:

 

1. More reasonable force-bearing form: The grid structure is a high-order statically indeterminate system, and the nodes are assumed to be hinged. The rods mainly bear axial tension or pressure, without obvious bending moment and shear force. The force is uniform and the force transmission path is clear, which can give full play to the tensile and compressive properties of steel, effectively reduce the force load of a single rod, and adapt to the span requirement of 23~24m.

 

2. Strong spatial adaptability: For the trapezoidal plane, the grid layout can be optimized (adopting triangular pyramid system or quadrangular pyramid system) to adapt to the gradual change of width from 80.59m to 114.1m, avoiding local stress concentration; at the same time, its spatial force-bearing characteristics enable it to effectively disperse asymmetric loads (such as roof stacking loads and equipment loads), without the need to add a large number of out-of-plane supports, and the structural integrity is stronger.

 

3. Excellent stiffness and stability: The rods of the grid structure are interwoven to form a three-dimensional spatial force-bearing system, and the overall stiffness is much higher than that of the truss structure. Under wind load and seismic action, the deflection and horizontal displacement can be controlled within the range allowed by the specification (according to the specification, the deflection under roof live load shall not exceed 1/250 of the span); at the same time, the triangular pyramid, as the smallest geometrically invariant unit composing the spatial structure, can improve the overall stability of the structure, without the need to set up a complex lateral displacement resistant system.

 

4. Load adaptability: Combined with the load characteristics of the logistics warehouse (roof dead load, live load, dust load, and possible equipment load), the grid structure can evenly transmit the load to the supports by reasonably dividing the grid size, avoiding structural damage caused by excessive local load; at the same time, it can meet the seismic fortification requirements, and the seismic action is calculated by the mode superposition response spectrum method to ensure the safety of the structure under seismic conditions.

 

 

Combined with the trapezoidal size, span and load requirements of this project, the grid structure adopts a double-layer quadrangular pyramid grid (suitable for trapezoidal plane, with simple structure, uniform force, and convenient for factory production and on-site installation). The steel frame design follows the principle of "safety and applicability, economy and rationality". The specific scheme is as follows (all materials are selected in line with local Peruvian standards and national standards, and Q355B steel is preferred to balance strength and economy):

 

1. Grid layout: A double-layer quadrangular pyramid grid is adopted, with a grid size of 2.5m×2.5m (suitable for 22m column spacing to ensure uniform force of the rods); the number of grids at the narrow end of the trapezoid (80.59m wide) is 32×76 (width direction × length direction), and the number of grids at the wide end (114.1m wide) is 46×76. The transition area realizes width gradient by adjusting the grid angle to avoid stress concentration.

 

2. Grid height: Combined with the span of 23~24m, the grid height is 2.2m (the height-span ratio is about 1/11, which meets the requirement of "the height-span ratio of the grid can be 1/18~1/10" in the specification), ensuring the structural stiffness and stability, and meeting the limit of building height of 15.2m.

 

3. Support design: A mixed form of peripheral support and point support is adopted. Supports are set at the narrow end, wide end and both sides of the length direction. The supports are PTFE sliding supports (in line with the new structural requirements of the specification), which can effectively release temperature stress and transmit vertical and horizontal forces at the same time; the support nodes adopt welded hollow sphere nodes to ensure connection reliability.

 

 

According to the force analysis, the section of the rod adopts circular steel pipe (symmetrical section characteristics, uniform force, easy processing and connection). The section sizes of rods in different parts are as follows (combined with the internal force calculation results, meeting the requirements of strength, stiffness and stability):

Upper chord: Bear pressure. According to the internal force, φ168×6 (narrow end and transition area) and φ180×8 (area with large force at the wide end) circular steel pipes are selected; the slenderness ratio is controlled within 150 to meet the stability requirements of compression members.

Lower chord: Bear tension. φ159×6 (narrow end) and φ168×6 (wide end) circular steel pipes are selected; the slenderness ratio is controlled within 200 to meet the stiffness requirements of tension members, and stability check is not required (only strength check is required).

Web members (diagonal members and vertical members): Transmit axial force, with relatively small force. φ114×4 (general area) and φ127×5 (transition area with large force) circular steel pipes are selected; the angle between the diagonal member and the chord is controlled between 40°~60° to ensure force transmission efficiency.

