by DO THE LIGHT THINGS LIGHT.
Concrete bridges are one of the most common forms of bridges in the world, using reinforced concrete as the main construction material..
It is highly popular due to its strong durability, reliability, flexibility, and economy..
How concrete structure bridges are verified in CDN? In the following serialization, we will provide a detailed introduction to bridge engineers..
The design section will be serialized in three parts: “upper, middle, and lower”. Please continue to follow us.
Important: After the model is successfully imported, the first step is to switch to the appropriate specification, and then the verification items need to be set..
This section contains the items that must be filled in the CDN specification settings.
“3D” (default): Design according to bending, shearing, and twisting components, considering Mz (but not Vy).
“Two dimensional”: designed according to bending and shearing components, without considering torque and Mz.
“Two dimensional+torque”: designed according to bending, shearing, and twisting components, without considering Mz and Vy.
Generally, the default selection for beam verification is “3D”. When conducting shear flexible beam grid or folded beam grid verification, the overall box beam has already been cut open and no longer conforms to the original shear flow. Therefore, according to personal understanding and needs, the impact of torque can be ignored..
Screenshot taken from “Design Specification for Highway Reinforced Concrete and Pre stressed Concrete Bridges and Culverts JTG 3362-2018”.
According to the type of geological exploration environment, the selection will affect the crack width calculation limit of RC components or PSC-B class components..
”Class A components: tensile stress is allowed, but cracks are not allowed for finite values of tensile stress..
Class B components: cracks are allowed, but the crack width is limited..
After selecting the corresponding design component type, the program will retrieve the corresponding formula for crack resistance calculation..
Screenshot taken from “Design Specification for Highway Reinforced Concrete and Pre stressed Concrete Bridges and Culverts JTG 3362-2018”.
The construction methods of “prefabrication” and “cast-in-place” can be selected according to the actual situation..
Screenshot taken from “Design Specification for Highway Reinforced Concrete and Pre stressed Concrete Bridges and Culverts JTG 3362-2018”.
When conducting torsional verification, it is necessary to calculate the torsional reduction coefficient β t. Among them, when calculating Wt, you can check “flange+web” or “web”..
For the box section in the specification JTG 3362-2018, as shown in the following figure, only the torsional resistance moment of the box compartment is considered, which is equivalent to checking the “web plate”;.
Screenshot taken from “Design Specification for Highway Reinforced Concrete and Pre stressed Concrete Bridges and Culverts JTG 3362-2018”.
This section includes calculation methods that can be considered or customizable during verification.
1> Bending resistance calculation – numerical cross-section for design is calculated using any cross-section.
After selecting this option, when checking the bending bearing capacity, the specific values of b and h are no longer obtained from the Z1-3 position of the section and substituted into the standard formula for calculation. Instead, the area of the compression zone is calculated by changing the formula. Unconventional cross-sections (such as fishbelly cross-sections) can be attempted by checking this option, while general cross-sections do not need to be checked..
Note: This method is not a standardized calculation method, so it is recommended to judge or compare the results on your own..
Screenshot taken from “Design Specification for Highway Reinforced Concrete and Pre stressed Concrete Bridges and Culverts JTG 3362-2018”.
According to Article 5.2.9 (3) of the JTG 3362-2018 specification in the above figure, when calculating the effective height h0, the longitudinal tensile steel bars can be disregarded as bent steel bars..
As shown in the figure below, the pink line represents the prestressed steel tendon. Due to the upward bending at the fulcrum position, h0 will gradually decrease, which may cause the section verification to fail first. According to the article description, we may not consider the bending of steel strands. For example, if this option is entered as 0.2h, the steel strands above the yellow dashed line in the figure will no longer be considered. The causes, detection methods, and reinforcement measures of cracks in concrete structures have been a major challenge for construction technicians for a long time. Cracks in concrete structures, especially in large volume buildings, are a common phenomenon. Therefore, engineering personnel are very concerned about the problem of cracks..
This article classifies cracks in reinforced concrete structures, analyzes the causes of cracks, commonly used detection methods, and various repair and reinforcement measures after cracks appear, for reference by engineering and technical personnel engaged in engineering construction..
The problem of cracks is a common concern for people, and for concrete structures, the existence of cracks is a very common phenomenon. A large amount of scientific research and practice have proven that cracks in concrete structures are inevitable, and the load when cracks appear is often 15% to 25% of the ultimate load..
Under normal usage loads, reinforced concrete structures generally work with cracks, with visible cracks ranging from 0.02 to 0.05mm. Cracks with a width less than 0.05mm are considered harmless cracks, and their impact on waterproofing, corrosion resistance, and load-bearing capacity can be ignored..
The current standard in China for controlling the maximum crack width of concrete structural components under normal operating conditions is 0.3mm. Therefore, from an economic and scientific perspective, a certain degree of cracking is acceptable..
But some cracks can cause a decrease in the load-bearing capacity of the structure and a decrease in its reliability; Although some may not have a significant impact on the bearing capacity, there may be issues such as the detachment of the protective layer on the concrete, accelerated corrosion of steel bars, and carbonation of concrete, which can reduce the durability of the structure or cause leakage, affecting its use..
