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# Beams - Supported at Both Ends - Continuous and Point Loads

## Supporting loads, stress and deflections.

The stress in a bending beam can be expressed as

σ = y M / I                                     (1)

where

σ = stress (Pa (N/m 2 ), N/mm 2 , psi)

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

M = bending moment (Nm, lb in)

I = moment of Inertia (m 4 , mm 4 , in 4 )

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 The moment in a beam with uniform load supported at both ends in position x can be expressed as

M x = q x (L - x) / 2                                          (2)

where

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

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

The maximum moment is at the center of the beam at distance L/2 and can be expressed as

M max = q L 2 / 8                                          (2a)

where

M max = 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)

#### Maximum Stress Equation 1 and 2a can be combined to express maximum stress in a beam with uniform load supported at both ends at distance L/2 as

σ max = y max q L 2 / (8 I)                                     (2b)

where

σ max = maximum stress (Pa (N/m 2 ), N/mm 2 , psi)

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

• 1 N/m 2 = 1x10 -6 N/mm 2 = 1 Pa = 1.4504x10 -4 psi
• 1 psi (lb/in 2 ) = 144 psf (lb f /ft 2 ) = 6,894.8 Pa (N/m 2 ) = 6.895x10 -3 N/mm 2

Maximum deflection :

δ max = 5 q L 4 / (384 E I)                                     (2c)

where

δ max = maximum deflection (m, mm, in)

E = Modulus of Elasticity (Pa (N/m 2 ), N/mm 2 , psi)

Deflection in position x:

δ x = q x (L 3 - 2 L x 2 + x 3 ) / (24 E I)                                     (2d)

Note! - deflection is often the limiting 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:

R 1 = R 2

= q L / 2                                      (2e)

where

R = reaction force (N, lb)

#### Example - Beam with Uniform Load, Metric Units

A 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 cm 4 (81960000 mm 4 ) and the modulus of elasticity for the steel used in the beam is 200 GPa (200000 N/mm 2 ) . 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

σ max = (150 mm) (6 N/mm) (5000 mm) 2 / (8 (81960000 mm 4 ))

= 34.3 N/mm 2

= 34.3 10 6 N/m 2 (Pa)

= 34.3 MPa

The maximum deflection in the beam can be calculated

δ max = 5 (6 N/mm) (5000 mm) 4 / ((200000 N/mm 2 ) (81960000 mm 4 ) 384)

= 2.98 mm

#### Uniform Load Beam Calculator - Metric Units

• 1 mm 4 = 10 -4 cm 4 = 10 -12 m 4
• 1 cm 4 = 10 -8 m = 10 4 mm
• 1 in 4 = 4.16x10 5 mm 4 = 41.6 cm 4
• 1 N/mm 2 = 10 6 N/m 2 (Pa)

#### Example - Beam with Uniform Load, Imperial Units

The maximum stress in a "W 12 x 35" Steel Wide Flange beam , 100 inches long, moment of inertia 285 in 4 , modulus of elasticity 29000000 psi , with uniform load 100 lb/in can be calculated as

σ max = y max q L 2 / (8 I)

= (6.25 in) (100 lb/in) (100 in) 2 / (8 (285 in 4 ))

= 2741 (lb/in 2 , psi)

The maximum deflection can be calculated as

δ max = 5 q L 4 / (E I 384)

= 5 (100 lb/in) (100 in) 4 / ((29000000 lb/in 2 ) (285 in 4 ) 384)

= 0.016 in

### Beam Supported at Both Ends - Load at Center Maximum moment in a beam with center load supported at both ends:

M max = F L / 4                                          (3a)

#### Maximum Stress

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

σ max = y max F L / (4 I)                                    (3b)

where

Maximum deflection can be expressed as

δ max = F L 3 / (48 E I)                                  (3c)

Forces acting on the ends:

R 1 = R 2

= F / 2                                      (3d)

#### 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 in 4 , modulus of elasticity 29000000 psi , with a center load 10000 lb can be calculated like

σ max = y max F L / (4 I)

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

= 5482 (lb/in 2 , psi)

The maximum deflection can be calculated as

δ max = F L 3 / E I 48

= (10000 lb) (100 in) 3 / ((29000000 lb/in 2 ) (285 in 4 ) 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:

M max = F a b / L                                          (4a)

#### Maximum Stress

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

σ max = y max F a b / (L I)                                    (4b)

Maximum deflection at point of load can be expressed as

δ F = F a 2 b 2 / (3 E I L)                                  (4c)

Forces acting on the ends:

R 1 = F b / L                                 (4d)

R 2 = F a / L                                 (4e)

### Beam Supported at Both Ends - Two Eccentric Loads M max = F a                                          (5a)

#### Maximum Stress

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

σ max = y max F a / I                                    (5b)

Maximum deflection at point of load can be expressed as

δ F = F a (3L 2 - 4 a 2 ) / (24 E I)                                  (5c)

Forces acting on the ends:

R 1 = R 2

= F                                  (5d) Insert beams to your Sketchup model with the Engineering ToolBox Sketchup Extension

### Beam Supported at Both Ends - Three Point Loads M max = F L / 2                                          (6a)

#### Maximum Stress

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

σ max = y max F L / (2 I)                                    (6b)

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

δ F = F L 3 / (20.22 E I)                               (6c)

Forces acting on the ends:

R 1 = R 2

= 1.5 F                                  (6d)

## Related Topics

• ### Beams and Columns

Deflection and stress, moment of inertia, section modulus and technical information of beams and columns.
• ### Mechanics

Forces, acceleration, displacement, vectors, motion, momentum, energy of objects and more.
• ### Statics

Loads - forces and torque, beams and columns.

## Related Documents

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American Standard Beams ASTM A6 - Imperial units.
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Dimensions and static parameters of American Standard Steel C Channels
• ### American Wide Flange Beams

American Wide Flange Beams ASTM A6 in metric units.
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Properties of HE-A profiled steel beams.
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Properties of HE-B profiled steel beams.
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Properties of HE-M profile steel beams.
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Convert between Elastic Section Modulus units.
• ### Square Hollow Structural Sections - HSS

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Dimensions and static parameters of steel angles with equal legs - imperial units.
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Dimensions and static parameters of steel angles with equal legs - metric units.
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Dimensions and static parameters of steel angles with unequal legs - imperial units.
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Dimensions and static parameters of steel angles with unequal legs - metric units.
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Stiffness is resistance to deflection.
• ### Stress

Stress is force applied on cross-sectional area.
• ### Three-Hinged Arches - Continuous and Point Loads

Support reactions and bending moments.
• ### Trusses

Common types of trusses.

• ### W-Beams - American Wide Flange Beams

Dimensions of American Wide Flange Beams ASTM A6 (or W-Beams) - Imperial units.
• ### Weight of Beams - Stress and Strain

Stress and deformation of vertical beams due to own weight.
• ### Wood Headers - Max. Supported Weight

Weight supported by a double or triple wood headers.

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## Citation

• The Engineering ToolBox (2009). Beams - Supported at Both Ends - Continuous and Point Loads. [online] Available at: https://www.engineeringtoolbox.com/beam-stress-deflection-d_1312.html [Accessed Day Month Year].

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12.8.9