To understand the specification of the allowable value of the deflection of the grid, it is necessary to analyze from the concept of deflection and the detection method of deflection.

The concept of deflection
Deflection is the linear displacement of the rod axis in the direction perpendicular to the axis or the linear displacement of the plate and shell midplane in the direction perpendicular to the midplane under force or non-uniform temperature changes.


Deflection detection method
Traditional bridge deflection measurement mostly uses dial indicator or displacement meter to directly measure it. At present, it is still widely used in my country's bridge maintenance, old bridge safety assessment or new bridge acceptance. The advantage of this method is that the equipment is simple, multi-point detection can be performed, the deflection value of each measuring point can be directly obtained, and the measurement result is stable and reliable. However, there are many shortcomings in the direct measurement method. This method requires drawing wires or erecting shelves at each measurement point, so direct measurement cannot be performed when there is water under the bridge; for overpass bridges, due to the influence of railway or highway traffic limits, this method also Unusable; high bridges crossing canyons and other high bridges cannot be measured by direct methods; in addition, direct methods are used for deflection measurement, which is complicated and time-consuming regardless of whether the instrument is deployed or removed.

To sum up, the allowable value of deflection of the grid should be calculated according to the requirements of the specific project and the span, radian of the grid and other factors. The standard specification is not perfect at present, but the basic safety must be ensured during the design. Claim!