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Architecture 314Structures I

Deflection of Structural Members

• Slope and Elastic Curve• Deflection Limits• Diagrams by Parts• Symmetrical Loading• Asymmetrical Loading• Deflection Equations and Superposition

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Deflection

Axial fiber deformation in flexure results in normal (vertical) deflection.

The change in lengths, top and bottom, results in the material straining. For a simple span with downward loading, the top is compressed and the bottom stretched.

The material strains result in corresponding stresses. By Hooke‘s Law, these stresses are proportional to the strains which are proportional to the change in length of the radial arcs of the beam “fibers“.

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Slope

• The curved shape of a deflected beam is called the elastic curve

• The angle of a tangent to the elastic curve is called the slope, and is measured in radians.

• Slope is influenced by the stiffness of the member:

– material stiffness E, the modulus of elasticity

– sectional stiffness I, the moment of inertia,

– as well as the length of the beam, L

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Deflection

• Deflection is the distance that a beam bends from its original horizontal position, when subjected to loads.

• The compressive and tensile forces above and below the neutral axis, result in a shortening (above n.a.) and lengthening (below n.a.) of the longitudinal fibers of a simple beam, resulting in a curvature which deflects from the original position.

Axial Stiffness

Flexural Stiffness

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Deflection Limits (serviceability)

• Various guidelines have been derived, based on usage, to determine maximum allowable deflection limits.

• Typically, a floor system with a LL deflection in excess of L/360 will feel bouncy or crack plaster.

• Flat roofs require a minimum slope of ¼” / ft to avoid ponding. “Ponding” refers to the retention of water due solely to deflection of relatively flat roofs. Progressive deflection due to progressively more impounded water can lead to collapse.

International Building Code - 2006

roof pondingfrom IRC, Josh 2014

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Relationships of Forces and DeformationsThere is a series of relationships involving forces and deformations along a beam, which can be useful in analysis. Using either the deflection or load as a starting point, the following characteristics can be discovered by taking successive derivatives or integrals of the beam equations.

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Slope and Deflection in Symmetrically Loaded Beams

• Maximum slope occurs at the ends of the beam

• A point of zero slope occurs at the center line. This is the point of maximum deflection.

• Moment is positive for gravity loads.

• Shear and slope have balanced + and - areas.

• Deflection is negative for gravity loads.

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Cantilever Beams

• One end fixed. One end free

• Fixed end has maximum moment, but zero slope and deflection.

• Free end has maximum slope and deflection, but zero moment.

• Slope is either downward (-) or upward (+) depending on which end is fixed.

• Shear sign also depends of which end is fixed.

• Moment is always negative for gravity loads.

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Methods to Calculate Deflection

Integrationcan use to derive equations

Diagramssymmetric load cases

Diagrams (by parts)asymmetric load cases

Diagrams (by shifting baseline)asymmetric load cases

Equationssingle load cases

Superposition of Equationsmultiple load cases

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Deflection by Integration

Load

Shear

Moment

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Deflection by Integration

Slope

Deflection

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Deflection by Diagrams

Load

Shear

Moment

Slope (EI)

Deflection (EI)

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Table D-24 I. Engel

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Deflection by Diagramsexample

Calculated as a triangle

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Methods to Calculate Deflection

Integrationcan use to derive equations

Diagramssymmetric load cases

Diagrams (by parts)asymmetric load cases

Diagrams (by shifting baseline)asymmetric load cases

Equationssingle load cases

Superposition of Equationsmultiple load cases

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Diagrams by Base Line ShiftAsymmetrically Loaded Beams

• The value of the slope at each of the endpoints is different.

• The exact location of zero slope (and maximum deflection) is unknown.

• Start out by assuming a location of zero slope (Choose a location with a known dimension from the loading diagram)

• With the arbitrary location of zero slope, the areas above and below the base line (“A” and “B”) are unequal

• Adjust the base line up or down by D distance in order to equate areas “A” and “B”. Shifting the base line will remove an area “a” from “A” and add an area “b” to “B”

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Asymmetrically Loaded Beams (continued)

• Compute distance D with the equation:

so,

• With the vertical shift of the base line, a horizontal shift occurs in the position of zero slope.

• The new position of zero slope will be the actual location of maximum deflection.

• Compute the area under the slope diagram between the endpoint and the new position of zero slope in order to compute the magnitude of the deflection.

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Example: Asymmetrical Loading – Diagram method

• Solve end reactions

• Construct shear diagram

• Construct moment diagram

• Choose point of θEI = 0Sum moment areas to each side to find θEI end values

• Calculate distance D

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Example: Asymmetrical Loading Diagram method

• Shift the baseline and find point of crossing. Find new areas

• Check that areas A and B balance. Any difference is just round-off error. (average the two to reduce error)

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Example: Asymmetrical Loading Diagram method

• Calculate the deflection in inches for a given member.

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Deflection: by Superposition of Equations

• Deflection can be determined by the use of equations for specific loading conditions

• See posted pages for more equations. A good source is the AISC Steel Manual.

• By “superposition” equations can be added for combination load cases. Care should be taken that added equations all give deflection at the same point, e.g. the center line.

• Note that if beam lengths and load (w) are entered in feet, a conversion factor of 1728 in3/ft3 must be applied in order to compute deflection in inches.

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Example: Equations Method – By Superposition

• To determine the total deflection of the beam for the given loading condition, begin by breaking up the loading diagram into parts, one part for each load case.

• Compute the total deflection by superposing the deflections from each of the individual loading conditions. In this example, use the equation for a mid-span point load and the equation for a uniform distributed load.

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Example: Equations Method• For a W18x55 with an

• E modulus of 30000 ksi • moment of inertia of 890 in4

• Using an allowable deflection limit of L / 240.

• Check deflection

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Example: Asymmetrical Loading – Superposition of Equations

Standard equations provide values of shear, moment and deflection at points along a beam.

Cases can be superposed or overlaid to obtain combined values.

To find the point of combined maximum deflection, the derivative of the combined deflection equation can be solved for 0. This gives the point with slope = 0 which is a max/min on the deflection curve.

Steel Construction ManualAISC 1989

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Example (same equations as above): Asymmetrical Loading – Superposition

Deflection equations for cases 5 + 8

Input actual dimensions

Reduce and write deflection equation in terms of x

Differentiate dy/dx to get the slopeequation

Set slope equation = 0

Solve for xThis will be the point of Δmax

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Deflection equations (5 + 8)

Input beam distances as before and reduce terms

Solve deflection for x=16.5’

Solve for specific section and material by dividing by EI of the section

Example (same as above): Asymmetrical Loading – Superposition

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Estimate: Asymmetrical Loading – Superposition of Equations

Or as an estimate…

It is also possible to estimate the deflection location and value without the more exact calculation of x.

If an equation for D max is given, use that (conservative).

Otherwise guess x near mid-span.

e.g. for example above

C.L. = 18 ft

D = 7344 k-ft3 = 2.15”Steel Construction Manual

AISC 1989

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Comparing the three methods

Baseline shift

x = 16.4’

D = 7341 k-ft3 = 2.14”

Equations (calculate x)

x = 16.5’

D = 7408 k-ft3 = 2.16”

Equations (estimate x @ C.L.)

x = 18’

D = 7344 k-ft3 = 2.15”

Steel Construction ManualAISC 1989