LOAD FACTORS AND LOAD COMBINATION It is impossible that all loads like live load, wind load and...
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Transcript of LOAD FACTORS AND LOAD COMBINATION It is impossible that all loads like live load, wind load and...
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LOAD FACTORS AND LOAD COMBINATION
It is impossible that all loads like live load, wind load and earthquake all occur together with their maximum intensity.
A load combination combines different types of loads depending on the probability of occurrence of these loads, considering their expected intensity in the combination compared with the maximum load intensity.
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The factors of safety are also included in the
LRFD load combinations and hence the
output of the expressions is a design load.
The alphabets used in the combinations
mean different types of nominal service
loads and the numerical values with them
are all the load factors.
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When intermediate floors have full live
loads, any type of roof load may be
considered equal to half of its normal service
load intensity.
Similarly, in case of maximum intensity
wind storm, live load may be half.
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The last combination, given afterwards, is very important for uplift of structure or reversal of forces.
The wind load on roof is upwards in majority of the cases and if the downward gravity load is less, the structure may be blown up or sagging bending may change into hogging bending.
A list of most commonly used combinations are as under:
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LRFD Load Combination
1. 1.4 (D+F)
2. 1.2 (D+F+T) + 1.6(L+H) +0.5(Lr or S or R)
3. 1.2D + 1.6(Lr or S or R) + (L or 0.8W)
4. 1.2D + 1.6W + 1.0L +0.5(Lr or S or R)
5. 1.2D + 1.0E + 1.0L + 0.2S
6. 0.9D + 1.6W + 1.6H
7. 0.9D + 1.0E + 1.6H5
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LRFD Load Combination D dead load L live load Lr roof live load W wind load S snow load E earthquake load R rainwater or ice load H load due to lateral earth pressure, ground
water pressure or pressure of bulk materials
F load due to fluids with well defined pressures and max heights.
T self-retaining force6
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LRFD Load Combination
Study the remaining discussion by yourself.
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ASD Load Combination1. D + F
2. D + H + F + L + T
3. D + H +F + (Lr or S or R)
4. D + H +F + 0.75(L + T) + 0.75 (Lr or S or R)
5. D + H +F + (W or 0.7E)
6. D + H +F + 0.75(W or 0.7E) + 0.75L + 0.75(Lr or S or R)
7. 0.6D +W +H
8. 0.6D + 0.7E + H8
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Simplified Load Combinations
When the loads S, R, H, F, E and T are taken
equal to zero and wind loads are taken from
the previous codes, the load combinations
are reduced to the following from:
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LRFD
1. 1.4D
2. 1.2D + 1.6L + 0.5Lr
3. 1.2D + 1.6Lr + (L or 0.8W)
4. 1.2D + 1.3W + 1.0L + 0.5Lr
5. 0.9D + 1.3W
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ASD
1. D
2. D + L
3. D + Lr
4. D + 0.75L + 0.75Lr
5. D + 0.8W
6. D + 0.6W + 0.75L + 0.75Lr
7. 0.6D + 0.8W 11
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Live Load Reduction
The intensity of live load may be reduced if
the contributory area for the live load
exceeds certain limit.
It is due to the fact that, under these
circumstances, all the area may not be
subjected to the full load.
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All uniform live loads, except the roof loads
(for which separate provisions are given in
ASCE-07), may be reduced as follows:
Where
Lo = the unreduced live load
AT = tributary area in m2
KLL = live load element factor 13
TLL AKLL
57.425.0
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a Interior columns and exterior columns without cantilever slabs.
KLL = 4
b Edge columns with cantilever slabs.
KLL = 3
c Corner columns with cantilever slabs, edge beams without cantilever slabs and interior beams.
KLL = 2
d All other members including slabs.
KLL = 1
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TYPES OF STRUCTURAL STEEL
Steels are divided into four categories depending on the carbon percentages (C) as following:
1. Low carbon steel C < 0.15%
2. Mild carbon steel C = 0.15 - 0.29%
3. Medium carbon steel C = 0.30 – 0.59%
4. High carbon steel C = 0.60 – 1.70%
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E-Value of steel = 185 GPa to 230 GPa
(Average 200 GPa)
Unit weight = 7850 kg/m3
= 77 kN/m3
= 7.85 g/cc
For comparison, the unit weight of concrete is 23.6 kN/m3.
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Most of the structural steel falls into the mild carbon steel or simply mild steel (MS) category.
Hot rolled structural shapes may be made to
conform to A36M, A529M, A572M,
A588M, A709M, A913M and A992M.
