LOAD FACTORS AND LOAD COMBINATION

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.

2

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.

3

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

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

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

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

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

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/ccFor 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.

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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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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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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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25

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

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

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.

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16.7% Slope

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

2. S-Shapes

• The width / depth ratio varies from about 0.25 to 0.85.

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16.7 % Slope

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%.43

16.7 % Slope

4. C-Shapes


• Previously called Standard or American Standard Channels.

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16.7 % Slope

4. C-Shapes


• 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

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



• If a = b, these are called equal angle sections.

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a

b



• If a ≠ b, these are called unequal angle sections.

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a

b


• 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


• 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


• 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

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.

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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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Zees

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I-Shaped Double Channels

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Angle

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Hat Sections

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.

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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.

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4-Angle Box Section

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Double Angle

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Two Channels connected back-to-back

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LOAD FACTORS AND LOAD COMBINATION