Design Concepts - SOLIDS & STRUCTURES

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2E4: SOLIDS & STRUCTURES

Lecture 16

Dr. Bidisha Ghosh

Notes:

http://www.tcd.ie/civileng/Staff/Bidisha.Ghosh/

Solids & Structures


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Types of Loading

There are four basic types of loading (in order of complexity).

Tension

Compression

Torsion

Bending

Sometimes, two or more basic types of loading can act

simultaneously on a member of a structure or machine.

This is a compression testing machine.

The different members are under different types of

loading.1. The specimen tested is under compression.

2. The two side bars (N) are under tension.

3. The screw is subjected to twist or torsion.

4. The crosshead is under bending.


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Types of Loading

Dead Loads : The dead load are the external loads that arerelatively constant over time, including the weight of the structureitself.

Live Loads: Loads that work over shorter durations, such as,weight of human beings, furniture, impact loading etc.

Environmental Load (Wind Loads, Snow Loads,Earthquake Loads etc.) :When structures block the flow of wind, the winds kinetic energyis converted into potential energy of pressure, which causes awind loading.

Earthquakes produce loadings on structure through itsinteraction with the ground and its response characteristics.

Other Loads (Hydrostatic and soil pressure etc.): Whenstructures are used to retain water, soil, or granular materials, the

pressure developed by these loadings becomes an important

criterion for their design.


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How things fail?

Buckling


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How things fail?

Tension


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How things fail?

Compression


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How things fail?

Bending


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How things fail?

Shear


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How things fail?

Corrosion


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How things fail?

Fatigue


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How things fail?

Torsion


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Concepts of Design

Main concept of design,

Load< resistance

There are mainly two types of design concepts:

1. Allowable Stress Method

This is also known as working stress method.

2. Load and Resistance Factor Design

This is also known as Limit State Design Method.


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Allowable Stress Design (ASD)

Allowable stress

Structures/machines are designed for stress below yield stress

or their ultimate strength to increase safety.

How?

Stress due to loading


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Working Stress Design (WSD)

Each material has some ultimate strength. But it is unsafe to load

a material to its ultimate strength as there can be uncertainties

regarding:

The quality of manufacture(fabrication / erection /

workmanship, etc.)

Load may be greater than anticipated

Material may be defective (existence of micro cracks fatigueetc.)

Other unforeseen situation (calculation errors, etc.)

In this case, the design stress (specifying the

strength of the material) is reduced from the yield orother specified maximum to get the allowable stress

.

Based on yield stress (elastic material) (2/3rdof yield

stress) or other predetermined strain amount (for an

inelastic materiale.g. for concrete, the stress at a


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Factor of Safety

Stress due to loading


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Load and Resistance Factor Design

This concept is based on the ultimate strength of materials.

Instead of reducing the material strength, factors are used for

accounting for uncertainty in the load and the materialresistance.

Factors are applied to increase load and to decreases

resistance,

Factored load factored strength

(Loads load factors) resistance resi stance facto rs

Load factors: e.g., 1.4xDL + 1.7xLL (for concrete design)

Resistance factors: 0.9xStrength of tension member

More rational and complex approach.

Started with concrete design, but now has been taken up by

steel and more recently for designing wood.
http://on.dot.wi.gov/dtid_bos/extranet/structures/LRFD/Training/LRFDvsASD_LFD-JerryD.pdfhttp://on.dot.wi.gov/dtid_bos/extranet/structures/LRFD/Training/LRFDvsASD_LFD-JerryD.pdfhttp://on.dot.wi.gov/dtid_bos/extranet/structures/LRFD/Training/LRFDvsASD_LFD-JerryD.pdf


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Load vs. Deformation


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FOS1


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FOS1


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FOS1


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FOS1


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FOS2


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Types of Bolt Joint

Lap joint

Butt joint


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FOS2

Double shear in 4 bolts on each side.

Hence, 4*2*shear force on each bolt surface = P

Shear stress should be calculated using FS.

Then allowable shear stress should be multiplied witharea of bolt.


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Moment of Inertia

Design Concepts - SOLIDS & STRUCTURES

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