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土建施工和机械施工的验收规范中常常规定了1‰这个值。龙门吊的上桁架工字钢梁(图1)的梁中起拱曲线值就规定了f=1/(1000)·l(图2)。如果这台龙门吊的轨距为16.5米,那么,工字钢梁的起拱值f=(16.5)/(1000)=16.5(毫米)。为什么要规定工字钢梁要往上拱呢?因为该梁按规定起重量承重时,梁会向下挠曲而变形。当然,这种变形为弹性变形,荷载一旦去掉后,工字钢梁又会恢复原状。但是,长期使用,工字钢梁会产生疲劳,原来规定的上拱值f也会起变
Acceptance codes for civil and mechanical construction often specify a value of 1‰. The value of the arching curve in the beam of the trussed I-beam (Fig. 1) of the gantry crane stipulates that f = 1/(1000)·l (Fig. 2). If the gauge of this gantry crane is 16.5 meters, the starting value of the I-beam is f=(16.5)/(1000)=16.5(mm). Why do we have to specify that the I-beam should be arched upwards? Because the beam will be weight-bearing according to the rules, the beam will bend downward and deform. Of course, this deformation is elastic deformation. Once the load is removed, the I-beam will be restored to its original shape. However, for long-term use, I-beams will be fatigued and the original prescribed upper arch value f will also change.