1 Problems needing attention in bridge design
1. 1 More attention should be paid to the durability of the structure. Since 1990s, our country has paid attention to the research of structural durability, and has made many achievements. Most of these studies are carried out from the perspective of materials and statistical analysis, but few people study how to improve the durability of bridges and how to make them easy for designers and builders to accept and operate from the perspective of structure and design. Moreover, for a long time, people have been focusing on the research of structural calculation methods, while ignoring the attention to the overall structure and details. There are essential differences between structural durability design and conventional structural design. At present, it is necessary to develop the research on durability from qualitative analysis to quantitative analysis. Research and practice at home and abroad show that structural durability plays a decisive role in the safe operation and economy of bridges.
1.2 Study on fatigue injury of diplopia. Vehicle load and wind load on bridge structure are both dynamic loads, which will produce cyclic stress in the structure, not only causing vibration of the structure, but also causing cumulative fatigue damage of the structure. Because the materials used in the bridge are uneven and discontinuous, there are actually many tiny defects. Under cyclic loading, these tiny defects will gradually develop and fuse to form damage, and gradually form macro cracks in the material. If macro cracks are not effectively controlled, it is very likely to cause brittle fracture of materials and structures. Early fatigue damage is often difficult to detect, but its consequences are often disastrous, so the research on fatigue damage needs to be paid enough attention.
1.3 Pay full attention to the problem of bridge overload. On the one hand, the overload of bridges may cause fatigue problems. Overload will increase the fatigue stress and damage of the bridge, and even some structural damage accidents caused by overload will occur. On the other hand, the internal damage of the bridge caused by overload cannot be recovered, which will change the working state of the bridge under normal load and may endanger the safety and durability of the bridge. Therefore, it is necessary to study and analyze the consequences of overload.
1.4 actively learn from foreign experience and achievements. The main problems existing in domestic bridge design are poor normal service performance, poor durability and poor safety (including short service life, high maintenance cost and frequent safety accidents). Although these problems are related to the low construction quality and management level in China, in all fairness, since this situation cannot be solved in a short time, as an engineering designer, we should face up to this problem, fully consider the current construction management level and material technology level, and adopt appropriate safety degree and appropriate design methods to ensure the performance of the bridge. This is a more active and effective method. In particular, the durability and safety of bridges are related to the unreasonable selection of structural systems or materials and improper handling of structural details.
2 overall design strategy of bridge steel structure
2. 1 lateral overturning stability design. Bridges with steel structures are generally light and have very high strength. However, in the design of small radius multi-lanes, its ability to resist lateral overturning is the focus of current research. In the early bridge construction, due to design reasons, the bridge body capsized during the bridge construction or use. Because the radius of continuous steel beam is relatively small, its span is relatively large. If the bridge deck is wider than the steel beam and the live load is not optimal, the external bearing of the beam will be stressed, while the internal bearing will not be stressed, so that the beam will be extremely uneven and the beam will overturn. In the design process, through reasonable calculation, the eccentric force of the beam is designed to meet the load requirements of the bridge and make the bridge bear uniform force. Sand filling measures are taken at the cross beam, which increases the overall stability of the multi-lane bridge under the condition of meeting the specifications.
2.2 Key points of welding structure integrity design. The integrity design of bridge welded structure is an important factor to ensure the overall stability of the bridge. Due to different forces, the forms of welded joints are also different. The stress at the joint leads to the difference of microstructure and mechanical properties of the base metal. At the same time, the stress can not be released by 100% in the welding process, and the welding stress usually leads to the deformation of the welded joint, resulting in a large number of defects in the welded joint, which can not meet the requirements of the overall design of the bridge. Therefore, the design of welded joints must be considered in the overall design of bridges. On the premise of meeting the relevant specifications, it is necessary to: choose the form according to local conditions, obtain the static and fatigue grades according to the requirements of weldability test, and determine the relevant forms of welds; In welding design, the key details must be designed in detail to ensure uniform stress and reduce stress as much as possible; The relevant requirements of welding inspection must be considered in the design, and the weld quality must be tested by relevant control indicators such as nondestructive testing.
2.3 Stiffening rib setting. Stiffening rib is a strip-shaped reinforcing rib set at the bearing or the place where the load is concentrated to ensure the local stability of the component and transfer the concentrated force. Usually, many people think that the design of stiffening ribs is dispensable, but in fact, it is necessary to decide whether to stiffen ribs through design calculation. Stiffening rib is determined by h0/δ value with web. If it is determined that stiffening ribs are needed, the vertical stiffening ribs are given priority, and their setting distance is determined by the thickness of the web and the related shear stress. When the vertical stiffening rib still cannot meet the requirements, horizontal stiffening rib can be set, which is a supplementary form of vertical stiffening rib.
Due to the lack of cross section of the original member, stiffening ribs are used to enhance the ability to resist bending moment and shear force. Because setting stiffening ribs can reduce the section size of the original member, effectively reduce the amount of steel used and reduce the compression cost, it is generally set on the original member in engineering to enhance the ability to resist bending moment and shear force.
2.4 Design of steel box girder. When the main road of the bridge is too wide, it is necessary to optimize the wide steel box girder in the driveway. In the design, it is very important to meet its vertical calculation requirements. For the span of the beam, from the calculation of the simply supported beam with double cantilever between supports, it can be seen that vertical stiffening ribs can be used at the supports. When the vertical stiffening rib cannot meet the requirements, the horizontal stiffening rib should be considered, and its calculation measures are similar to those of the vertical stiffening rib.
2.5 Construction manhole setting. Manholes are an important part in the overall design of bridges. For the convenience of construction, manholes are usually set on the top plate and web of bridge box girder. The concrete position of the roof manhole can be set at the span 1.5, while the concrete position of the web manhole must be set at a relatively weak place, such as a simply supported beam, whose web manhole can be set in the middle of the span, while the continuous beam must accurately calculate the shear force and choose the place with the minimum shear force. Sometimes there is more than one inspection well design, so it is impossible to distribute all inspection wells on the same section and stagger them. When the stress is large, it is necessary to add installation holes and take strengthening measures.
2.6 Calculation of internal force of structure. The internal force of the structure is calculated by single cantilever with side hole and simply supported cantilever with middle hole. Divide the bridge into several units longitudinally, number the section of each unit, and then input the original data of the project. The input data information includes: general information of the project, unit characteristic information, prestressed steel beam information, construction stage and use stage information. Check the safety of the whole bridge structure according to the fully prestressed members, including prestress, shrinkage and creep and live load calculation. The abutment is provided with a sliding bearing, and the pier is provided with a fixed bearing. There is a master-slave constraint between the anchor beam and the suspension beam. One end of the hanging beam is provided with a fixed bearing, and the other end is provided with a sliding bearing.
3 Conclusion
The acceleration of infrastructure construction in China has promoted the rapid development of bridge technology. Under the current situation, the overall application of bridge steel structure is also very extensive, mainly in the design process to ensure the integrity and stability of bridge steel structure. It is necessary to comprehensively analyze the stress of the bridge from the perspective of integrity and strengthen the optimal design of welding forms to ensure the overall quality of the bridge steel structure.
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