Microsoft Word tn292 floor deflection 032109. doc

                                                                            

                   

Technical Note

 

 

 

 

3

One-Way Conventionally Reinforced Slabs and Beams 

 

TABLE  2   MINIMUM THICKNESS OF CONVENTIONALLY 

        REINFORCED BEAMS OR ONE-WAY SLABS

 

Member 

Simply 

supported 

One end 

continuous 

Both ends 

continuous 

Cantilever 

Solid one-way slabs 

L/20 

L/24 

L/28 

L/10 

Beams or ribbed  

one-way slabs 

L/16 L/18.5  L/21 

L/8 

Notes: 

L =span length 

Values given shall be used directly for members with normal weight concrete and 

Grade 60 ksi (400 MPa) reinforcement. For other conditions, the values shall be 

modified as follows: 

a)  For lightweight concrete having equilibrium density, wc, in the range of 90 to  

    115 lb/ft

3

 (1440-1840 kg/m

3

), the values shall be multiplied by (1.65-0.005 wc)  

    but not less than 1.09 [in SI units, (1.65-0.003

γ

c

) but not less than 1, where 

γ

c

 is  

    the density in kg/m

3

]. 

b)  For fy other than 60,000 psi(400 MPa), the values shall be multiplied by  

    (0.4+fy/100,000) [in SI units (0.4+fy/670)]. 

 

 

Two-Way Conventionally Reinforced Slabs and Beams 

 

TABLE 3  MINIMUM THICKNESS OF SLABS WITHOUT INTERIOR BEAMS* 

 

fy  

 

psi** 

Without drop panels*** 

With drop panels*** 

Exterior panels 

Interior 

panels 

Exterior panels 

Interior 

panels 

Without 

edge 

beams

 

With 

edge 

beams**** 

 

Without 

edge beams 

With 

edge 

beams**** 

 

40,000 Ln/33 

Ln/36 

Ln/36 

Ln/36 

Ln/40  Ln/40 

60,000 Ln/30 

Ln/33 

Ln/33 

Ln/33 

Ln/36  Ln/36 

75,000 Ln/28 

Ln/31 

Ln/31 

Ln/31 

Ln/34  Ln/34 

Notes: 

* For two-way construction, Ln is the length of clear span in the long direction, measured face-to-face  

  of supports in slabs without beams and face-to-face of beams or other supports in other cases. 

** For fy between the values given in the table, minimum thickness shall be determined by linear  

   interpolation. 

*** Drop panels are defined as extension of slab thickening into span not less than span/6, and 

extension  

     of thickening below slab not less than slab thickness/4. 

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Technical Note

 

 

 

 

4

**** Slabs with beams between columns along exterior edges. The ratio of edge beam stiffness to   

the stiffness of the edge beam’s design strip shall not be less than 0.44 

 

Post-Tensioned Members 

 

For post-tensioned beams and slabs, the recommended values by the Post-Tensioning Institute [PTI, 

1990 are as follows: 

 

 

TABLE 4   RECOMMENED SPAN TO DEPTH RATIOS FOR 

POST-TENSIONED MEMBERS 

 

Continuous 

Spans

 

Simple  

Spans

 

 

Roof Floor Roof Floor 

One-way solid slabs 

50 45 45 40 

Two-way solid slabs (supported 

on columns only) 

45-48 40-45 

 

 

 

Two-way waffle slabs (1m pans) 

40 35 35 30 

Beams 

35 30 30 26 

One-way joists 

42 38 38 35 

  Note:   The above ratios may be increased if calculations verify that 

                    deflection, camber, and vibrations are not objectionable. 

 

 

DEFLECTION CALCULATIONS 

 

Under otherwise unchanged conditions, the deformation of an exposed and loaded concrete member 

continues to increase. The increase is due to creep under applied load and shrinkage from loss of 

moisture. The engineering approach to estimating of deflection is to determine the instantaneous 

response of a structure under an applied load, and magnify the instantaneous displacement due to the 

time-dependent  factors of creep and shrinkage. With time, the rate of change in displacement reduces. 

For building structure it is assumed that five years is sufficient time for the deflections to have reached 

their final values.  While it is practical to calculate the time-dependent deflection for any time interval, 

the common practice is to estimate the total value at five years and use this value in the design. 

 

Instantaneous Deflection 

Instantaneous deflection is generally calculated using concrete’s modulus of elasticity at 28 days, 

gross-cross sectional area and linear elastic theory. The calculated deflection may require adjustment, 

if the member is likely to crack, when subjected to the design load.  Cracking reduces the stiffness of a 

member and results in increased deflection. The options for calculating instantaneous deflection with 

due allowance to cracking are: 

 

  Closed form formulas or tables, available primarily for uncracked sections; 

  Use of equivalent moment of inertia (Ie) and simplified averaging (ACI-318’s simplified 

procedure); 

  Use of equivalent moment of inertia (Ie) combined with numerical integration; and 

  Use of Finite Element floor programs that allow for cracking. 

 

Each of the above procedures is briefly discussed in the following section. 

 

 

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