What are the advantages of pre-arching?
Control the deflection of the beam : Pre-arching can offset part of the deflection caused by the load, so that the beam maintains a relatively straight shape when in use.
Improve aesthetics : reduce visible sagging deformation, maintain the flatness of the floor or roof, and enhance the visual effect of the building.
Save materials : Pre-arching can use beams with smaller cross-sections to meet the same deflection requirements, thereby saving steel usage; making the floor slab thickness consistent and reducing the volume of poured concrete due to the deflection of the steel beams. Without pre-arching, the volume of floor slab concrete may increase by 10-15%.
How much deflection does the pre-arch need to compensate?
Generally speaking, the design can be divided into the following three situations:
1. The deflection under a certain percentage of dead load.
2. Deflection under total dead load.
3. Deflection under the action of total dead load and a certain percentage of live load.
The value "1/500" mentioned above refers to roof trusses and trusses. No specific value is given for solid steel beams, and it is not clear whether it can be used as a reference.
"Depends on actual needs" is also a very general statement. What specific practical situations will we encounter?
If the pre-arching is not enough, the expected effect cannot be achieved. If the pre-arching is too much, the floor thickness may be insufficient, the studs may be exposed or the protective layer may be insufficient; or the floor thickness is sufficient, but the top elevation of the floor exceeds the limit.
The general experience value given in "Design Considerations for Camber" is 80% of the dead load . If the thickness of the floor slab changes significantly, the proportion still needs to be adjusted.
The practice of offsetting 80% of the expected dead load-referred to here as the 80% rule-has been applied to a broad spectrum of projects and, in most cases, appears to result in typical interior beams that perform well for "average" projects.
What to do with the increased costs caused by pre-arching?
Small steel mills are small in scale and usually have less bargaining power. For the process of pre-bending steel beams, the situation is more complicated: manufacturers with professional equipment use cold bending, but this increases transportation costs; those who lack equipment use hot bending, which increases time and labor costs. Additional quality control and measurement procedures may also be required.
As a result, the unit price of the steel structure remains unchanged, while the processing cost increases. How to absorb these additional costs and maintain profitability is indeed a difficult problem! According to the factory's calculations, the pre-arching of the steel beam increases the processing cost by about 5-10%.
Pre-cambering may also result in cost savings :
The cost of additional concrete due to beam deflection is reduced.
The use of heavier steel beam sections is avoided.
This increase and decrease may benefit the project in the end, but the unilateral manufacturing cost of the steel structure will definitely increase.
In order to absorb the increased costs and ensure normal profits, will steel mills choose to "optimize" in other aspects? We will talk about this later, some phenomena of "wrong thinking".
What if the design does not take pre-arching into consideration?
The arching of steel structure is more complicated, unlike concrete, which is as simple as adjusting the wooden formwork. If the design drawings are not clear and the deepening is not considered, then this problem will be transferred to the site for processing. What should the construction do?
In this case, consider communicating with the designer to confirm whether pre-arching is required. If necessary, ask the designer to provide specific arching values and requirements. If the quantity is small, we can find a way to solve it on site.
If pre-arching is really necessary but site conditions do not allow it, temporary support can be considered to support the steel beam during concrete pouring and curing to reduce initial deformation.
If the increased amount of concrete pouring is acceptable, directly pour the variable thickness floor slab or increase the cross-section of the steel beam.
In what situations is pre-arching not necessary?
- Beams requiring less than 20 mm of pre-camber: loss of camber may render them ineffective.
- Beams with a total length of less than 7 meters.
- The web thickness is less than or equal to 6 mm.
- Beams connected to exterior walls: Exterior wall system connections are difficult to coordinate.
- Beams in Rigid Frames: For connections that need to be both rigid and maintain 90 degrees, node assembly problems arise.
- Beams subjected to significant asymmetric loads: The pre-arch design is primarily used to counteract uniform loads.
