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Beams - Supported at Both Ends - Continuous and Point Loads

Support loads, stress and deflections

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Stress in a bending beam can be expressed as

σ = y M / I                                     (1)            

where

σ = stress (Pa (N/m2), N/mm2, psi)

y = distance to point from neutral axis (m, mm, in)

M = bending moment (Nm, lb in)

I = moment of Inertia (m4, mm4, in4)

The calculator below can be used to calculate maximum stress and deflection of beams with one single or uniform distributed loads.

Beam Supported at Both Ends - Uniform Continuous Distributed Load

Maximum moment in a beam with uniform load supported at both ends:

Mmax = q L2 / 8                                          (1a)

where

Mmax = maximum moment (Nm, lb in)

q = uniform load per length unit of beam (N/m, N/mm, lb/in)

L = length of beam (m, mm, in)

Moment in position x:

Mx = q x (L - x) / 2                                          (1b)

where

Mx = moment in position x (Nm, lb in)

x = distance from end (m, mm, in)

Maximum Stress

 

Maximum stress in a beam with uniform load supported at both ends:

σmax = ymax q L2 / (8 I)                                     (1c)

where

σmax= maximum stress (Pa (N/m2), N/mm2, psi)

ymax = distance to extreme point from neutral axis (m, mm, in)

  • 1 N/m2 = 1x10-6 N/mm2 = 1 Pa = 1.4504x10-4 psi
  • 1 psi (lb/in2) = 144 psf (lbf/ft2) = 6,894.8 Pa (N/m2) = 6.895x10-3 N/mm2

Maximum deflection:

δmax = 5 q L4 / (384 E I)                                     (1d)

where

δmax = maximum deflection (m, mm, in)

E = Modulus of Elasticity (Pa (N/m2), N/mm2, psi)

Deflection in position x:

δx = q x (L3 - 2 L x2 + x3/ (24 E I)                                     (1e)

Note! - deflection is often the limit factor in beam design. For some applications beams must be stronger than required by maximum loads, to avoid unacceptable deflections.

Forces acting on the ends:

R1 = R2

     = q L / 2                                      (1f)

where 

R = reaction force (N, lb)   

Example - Beam with Uniform Load, Metric Units

UB 305 x 127 x 42 beam with length 5000 mm carries a uniform load of 6 N/mm. The moment of inertia for the beam is 8196 cm4 (81960000 mm4) and the modulus of elasticity for the steel used in the beam is 200 GPa (200000 N/mm2). The height of the beam is 300 mm (the distance of the extreme point to the neutral axis is 150 mm). 

The maximum stress in the beam can be calculated

σmax = (150 mm) (6 N/mm) (5000 mm)2 / (8 (81960000 mm4))

  = 34.3 N/mm2

  = 34.3 106 N/m2 (Pa)

  = 34.3 MPa

The maximum deflection in the beam can be calculated

δmax = 5 (6 N/mm) (5000 mm)4 / ((200000 N/mm2) (81960000 mm4) 384)

   = 2.98 mm

Uniform Load Beam Calculator - Metric Units

q - Uniform load (N/mm)

L - Length of Beam (mm)

I - Moment of Inertia (mm4)

E - Modulus of Elasticity (N/mm2)

y - Distance of extreme point off neutral axis (mm)


Load Calculator!

  • 1 mm4 = 10-4 cm4 = 10-12 m4
  • 1 cm4 = 10-8 m = 104 mm
  • 1 in4 = 4.16x105 mm4 = 41.6 cm4
  • 1 N/mm2 = 106 N/m2 (Pa)

Uniform Load Beam Calculator - Imperial Units

q - Load (lb/in)

L - Length of Beam (in)

I - Moment of Inertia (in4)

E - Modulus of Elasticity (psi)

y - Distance of extreme point off neutral axis(in)


Load Calculator!

