Deflection in Beam – Causes ,Permissible Deflection & prevention

Deflection of concrete beam is acceptable to a some limit which are specified by our Codes . These codes specify the amount of deflection in Beam are acceptable. And Deflection limits depend on the codes being used for design of structures (ACI ,Eurocode and Indian Standard )

There are various causes that lead to excessive deflections in concrete Beam and it decreases the life of the structure

Type of Deflection :-

There are many type of deflections but mainly two type of deflection seen in beams are describe as following

Immediate Elastic Deflection : This occurs due to the live load and
elastic behaviour of structure under load. Permissible calculation calculated by below formula :-

• Long Term Plastic deflection
• Immediate Elastic Deflection

By American Concrete Institute  :- Span/180

Long Term Elasto – Plastic deflection : This occurs due to plastic behaviour of concrete means the deflection due to effect of temperature , creep and shrinkage

By American Concrete Institute :- Span/360

Minimum Depth of members as per ACI Code are as follows:

Causes of Excessive Deflections in Beam

• Material used in the Beam
• Ignorance in design of beam when we select small depth for beam but in most cases lead to excessive deflection 
• Decrease in thickness of the beam
• It happens when we provide no reinforcement in Tension Zone of the beam
• when we avoid permissible deflection criteria
• Creep and shrinkage in flexural members

How to Reduce Deflection in Beam

• Adding tension bars are very effective process this will decrease the deflection
• Analyze the deflection permissible criteria can decrease the chance of deflection
• Always provide doubly reinforced beam ( both tension and compression reinforcement
• Always make the beam section deeper according to the code standard
• we should take care of the fact that the material we are using in beams (steel,concrete) should not cross it’s elastic limit while deflection because after that limit it would permanently deformed and will not come back to its original position,hence the member can fail.