When the crack width reaches a certain value, it may also endanger the safety of the structure. Therefore, how to evaluate, identify, and repair cracks in concrete structures is of great practical significance for the use and maintenance of the structure..
The causes of crack formation are generally divided into two categories: structural cracks and non structural cracks..
Cracks caused by various static and dynamic loads directly applied. The characteristic of structural failure is that the stress reaches the limit due to insufficient bearing capacity of the structure. This type of crack is quite dangerous, and if not treated properly, it will pose a hidden danger to the safety of the structure..
Cracks caused by indirect effects such as temperature changes, shrinkage, and uneven settlement that constrain the deformation of the structure. This type of crack has little impact on the structural bearing capacity, and repair measures can be taken according to the requirements of structural durability, impermeability, earthquake resistance, and use..
In actual engineering structures, cracks caused by loads only account for about 20% of the total number, while cracks caused by indirect effects account for about 80% of the total number of cracks..
The causes of cracks are complex, and their impact on the structure varies greatly. Only by understanding the structural stress state and the impact of cracks on the structure can corresponding repair measures be determined..
The investigation of crack causes includes investigations into materials and construction quality, design calculation and construction, usage environment and load, etc., which provides a basis for crack analysis..
Determine whether it is a structural crack or a non structural crack through observation of the current situation of cracks and investigation of their causes..
Cracks with a constant width and length belong to stable cracks. As long as their width is not large and meets the requirements of the regulations, their danger is relatively small and they are considered safe components..
The width and length of cracks continue to expand over time, indicating that the stress on the steel bars may approach or reach the flow limit, which has a serious impact on the bearing capacity. Measures should be taken in a timely manner..
Crack detection is the inspection of the current situation of cracks, which provides a basis for crack analysis and hazard assessment by detecting the current situation and drawing a crack distribution map..
The commonly used instruments for crack appearance detection include graduated magnifying glasses, crack comparison cards, etc. The depth of cracks is mainly detected by ultrasonic method or direct core drilling method. The general steps for detection are as follows:.
First, draw the shape of the component that produces the crack, then mark the location and length of the crack on the diagram, and number each crack and indicate the time of crack occurrence. What is the minimum grade requirement for concrete strength in different building structures? The selection of strength grade for structural concrete should meet the bearing capacity, stiffness, and durability requirements of the engineering structure. For concrete structures with a design service life of 50 years, the minimum requirement for the strength grade of structural concrete should also comply with the following regulations:.
The prestressed concrete floor structure should not be lower than C30, and other prestressed concrete structural components should not be lower than C40;.
Structural components that withstand repeated loads should not be lower than C30..
Reinforced concrete structures with seismic resistance levels not lower than Level 2 should not be lower than C30..
Reinforced concrete structures using 500MPa and above grade steel bars should not be lower than C30..
For concrete structures with a design service life greater than 50 years, the minimum strength grade of structural concrete should be increased compared to the above regulations..
The selection of strength grade for concrete structures should consider the characteristics of the engineering structure. Firstly, the bearing capacity, stiffness, and durability requirements of the structure should be met. The design strength should be determined by design calculations, but the minimum strength grade requirements specified in this article should be met to ensure the basic safety and durability of the engineering structure..
The minimum concrete strength grade requirements for concrete structures with a design service life of 50 years are mostly higher than the current relevant standards, in order to appropriately improve the safety and durability of concrete structures:.
1) The minimum strength grade of concrete for plain concrete structures has been increased from C15 to C20, and the minimum strength grade of concrete for reinforced concrete structures has been increased from C20 to C25..
2) For prestressed concrete structural components, the concrete strength grade C30 is the minimum requirement, mainly applicable to floor slabs and other components of building structures (including prefabricated composite floor slabs); For other prestressed concrete structural components (such as bridge structures and beams, columns, etc.), the strength grade of concrete should be increased and should not be lower than C40..
3) For steel-concrete composite structural components, in order to better utilize the efficiency of the two materials, the requirement is proposed that the concrete strength grade should not be lower than C30..
4) For reinforced concrete components with seismic resistance levels not lower than level 2 (including level 2, level 1, and special level 1 in current standards), the requirement that the concrete strength level should not be lower than C30 has been proposed. Compared with the current standards, the requirements for level 2 seismic resistance level components have been appropriately increased..
5) For concrete structures using high-strength steel bars of 500MPa and above, in order to better utilize the performance of high-strength steel bars, the strength grade of concrete should be correspondingly increased. This clause proposes a requirement of no less than C30, which is higher than the current standard C25..
The durability performance of concrete structures during service is related to the design service life of the structure and the environmental conditions exposed to the concrete. Design concrete structures with a working life longer than 50 years, as the durability requirements of the structure are higher, the minimum strength grade of the structural concrete should be further appropriately increased..
The plain concrete structure referred to in this article generally does not include the plain concrete cushion layer of basements or other underground structures; The minimum concrete strength grade of the plain concrete cushion layer should be determined based on the actual engineering situation (including the geotechnical properties of the foundation, etc.). If the depth of the reinforcement is greater than the size of the component and it is not processed, it can cause safety hazards. “Tumumantan” – different perspectives, equally exciting. Click on the blue text “Tumumantan” below the title to follow, and we will provide you with meaningful and valuable knowledge sharing of building structures..
