2E4: SOLIDS & STRUCTURES

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Nov 29, 2013 (3 years and 4 months ago)

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2E4: SOLIDS & STRUCTURES


Lecture 16

Dr.
Bidisha

Ghosh

Notes:
http://www.tcd.ie/civileng/Staff/Bidisha.Ghosh/So
lids & Structures



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.

Types of Loading

Dead Loads :
The dead load are the external loads that are relatively
constant over time, including the weight of the structure itself.


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 wind’s kinetic energy is
converted into potential energy of pressure, which causes a wind loading.

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



Other Loads (Hydrostatic and soil pressure etc.):
When
structures are used to retain water, soil, or granular materials, the pressure
developed by these loadings becomes an important criterion for their design.


How things fail?

Buckling

How things fail?

Tension

How things fail?

Compression

How things fail?

Bending

How things fail?

Shear

How things fail?

Corrosion

How things fail?

Fatigue

How things fail?

Torsion

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.

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 <= factor of safety*yield stress

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 fatigue etc
.)


Other
unforeseen situation (calculation errors,
etc.)



In this case, the design stress (specifying the strength of the
material) is reduced from the yield or other specified maximum
to get the “allowable stress” .


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

of yield stress) or
other
predetermined strain amount (for an
inelastic
material

e.g. for concrete, the stress
at
a strain of 0.3
%, allowable stress
is defined.


This
is the earliest and most tradition design
method
, also least
involved computationally.

Factor of Safety

Stress due to loading <= factor of safety*yield
stress


Factor of safety (FS),


,(for stresses)
,(for loads)
failure failure
actual actual
failure failure
actual actual
or
P V
or
P V
 
 
 
 
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 material resistance.


Factors are applied to increase load and to decreases resistance,


Factored load ≤ factored
strength


∑ (Loads
×

load factors) ≤ resistance
×

resistance factors


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.pdf



Load vs. Deformation

FOS1

FOS1

FOS1

FOS1

FOS2

Types of Bolt Joint

Lap joint






Butt joint

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 with area
of bolt.

Moment of Inertia