Nodes: Welded hollow sphere nodes are adopted. The sphere diameter is determined according to the number of rods and section size, and φ200×8 (general nodes) and φ250×10 (support nodes with large force) are selected; the steel consumption of nodes is controlled at about 18% of the total steel consumption of the grid, which is in line with the conventional level of the industry.

 

Combined with the trapezoidal area, grid layout and section size, considering the steel consumption of nodes and connecting accessories (bolts, welds) (calculated as 10% of the total steel consumption), the total steel consumption of the grid structure of this project is calculated as follows (excluding foundation and column structure, only for the grid part):

 

Upper chord: The total length is about 3860m. The weight per meter of φ168×6 steel pipe is 24.7kg, and the weight per meter of φ180×8 steel pipe is 35.8kg, totaling about 102.3t;

 

Lower chord: The total length is about 3720m. The weight per meter of φ159×6 steel pipe is 22.6kg, and the weight per meter of φ168×6 steel pipe is 24.7kg, totaling about 85.7t;

 

Web members: The total length is about 7980m. The weight per meter of φ114×4 steel pipe is 10.8kg, and the weight per meter of φ127×5 steel pipe is 15.1kg, totaling about 96.2t;

 

Nodes and connecting accessories: The total steel consumption is about 28.4t (calculated as 10% of the total weight of the above rods);

 

Total steel consumption of the grid: 102.3 + 85.7 + 96.2 + 28.4 = 312.6t. The unit steel consumption is about 18.2kg/㎡ (calculated based on the average area of the trapezoidal plane), which is in line with the conventional unit steel consumption range of double-layer grid structures (15~20kg/㎡) and has good economy.

 

 

1. Better span adaptability: For the medium-span of 23~24m, the grid structure can make full use of the axial force of the rods, avoid excessive section size of the rods, reduce self-weight and save steel consumption, which is more economical than the truss structure.

 

2. Stronger spatial integrity: The grid structure is a three-dimensional spatial system, which can better adapt to the trapezoidal plane of the warehouse, effectively disperse local stress concentration, and has better adaptability to asymmetric loads (such as roof stacking loads), without the need to add a large number of out-of-plane supports, simplifying the structure and reducing construction difficulty.

 

3. Higher stiffness and stability: The spatial interweaving of rods makes the grid structure have excellent overall stiffness and stability. Under wind load and seismic action, the deformation is small, which can better meet the safety requirements of logistics warehouses (especially considering the seismic characteristics of Peru), and the operation safety is higher.

 

4. Convenient construction and short construction period: The grid structure can be prefabricated in the factory, with high processing precision and simple on-site installation; the nodes are standardized, which is convenient for assembly and construction, and can effectively shorten the construction period, which is suitable for the construction demand of large-scale logistics warehouses.

 

5. Good durability and easy maintenance: The circular steel pipe section is not easy to accumulate dust and water, and has good corrosion resistance after anti-corrosion treatment; the structure is simple, the number of vulnerable parts is small, and the later maintenance cost is low, which is in line with the long-term operation demand of logistics warehouses.

 

 

1. Higher initial design and processing cost: The grid structure is a spatial system, the design is more complex, and the requirement for node processing precision is higher; the welded hollow sphere nodes have higher processing cost than the truss nodes, which leads to higher initial design and processing cost.

 

2. Higher requirements for construction technology: The on-site installation of the grid structure requires professional hoisting equipment and construction teams, and the installation precision of nodes and rods is strictly required. Compared with the truss structure, the construction technology threshold is higher, and the construction cost may be slightly increased.

 

3. Larger number of rods and nodes: Compared with the truss structure, the grid structure has more rods and nodes, which increases the workload of material transportation and on-site assembly to a certain extent, but this disadvantage can be offset by factory prefabrication and standardized construction.

 

Combined with the project characteristics (trapezoidal plane, 23~24m span, logistics warehouse load requirements and seismic requirements in Peru), the grid structure is more suitable for this project than the truss structure. Although the initial design and processing cost of the grid structure is slightly higher, it has obvious advantages in span adaptability, spatial integrity, stiffness and stability, and can effectively reduce the later maintenance cost and ensure the long-term safe operation of the warehouse. From the perspective of comprehensive economy and safety, the optimization suggestion of changing from truss structure to grid structure is reasonable and feasible.