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Basic structural concepts

1. Force

Force is an influence/action on object which causes/attempts to cause movement

Is a vector

Leng

th =

Mag

nitu

de

Point of action

Arrow = direction

jo

Unit: Newtons (N); kiloNewtons (kN)

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2. Mass & Weight

Mass = amount of matter in an object

Mass unit: grams (g); kilograms (kg)

Weight = Mass × Acceleration due to Gravity

Mass 1kg is subjected to acceleration of 9.81m/sec2 ( 10 m/sec2), weighs (1×10)=10 N

Facts:

10N = 1kg (a bag of sugar)

1kN = 100kg

1000kg = 10kN (a weight of a small car)

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3. Density and unit weight

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Load

1. Types of loads

Load is a force on a part of a structure

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1.1 Dead load

Permanent load, always present

The weight of permanent portions of a building - self-weight

Material Load

Reinforced concrete Unit weight: 24 kN/m3

Therefore a 100 mm thick concrete wall weighs 2.4 kN/m2

Blockwork/Brickwork Unit weight: 22 kN/m3 Therefore a 100 mm thick blockwork wall weighs 2.2 kN/m2

Steel Unit weight: 78.5 kN/m3 Steel beams weigh between 0.2 and 2.0 kN/m

Aluminium Unit weight: 27.7 kN/m3 (2771 kg/m3)

Timber Unit weight: Softwood: 5.9 kN/m3, Hardwood: 12.5 kN/m3 Therefore a 50mm x 200mm (two by eight') softwood joist weighs 0.06 kN/m

Glass Unit weight: 25 kN/m3 Therefore the weight of glass is 0.025 kN per millimetre thick.

Water Unit weight: 10 kN/m3

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1.2 Live load

Not always present, fluctuate

Source: Occupants of the building, books, goods, furniture etc

Values of live load depend on the use of the building (or part of the building) concerned. A full listing appears in British Standard. BS 6399 Part 1. Some values are given below:

Space Live (kN/m2)

Domestic 1.5

Offices 2.5

Cafes/restaurants 2 .0

Classrooms 3.0

Assembly: fixed seating 4.0

Corridors/stairs in hotels, etc. 4.0

Exhibitions 4.0

Gyms 5.0

Bars, concert halls, etc. 5.0

Stages 7.5

Shops 4.0

Parking (cars) 2.5

Plant rooms 75

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1.3 Wind load

Not critical for low rise buildings

1.4 Others

Lateral load from soil pressure and water pressure

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2. Nature of load

Loads could be one of three types:

Point load (titik)

Uniformly distributed load (teragih segaya)

Uniformly varying load (berubah segaya)

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Point load

Elevation of column

supported on beam

Symbolic representation

of point load

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Uniform distributed load

Representation of uniformly-distributed loads on beams (UDLs)

Span =5m

7kN/m

Span =5m

35kN

5m

7x5=35kN

2.5m

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Uniformly varying load

Retained earth

pushes horizontally

behind the wall

Symbolic representation of a uniformly

varying load on a retaining wall

12kN/m

6.0m

2.0m

0.5x6x12=36kN

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3. Load Path

A B C

DE

F

A B C

DEF

Part of structural floor

planLoad path

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4. Stress and Strain

Stress: Intensity of force acting on an object (N/mm2)

(A) Area

(P) Force Stress

Strain: Deformation occured to an object due to force (no unit).

(L)l length Original

L)( length in Change Strain

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5. Relationship between stress and strain (tegasan & keterikan)

W1W2W3

Load will be

added

progressivelyRo

d o

rig

ina

l le

ng

th

Elo

ng

atio

n

Rod cross

sectional

Area = A

Load (P)

Stress

(=P/A)

Elongation

l

Strain

=L/L

W1 1 L1 1

W1+W2 2 L2 2

W1 +W2+W3

.

.

.

.

3

.

.

.

.

L3

.

.

.

.

3

.

.

.

.

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Proportional Limit

within the proportional limit, the stress is directly proportional to strain - Hooke’s Law

or = k

k is called the Modulus of Elasticity (E) or Young’s Modulus

= k or = E

a stiffer material will have a higher elasticity modulus

Young’ Modulus of a material is fairly constant:

Esteel = 205kN/mm2

Econc 17kN/mm2

Elastic Limit

beyond this limit the material will no longer go back to its original shape when the load is removed

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Yield Point

The point at which the material will have an appreciable elongation or yielding without any increase in load. (permanent deformation).

Ultimate Strength

the maximum ordinate in the stress-strain diagram (ultimate strength)

Rapture Strength

the strength of the material at rupture (the breaking strength)

Working Stress, Allowable Stress, and Factor of Safety

Working stress = the actual stress of a material under a given loading

Allowable stress = The maximum safe stress that a material can carry (limited to values not exceeding the proportional limit) Proportional limit is difficult to determine accurately, the allowable stress is taken as either the yield point or ultimate strength divided by a factor of safety.

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Relationship between stress and strain can be used to examine the effect of load on structure, example:

31.5kg

Dcable=2mm

Ecable=200kN/mm2

Elo

ng

atio

nC

ab

le o

rig

ina

l le

ng

th

Before

Loading

After

Loading

Estimate the elongation of the cable after loading.

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Solution:

mm 2(D)diameter Cable

mm 1200(L) length Cable

N 315

1031.5(P) Load

kg 31.5Mass

2

2

2

mm 3.144

23.144

D(A) area sectional cross Cable

2N/mm 1003.14

315A

Pσ

1200

Elongationlength Original

Elongationε

2cable kN/mm 200E

mm

elongation kN/mm N/mm

εEσ

12001000200100 22

mm 0.61000200

1200100Elongation

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Tutorial

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6. Rule of Thumb

is a principle with broad application that is not intended to be strictly accurate or reliable

Can be used to determine the initial sizing of structural elements

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