When serviceability is concern, limiting cracks in reinforced concrete flexure member is not enough alone, additionally we have to pay attention to apply some controls in flexure deflection of beam. Excessive deflection is not only an esthetic issue but also this creates many problems like:
• Formation of cracks in supported partitions and walls
• Ill-fitting or jamming windows and doors
• Poor drainage on roof (may be a structural issue when accumulated water impose load on structural members)
• Misalignment of sophisticated equipment and machinery
• Offensive sag
Thus, it is essential to keep control on deflections of members by any means, so that they perform well under various service conditions, though they are primarily designed considering strength at expected overloading.
In present practice to achieve serviceability, two approaches are available to control deflection:
First approach is quite indirect which consist of applying suitable maximum limits for span to depth ratio for a member. This approach is very simple and found satisfactory in many cases. But some conditions of structural system and loading have to be fulfilled.
The loads and distribution of them, spans, member proportions (say width to depth ratio) and sizes should be in usual ranges.
The second approach is to calculate deflections under actual loading conditions and member proportioning which is compared to specific limitations that may be prescribed by codes or special requirements. Thus second approach is used where greater accuracy is required and unusual conditions prevail.
Now we will discuss about accuracy of calculation, it is very difficult to calculate actual deflections and a probable value can be determined. This is due to
• Uncertainty about material properties
• Influence of existing cracking
• History of loading on member of interest
Thus extreme precision is, therefore, never justified, as it is unlikely to produce a result with high accuracy. However, in general, it is enough to have idea about range of deflection say deflection due to certain load would be about half inch instead of two inches, but it is not very important to know whether deflection would be 5/8 inch instead of ½ inch.
The deflections of our interest are those that are generally occur during service life of member. During service life, a member is subjected to total dead load with a fraction of live load at this duration or sometimes all live load may be applied on it.
According to ACI code, both components of reinforced concrete member (steel and concrete) should be stressed within their elastic range when they are loaded to their full service load. This safety provision of code is also followed by other equivalent design specifications.Therefore, deflections produced just after application of load can be determined depending on two considerations; one is properties of elastic member which is not cracks and another is considering a cracked member. Sometimes combination of both is required. This deflection is called immediate deflections.
In addition to immediate deflections of concrete member upon applying load, there have other deformations which take time to appear and gradually become significant after extended period of loading. The time-dependent deformations are mainly due to shrinkage and creep of concrete.
The consequence of these type of deformations is continuing deflection of reinforced concrete members with time; these long-term deflection usually continue for several years. The significance of these is not only time but also they may become 2-3 times or even more, in many cases, than immediate deflection within elastic range.
So it is essential to know how to predict both immediate deflections and long-term deflections. ACI code 9.5.2 specifies us all requirements to control deflections namely:
• Minimum depth/span ratio
• Calculations for immediate deflection
• Long-term deflection multipliers
• Treatment for continuous span.