02-simple structure design

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Mechanics of MaterialSimple Structure Design

Eng. Ahmad Hammad Germ


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Factor of safety is the ratio of the failure load, Fail, divided by the a

load, Fallow:

. .

F.S. – Factor of Safety

For safety consideration, the applied load should be less than the l

member can fully support, i.e.,

Ffail > Fallow F.S. > 1

Load that causesmember Failure

Design load

Generally, Ffail is found experimentally and F.S. is chosen based on



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Allowable Stress

If the load applied to the member is linearly related to the stress dev

within the member, i.e., uniformly distributed stress:

&

=

Then:

. .

..



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Factor of Safety


5/32Eng. Ahmad Hammad Germ

General Recommendation


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Example: If the pin at point B is subjected to double shear , and it has an all

stress, , of 10 MPa, what is the minimum required diameter for the pi



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660 N



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Front View Side View

Double Shea

660 N, from

support to beam

660 N, from beam

to support 660 N

330 N330 N



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21



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What if the pin is su

single shear instead



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Front View Side View

Single Shea

660 N, from

support to beam

660 N, from beam

to support



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P1-71. The joint is fastened together using two bolts.

Determine the required diameter of the bolts if the

failure shear stress for the bolts is 350

Use a factor of safety for shear of F.S. = 2.5.



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Determine the required minimum thickness t of

member AB and edge distance b of the frame if P = 40

kN and the factor of safety against failure is 2. The

wood has a normal failure stress ofσ fail =42 MPa and

shear failure of τ fail = 10.5 MPa.

Exam Question (1st 2015/2016)


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Types of Load

StaticLoad

Dynamic

Load

Load is applied relatively slowly and resulting a ststate response

Suddenly applied, and not steady state loads resulrapid change in magnitude, direction and locati



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Types of Loads

Dead Loads Live Loa

Wind Loads

- Earthquake Loads

- Earth Pressure loads (bas

- Liquid Loads (Bridge pie

- Thermal Loads



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Dead Loads

- Permanent fixed loads acting on the structure Loads

- Could be determined with high certainty


- Vertical Load


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Live Loads

- Any temporary or transient loads that act on structure


- Vertical Load


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Wind Loads


- Can act in any direction, inside or outside

- Affected by structure size and shape

- Affected by the height from the ground

- Affected by the location of the structure

- Affected by Surrounding obstruction


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Wind Loads


The more streamlined the object, the les

30 St Mary Axe, London


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The Load Path


Is a route along which the applied loads are transferred to the support (grou

Body Seat Legs

Building walls

Foundation

Must be NO FAILURE occurs at this path


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Bridge 9340 Failure, Minneapolis 2007

300 m of the deck truss collapsed

13 people dead and 145 were injured

The main span fell 32m into river


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Minneapolis Bridge Failure, 2007

Failure was due the increase loading

On the day of failure, a large amount of repair equipment and material wer

the bridge’s weakest point. The live load was estimated to be 262 ton.

More than 5 cm of asphaltic concrete were added to the road surface over

increasing the dead load by 20%


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Observed live load

office buildings va

250 kg/m280 kg/m2


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We define ALL our loa

5% chance of being exc

them characteristic load


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The accepted practice is to choose a value of the live load that has a 5%

being exceeded once in 50 years (design life of most buildings)

But does a percentage of 5% percent is acceptable RISK?

Characteristic loads is the load that have 5% of being exceeded

We applied the load factor to the characteristic loads to increase the safe

Design loads = load factors * characteristic loads


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The limit state of a structure is a term that describes when the structure is o

of becoming unfit for use.

The design using limit state design method requires the structure to satisfycriteria

The Ultimate limit state (ULS) The Serviceability limi

The structure must not collapse when

subjected to the peak design load forwhich it was designed

Ex: Design loading = 1.2 D + 1.5 L

These load factors reduce the chance of

the load being exceeded from 5% to

0.01%

Considering only the perform

Structure

Ex: Design loading = D + 0

These load factors increases

the load being exceeded from


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Resistance Factor

Resistance factor represents the confidence of material behavior

Design Stress = Resistance Factor * Failure Stress

For stress Resistance Factor = 0.9 while for concrete = 0.65

For LSD:

Resistance Factor * Stress = Load factors * loads



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Suggested Problem: 1-74, 1-77, 1-84, 1-88, 1-89, 1-90,


Remember:“It is impossible to make anything foolproof because fools are so ingenious

02-simple structure design

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