Serviceability of a flexural beam can be achieved by limiting cracking. And the limitations on cracking can be achieved by imposing certain controls on deflections of beams. Deflection control ensures serviceability. If the deflection value is very large then it leads to cracking of doors, windows, supported and partition walls, drainage in roof slab and misalignment of sensitive machinery that cause excessive vibration. This demands the control on deflection.
There are two methods through which deflection can be controlled. The first method is the indirect method in which an upper limit is defined for the span-depth ratio. This method is very simple and satisfactory for cases where service load, span length and the load distribution falls within the defined ranges. If the case is not like above, then there is a second method. In this method calculation of deflection is very necessary. After calculation, compare the results with those of predicted values that are specified from limitations in ACI code.
Calculation of deflection provides guidance to the actual deflections. This is because, there are many uncertainties exists regarding the properties of the material and effect of cracking. Therefore, the calculation of accurate result is not likely being possible even during the extreme precisions in the calculation.
During the service period, a member carries the full dead load plus some fraction of live load or maybe all the live load. ACI code for safety provisions has ensure that under service loads, stresses in concrete and steel remains within the elastic limit. Consequently, there are loads that occur once during service are so called immediate deflections/elastic deflections can be calculated based on two assumptions;
Un-cracked elastic member
Cracked elastic member