The design of steel truss must first follow the principle of force balance in statics, which is the basis for ensuring that the structure remains stable when bearing loads. Any steel truss structure is subject to external loads and internal constraints in working condition, and these forces need to be balanced in all directions. For example, when the truss bears a uniformly distributed load on the top, the load will be transmitted to the support through each rod. The force on each node must be calculated during design to ensure that the resultant force in the horizontal and vertical directions is zero, so as to avoid translation or rotation of the structure due to unbalanced forces, thereby ensuring that the entire truss can stand stably under load without overall instability.
The strength principle in material mechanics is an important point that cannot be ignored in steel truss design. It is related to whether the structure can withstand loads without damage. Different steels have different tensile, compressive and shear strength limits. When designing, it is necessary to select the appropriate steel model according to the type and size of the force on each rod of the truss, and ensure that the cross-sectional size of the rod meets the strength requirements. For example, the tension rods need to have sufficient tensile strength to avoid being broken under the action of tension; the compression rods need to consider the compressive strength to prevent being crushed. Only by controlling the stress of each rod within the allowable range of the material can the entire truss structure have sufficient bearing capacity and will not be damaged due to insufficient material strength.
The principle of simplified rod force in structural mechanics plays an important role in steel truss design. It can simplify the calculation process and improve design efficiency. Ideally, the nodes of steel truss are assumed to be hinge nodes, that is, only axial forces can be transmitted between rods, and there is no bending moment and shear force. This simplification makes each rod only bear tension or pressure, which greatly simplifies the complexity of internal force calculation. Although the nodes in actual engineering are not completely ideal hinge nodes, through reasonable structural design, the stress state of the nodes can be made close to the assumption of hinge nodes, so that internal force analysis and rod design can be carried out based on this simplification. Without such simplification, the bending moment and shear force at the node are included in the calculation, which will make the design process extremely cumbersome, increase the difficulty of calculation and workload. However, following this principle can make the design work more efficient while ensuring the design accuracy.
The stability principle is the key to ensuring structural safety in steel truss design, especially for the stability control of compression members and the overall structure. When compression members are under pressure, they may buckle and become unstable before reaching the material strength limit. This instability often occurs suddenly and has serious consequences. Therefore, when designing compression members, it is necessary to calculate their stable bearing capacity based on parameters such as their slenderness ratio to ensure that they will not buckle under pressure. At the same time, the overall stability of the entire steel truss structure also needs to be considered. For example, under the action of lateral loads, the structure should be prevented from overturning or lateral bending. By setting supports and other measures, the overall stiffness of the structure can be enhanced to improve its ability to resist instability. Only by fully considering the stability principle can the steel truss be prevented from being destroyed prematurely due to instability during use.
The principle of reasonable force transmission path has important guiding significance for the structural layout and rod connection design of steel truss. When steel truss bears load, the force will be transmitted from the load point to the support along a certain path. When designing, it is necessary to make this transmission path clear and direct to avoid the situation of circuitous or concentrated force transmission. For example, when designing a roof steel truss, the roof load should be transmitted to the upper chord node of the truss through the purlin, and then transmitted to the lower chord node by the upper chord member and the web member, and finally transmitted to the foundation through the support. If the force transmission path is unreasonable, some rods may bear additional additional force, resulting in excessive force, or unnecessary stress concentration at the node, affecting the safety and durability of the structure. Following this principle, the force can be transmitted evenly and smoothly inside the truss, making the force of each part of the structure more reasonable.
The stiffness principle is used in steel truss design to control the deformation of the structure to ensure that it meets the requirements of the use function. Steel trusses will produce certain deformations under loads, such as deflection under vertical loads. If the deflection is too large, it may affect the use space below the truss, or cause additional stress in other components connected to the truss, and even affect the normal use of the structure. When designing, it is necessary to specify a reasonable deflection limit according to the use scenario of the truss, and ensure that the maximum deflection of the truss under load does not exceed this limit through calculation. In order to meet the stiffness requirements, it may be necessary to adjust the cross-sectional size or layout of the rods to increase the overall stiffness of the structure and reduce the deformation. For example, for a large-span steel truss, appropriately increasing the cross-sectional area of the upper and lower chords or setting more webs can effectively improve the stiffness of the truss and control its deflection within the allowable range.
The principle of collaborative work requires that the various components of the steel truss cooperate with each other, bear the load together, and exert the bearing capacity of the overall structure. The steel truss is a whole composed of multiple rods connected by nodes. Each rod is a part of the structure. When designing, the force of a certain rod cannot be considered in isolation, but the synergy of the entire structure should be paid attention to. For example, when a part of the truss is subjected to load, the load will be transferred to other parts through the interaction between the rods, and the rods will share the load together to prevent a single rod from being damaged due to excessive force. In the design, it is necessary to ensure the reliability of the node connection so that the force can be effectively transmitted between the rods and the coordinated work of the rods during the force process. If the node connection is not firm, it may cause a rod to be unable to participate in the force normally, causing other rods to bear additional loads, destroying the coordinated working state of the structure, and thus affecting the bearing capacity and safety of the entire truss.
