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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.

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LOAD FACTORS AND LOAD COMBINATION

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  1. 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.

  2. 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.

  3. 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.

  4. 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:

  5. LRFD Load Combination • 1.4 (D+F) • 1.2 (D+F+T) + 1.6(L+H) +0.5(Lror S or R) • 1.2D + 1.6(Lr or S or R) + (L or 0.8W) • 1.2D + 1.6W + 1.0L +0.5(Lr or S or R) • 1.2D + 1.0E + 1.0L + 0.2S • 0.9D + 1.6W + 1.6H • 0.9D + 1.0E + 1.6H

  6. 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 force

  7. LRFD Load Combination • Study the remaining discussion by yourself.

  8. ASD Load Combination • D + F • D + H + F + L + T • D + H +F + (Lr or S or R) • D + H +F + 0.75(L + T) + 0.75 (Lr or S or R) • D + H +F + (W or 0.7E) • D + H +F + 0.75(W or 0.7E) + 0.75L + 0.75(Lr or S or R) • 0.6D +W +H • 0.6D + 0.7E + H

  9. 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:

  10. LRFD • 1.4D • 1.2D + 1.6L + 0.5Lr • 1.2D + 1.6Lr + (L or 0.8W) • 1.2D + 1.3W + 1.0L + 0.5Lr • 0.9D + 1.3W

  11. ASD • D • D + L • D + Lr • D + 0.75L + 0.75Lr • D + 0.8W • D + 0.6W + 0.75L + 0.75Lr • 0.6D + 0.8W

  12. 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.

  13. 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

  14. TYPES OF STRUCTURAL STEEL Steels are divided into four categories depending on the carbon percentages (C) as following: • Low carbon steel C < 0.15% • Mild carbon steel C = 0.15 - 0.29% • Medium carbon steel C = 0.30 – 0.59% • High carbon steel C = 0.60 – 1.70%

  15. 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.

  16. 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.

  17. 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.

  18. Most commonly used structural steel is A36M having the following properties: Fy = 250 MPa Fu = 400 MPa E = 200 GPa

  19. 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. 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 HPh x w is a bearing pile section, which is approximately h mmdeep weighing w kgs/m.

  21. 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.

  22. 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.

  23. Figure 1.2 Slope ≈ 0° W-Section

  24. Figure 1.2 16.7% Slope S-Section

  25. Figure 1.2 Angle-Section

  26. Figure 1.2 16.7% Slope Channel-Section

  27. Figure 1.2 Tee-Section

  28. Figure 1.2 Thicker than flange HP-Section

  29. Figure 1.3 Pipe Section

  30. Figure 1.3 Structural Tubing

  31. Figure 1.3 Bars

  32. Figure 1.3 Plates

  33. 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.

  34. 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.

  35. Less or no slope Flange 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. Web

  36. This kilogram-force weight per unit length may be converted in kN/m by multiplying it with the factor 9.81/1000.

  37. 16.7% Slope 2. S-Shapes • Doubly symmetrical I-shapes. • Previously called standard I-beams or American Standard Beam.

  38. 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.

  39. 16.7 % Slope 2. S-Shapes • The width / depth ratio varies from about 0.25 to 0.85.

  40. 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.

  41. 4. C-Shapes The C-shapes have the following distinguishing features: • Channel shapes with standard proportions. • Inner flange slope is 16.7%. 16.7 % Slope

  42. 4. C-Shapes The C-shapes have the following distinguishing features: • Previously called Standard or American Standard Channels. 16.7 % Slope

  43. 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. 16.7 % Slope

  44. 5. MC-Shapes These sections have the following properties: • Channels not classified as C-shapes. • Previously called Shipbuilding or Miscellaneous Channels.

  45. 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’. a b

  46. 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. a b

  47. 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. a b

  48. a 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. b

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