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Sheets are manufactured according to the standards ASTM A606, A1011MSS, HSLAS and HSLAS-F.
Bolts are made according to ASTM
standards A307, A325M, A449, A40M and
F1852.
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Most commonly used structural steel is
A36M having the following properties:
Fy = 250 MPa
Fu = 400 MPa
E = 200 GPa
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Weld Electrode And Filler Material
Weld electrodes are classified as E60, E70,
E80, E100 and E110.
The letter E denotes electrodes.
The two digits indicate the ultimate tensile
strength in ksi. The corresponding SI
equivalents are E425, E495, E550, E690 and
E760.20
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HOT ROLLED STRUCTURAL SHAPES
These are the steel cross-sectional shapes that are hot rolled in the mills. Some of these shapes are shown in Figure 1.2, whereas, the steel bars, plates and hollow sections are reproduced in Figure 1.3.
An HP h x w is a bearing pile section, which is approximately h mm deep weighing w kgs/m.
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HOT ROLLED STRUCTURAL SHAPES
Bearing piles are made with the regular W
rolls but with thicker web to provide better
resistance to the impact of pile driving.
HSS are hollow structural sections that are
prismatic square, rectangular or round
products of a pipe or tubing.
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HOT ROLLED STRUCTURAL SHAPES
Every hot rolled shape has its unique
standard designation, which not only tells
about the type of cross-sectional shape but
also about its size.
The details about some of the common hot
rolled shapes are given in the next slides.
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Slope ≈ 0°
W-Section
Figure 1.2
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16.7% Slope
S-Section
Figure 1.2
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Angle-Section
Figure 1.2
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Channel-Section
16.7% Slope
Figure 1.2
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Tee-Section
Figure 1.2
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HP-Section
Thicker than flange
Figure 1.2
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Pipe Section
Figure 1.3
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Structural Tubing
Figure 1.3
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Bars
Figure 1.3
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Plates
Figure 1.3
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1. W-Shapes
The letter ‘W’ stands for an I-shape with wide flange. The cross-section is doubly symmetric in the form of the letter “I”. The width / depth ratio varies from about 0.3 to 1.0.
The US Customary designation W 16 x 40 means that the nominal depth of the section is 16 in and the weight per unit length of the section is 40 lbs/ft.
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1. W-Shapes
Nominal height is the rounded off height to
be used for common use.
Actual depth of the section may be in
decimals and somewhat different from this
depth.
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The equivalent SI designation W410 x 60
means that the W-section has a nominal
depth of 410 mm and a weight of 60
kgf/m.
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Less or no slope Flange
Web
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This kilogram-force weight per unit length
may be converted in kN/m by multiplying
it with the factor 9.81/1000.
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2. S-Shapes
• Doubly symmetrical I-shapes.
• Previously called standard I-beams or
American Standard Beam.39
16.7% Slope
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2. S-Shapes
• The inner edge of the flange has a slope of approximately 16.7%.
• An S510 x 112 section means that the section is S-shape having nominal depth of 510 mm and weight of 112 kgf/m.
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16.7 % Slope
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2. S-Shapes
• The width / depth ratio varies from about 0.25 to 0.85.
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16.7 % Slope
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3. M-Shapes
• Miscellaneous I-shapes.
• Doubly symmetrical I-shapes not classified as W or S shapes.
• Relatively lightweight used for smaller spans and lesser loads.
• An M310 x 17.6 means that it is M-shape section having nominal depth of 310 mm and weight of 17.6 kgf/m.
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4. C-Shapes
The C-shapes have the following distinguishing features:
• Channel shapes with standard proportions.
• Inner flange slope is 16.7%.
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16.7 % Slope
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4. C-Shapes
The C-shapes have the following distinguishing features:
• Previously called Standard or American Standard Channels.
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16.7 % Slope
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4. C-Shapes
The C-shapes have the following distinguishing features:
• A C150 x 19.3 is a standard channel shape with a nominal depth of 150mm and a weight of 19.3 kgf/m.
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16.7 % Slope
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5. MC-Shapes
These sections have the following properties:
• Channels not classified as C-shapes.
• Previously called Shipbuilding or Miscellaneous Channels.
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6. L-Shapes or Angle Sections
The various types of angle sections are shown in Figure and their salient features are given below:
• The single angle sections are in the form of letter ‘L’.
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a
b
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6. L-Shapes or Angle Sections
The various types of angle sections are shown in Figure and their salient features are given below:
• If a = b, these are called equal angle sections.