Example - Beam with Uniform Load, English Units

The maximum stress in a "W 12 x 35" Steel Wide Flange beam, 100 inches long, moment of inertia 285 in4, modulus of elasticity 29000000 psi, with uniform load 100 lb/in can be calculated as

σmax = ymax q L2 / (8 I)

    = (6.25 in) (100 lb/in) (100 in)2 / (8 (285 in4))

    = 2741 (lb/in2, psi)

The maximum deflection can be calculated as

δmax = 5 q L4 / (E I 384)

    = 5 (100 lb/in) (100 in)4 / ((29000000 lb/in2) (285 in4) 384)

    = 0.016 in

Beam Supported at Both Ends - Load at Center

Maximum moment in a beam with center load supported at both ends:

Mmax = F L / 4                                          (2a)

Maximum Stress

Maximum stress in a beam with single center load supported at both ends:

σmax = ymax F L / (4 I)                                    (2b)

where

F = load (N, lb)

Maximum deflection can be expressed as

δmax = F L3 / (48 E I)                                  (2c)

Forces acting on the ends:

R1 = R2

     = F / 2                                      (2d)

Single Center Load Beam Calculator - Metric Units

F - Load (N)

L - Length of Beam (mm)

I - Moment of Inertia (mm4)

E - Modulus of Elasticity (N/mm2)

y - Distance of extreme point off neutral axis (mm)


Load Calculator!

Single Center Load Beam Calculator - Imperial Units

F - Load (lb)

L - Length of Beam (in)

I - Moment of Inertia (in4)

E - Modulus of Elasticity (psi)

y - Distance of extreme point off neutral axis (in)


Load Calculator!

Example - Beam with a Single Center Load

The maximum stress in a "W 12 x 35" Steel Wide Flange beam, 100 inches long, moment of inertia 285 in4, modulus of elasticity 29000000 psi, with a center load 10000 lb can be calculated like

σmax = ymax F L / (4 I)

    = (6.25 in) (10000 lb) (100 in) / (4 (285 in4))

    = 5482 (lb/in2, psi)

The maximum deflection can be calculated as

δmax = F L3 / E I 48

    = (10000 lb/in) (100 in)3 / ((29000000 lb/in2) (285 in4) 48)

    = 0.025 in

Some Typical Vertical Deflection Limits

  • total deflection : span/250
  • live load deflection : span/360
  • cantilevers : span/180
  • domestic timber floor joists : span/330 (max 14 mm)
  • brittle elements : span/500
  • crane girders : span/600

Beam Supported at Both Ends - Eccentric Load

Maximum moment in a beam with single eccentric load at point of load:

Mmax = F a b / L                                          (3a)

Maximum Stress

Maximum stress in a beam with single center load supported at both ends:

σmax = ymax F a b / (L I)                                    (3b)

Maximum deflection at point of load can be expressed as

δF = F a2 b2 / (3 E I L)                                  (3c)

Forces acting on the ends:

R1 = F b / L                                 (3d)

R2 = F a / L                                 (3e)

Beam Supported at Both Ends - Two Eccentric Loads

Maximum moment (between loads) in a beam with two eccentric loads:

Mmax = F a                                          (4a)

Maximum Stress

Maximum stress in a beam with two eccentric loads supported at both ends:

σmax = ymax F a / I                                    (4b)

Maximum deflection at point of load can be expressed as

δF = F a (3L2 - 4 a2) / (24 E I)                                  (4c)

Forces acting on the ends:

R1 = R2

     = F                                  (4d)

Insert beams to your Sketchup model with the Engineering ToolBox Sketchup Extension

Beam Supported at Both Ends - Three Point Loads

Maximum moment (between loads) in a beam with three point loads:

Mmax = F L / 2                                          (5a)

Maximum Stress

Maximum stress in a beam with three point loads supported at both ends:

σmax = ymax F L / (2 I)                                    (5b)

Maximum deflection at the center of the beam can be expressed as

δF = F L3 / (20.22 E I)                               (5c)

Forces acting on the ends:

R1 = R2

     = 1.5 F                                  (5d)

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