Where:

L = Floor span in inches

(2) Calculate the predicted deflection of a single joist,

due to a 255 lb concentrated

ot

load at midspan:

255L3

(Eq 2-9)

ot

48EI

Where:

L = Floor span in inches.

E = Modulus of Elasticity of the joist.

I = Moment of Inertia of a single joist.

from the Steel Joist Institute (SJI)

(3) Calculate the number of effective joist, N

eff

equation:

x

1 2 cos

(Eq 2-10)

Neff

2x o

Where:

x = Distance from the center joist to the joist under consideration (inches).

x = Distance from center joist to the edge of the effective floor = 1.06eL (inches).

o

L = Joist span (inches).

0.25

e = (D /D )

x

y

3

D = Flexural stiffness perpendicular to the joist = E t /12

x

c

D = Flexural stiffness parallel to the floor joists = EI /S

y

t

E = Modulus of elasticity of the sub-flooring.

c

E = Modulus of elasticity of the joists.

t = sub-flooring thickness.

I = Moment of inertia of joists alone.

t

S = Joist Spacing.

:

(4) Calculate the predicted central floor deflection,

o

ot

(Eq 2-11)

o

Neff

Where:

= Deflection of floor at mid-bay.

o

= Deflection of a single joist due to 255 lb. concentrated load at midspan.

ot

N

= Number of effective joists in the floor system.

eff

(5) Compare the

value to the critical deflection, y :

o

crit

If

<y :

Acceptable

o

crit

If

y

<

1.1 y :

Marginal

crit

o

crit

If

> 1.1 y :

Unacceptable

o

crit

2-11

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