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a
b
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6. L-Shapes or Angle Sections
The various types of angle sections are shown in Figure and their salient features are given below:
• If a ≠ b, these are called unequal angle sections.
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a
b
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6. L-Shapes or Angle Sections
• Sides of the angle are called ‘legs’ or ‘arms’.
• L89 x 76 x 12.7 is an unequal leg angle with longer leg dimension of 89mm and shorter leg dimension of 76mm with a leg thickness of 12.7mm.
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a
b
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6. L-Shapes or Angle Sections
• Double angle sections are combination of two angles with longer or shorter sides close to each other.
• Double angle sections are denoted by 2Ls.
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a
b
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6. L-Shapes or Angle Sections
• 2L89 x 76 x 12.7 means two angles L89 x
76 x 12.7 placed side by side in one of the
ways shown in the figure.
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a
b
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7. T-Shapes
• These are called structural tees.
• These are obtained by splitting W, S or M
shapes and are called WT, ST, or MT
shapes, respectively.53
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7. T-Shapes
• A WT205 x 30 is a structural tee with a
nominal depth of 205mm and a weight of
30kgf/m and is obtained by splitting the
W410 x 60 section.
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COLD – FORMED SHAPES
These sections are formed from thin high
strength steel alloy plates under normal
temperature.
Some of the common shapes of these
sections are drawn in Figure:
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Channels
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57
Zees
![Page 58: LOAD FACTORS AND LOAD COMBINATION It is impossible that all loads like live load, wind load and earthquake all occur together with their maximum intensity.](https://reader036.fdocuments.net/reader036/viewer/2022062606/5697c0261a28abf838cd5d63/html5/thumbnails/58.jpg)
58
I-Shaped Double Channels
![Page 59: LOAD FACTORS AND LOAD COMBINATION It is impossible that all loads like live load, wind load and earthquake all occur together with their maximum intensity.](https://reader036.fdocuments.net/reader036/viewer/2022062606/5697c0261a28abf838cd5d63/html5/thumbnails/59.jpg)
59
Angle
![Page 60: LOAD FACTORS AND LOAD COMBINATION It is impossible that all loads like live load, wind load and earthquake all occur together with their maximum intensity.](https://reader036.fdocuments.net/reader036/viewer/2022062606/5697c0261a28abf838cd5d63/html5/thumbnails/60.jpg)
60
Hat Sections
![Page 61: LOAD FACTORS AND LOAD COMBINATION It is impossible that all loads like live load, wind load and earthquake all occur together with their maximum intensity.](https://reader036.fdocuments.net/reader036/viewer/2022062606/5697c0261a28abf838cd5d63/html5/thumbnails/61.jpg)
BUILT-UP SECTIONS
Sections made by combining two or more
standard hot rolled sections, joined
together at intervals with the help of direct
welding, stay plates or lacing, are called
built-up sections.
Examples are four angles section, double
angle section and double channel section
shown in Figure.61
![Page 62: LOAD FACTORS AND LOAD COMBINATION It is impossible that all loads like live load, wind load and earthquake all occur together with their maximum intensity.](https://reader036.fdocuments.net/reader036/viewer/2022062606/5697c0261a28abf838cd5d63/html5/thumbnails/62.jpg)
BUILT-UP SECTIONS
However, double angle section is
sometimes excluded from built-up section
category and is considered as a regular hot
rolled member because of difference of its
behavior from other built-up sections.
62
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63
4-Angle Box Section
![Page 64: LOAD FACTORS AND LOAD COMBINATION It is impossible that all loads like live load, wind load and earthquake all occur together with their maximum intensity.](https://reader036.fdocuments.net/reader036/viewer/2022062606/5697c0261a28abf838cd5d63/html5/thumbnails/64.jpg)
64
Double Angle
![Page 65: LOAD FACTORS AND LOAD COMBINATION It is impossible that all loads like live load, wind load and earthquake all occur together with their maximum intensity.](https://reader036.fdocuments.net/reader036/viewer/2022062606/5697c0261a28abf838cd5d63/html5/thumbnails/65.jpg)
65
Two Channels connected back-to-back
![Page 66: LOAD FACTORS AND LOAD COMBINATION It is impossible that all loads like live load, wind load and earthquake all occur together with their maximum intensity.](https://reader036.fdocuments.net/reader036/viewer/2022062606/5697c0261a28abf838cd5d63/html5/thumbnails/66.jpg)
66
Question?