Microsoft Word tn292 floor deflection 032109. doc

---

 

 

 

                                          

Technical Note 

   Your Partner in Structural Concrete Design

 

 

[e-mail]   www.adaptsoft.com 

ADAPT Corporation, Redwood City, California, USA, Tel: [phone]   Fax [phone] 

ADAPT International Pvt. Ltd, Kolkata, India, Tel: [phone]  Fax: [phone] 

 

 

TN292_Floor_deflection_032109 

 

 

 

DEFLECTION OF CONCRETE FLOOR SYSTEMS 

FOR SERVICEABILITY

1

 

 

Bijan O Aalami

2

 

 

 

Deflection control is a central considerations in serviceability of floor systems. This Technical Note 

reviews the levels of acceptable deflections and the currently available methods for their estimate. 

 

OVERVIEW 

.  

There are several reasons to control deflection. 

 

  A concrete floor should have adequate stiffness to prevent changes in deflection that would 

damage attached partitions or other construction elements likely to be damaged by large 

deflections. 

  The deflection of a floor should not be noticeable by occupants  such as to convey a sense of 

inadequacy or safety concerns.  

  Since, in some instances, deflection is used as a measure leading of undesirable vibration in a 

floor, its value must be controlled.  

 

 

LIMITS FOR ACCEPTABLE DEFLECTION 

 

Aesthetics and Sense of Comfort 

In considering aesthetics and sense of comfort for occupants, the most important criterion is the out-of-

level condition of a floor, as opposed to its stiffness. Sensitive individuals, when walking over or viewing 

a floor in elevation, are claimed to perceive a floor’s sag when the vertical out-of-level to span ratio is in 

excess of 1/250, and for cantilevers in excess of 1/125. The out-of-level condition of a floor system can 

be controlled through camber at the time of construction, upon estimating the long-term deflection. 

 

Deflection Limits to Mitigate Damage to Non-structural Construction 

It is important to note that  ACI [ACI 318, 2008] does not impose a limit to deflection under selfweight. 

ACI’s recommendations address the amount of deflection subsequent to the installation of non-

structural elements likely to be damaged.   

 

The following table lists the ACI’s stipulation on deflections (TABLE 9.5(b)). In the application of ACI’s 

recommended deflection limits, it is important to recognize that the given values are to be compared 

with “computed” values, not measured out-of-plane amounts. 

 

 

TABLE 1   MAXIMUM PERMISSIBLE COMPUTED DEFLECTIONS 

                                                 

1

 Copyright ADAPT Corporation, 2008 

2

 Professor Emeritus, San Francisco State University; Principal, ADAPT Corporation 

 

 

---

                                                                            

                   

Technical Note

 

 

 

 

2

 

Type of member 

Deflection to be considered 

Deflection 

limitation 

Flat roofs  not supporting or attached to  

nonstructural elements likely to be  

damaged by large deflection 

Immediate deflection due to live load 

L/180 * 

Floors not supporting or attached to  

nonstructural elements likely to be  

damaged by large deflection 

Immediate deflection due to live load  

L/360 

Roof or floor construction supporting or  

attached to nonstructural elements likely 

to be damaged by large deflection 

That part of the total deflection occurring 

after attachment of nonstructural 

elements(sum of the long-time deflection 

due to all sustained loads and the immediate 

deflection due to any additional live load)**** 

L/480 ** 

Roof or floor construction supporting or  

attached to nonstructural elements not 

likely to be damaged by large deflection 

L/240 *** 

Notes:  

*     Limit not intended to safeguard against ponding. Ponding should be checked by suitable  

      calculations of deflection, including added deflections due to ponding of water, and considering 

      long-term effects of all sustained loads, camber, construction tolerances, and reliability of 

      provisions for drainage. 

**   Limit may be exceeded if adequate measures are taken to prevent damage to supported or 

      attached elements. 

***  But not greater than tolerance provided for nonstructural elements. Limit may be exceeded if  

      camber is provided so that total deflection minus camber does not exceed limit. 

**** Long-time deflection shall be determined using established procedures,  but may be reduced  

       by amount of deflection calculated to occur before attachment of nonstructural elements. This  

       amount shall be determined on basis of accepted engineering data relating to time-deflection  

       characteristics of members similar to those being considered. 

 

 

DEFLECTION CONTROL THROUGH LIMITATIONS ON SPAN TO DEPTH RATIOS 

 

For common residential and commercial buildings, designers can forego deflection calculation,  if the 

stiffness of the member selected is large enough. Deflection calculation requirements are governed 

through recommended span-to-depth ratios for different types of floor members.  ACI 318 has the 

recommendations given in Tables 2 and 3. 

 

 

 

 

 

 

 

 

---