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Design of Steel Structures (Limit State Design) |
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Module
1: Introduction and design of connections
Module
2: Design of Tension Member
Module
3: Design of Compression Member
Module
4: Design of Beams
Module
5: Design of Plate and Gantry Girders
Unit 1: Introduction and design of connections
Performance Expected from a Structure
- Stresses must be within limit
- Deflection must be within limit
- Structure must be stable
- Structure must have adequate life
- Structure must not vibrate dangerously
- Structure must be easy to maintain
- Structure must have pleasing appearance
- Structure must be easy to construct
- Structure must have enough ductility
What are Steel Structures?
The various structures which come under the realm of civil engineering such as; buildings, bridges, dams, towers are constructed with a variety of materials such as concrete, steel, masonry, timber, cast iron, and plastic. The structures which are constructed using structural steel are called as steel structures such as; steel buildings (ware houses, factories, super market and offices), steel bridges, and steel towers [K. S. Sai Ram, Pearson 2010]
What does Structural Steel Consists of ?
- Tension Member (Carrying Tensile Force)
- Compression Member (Carrying Compressive Force)
- Beams and Girders (Carrying Flexural Forces)
- Beam-Column (Combined action of Bending and Bearing)
- Column bases and caps (Combined action of Compression and Flexure)
- Brackets (Shear, Twist and Bending)
- Connections (Pinned, Semi-rigid, and Rigid)
- Trusses (Axial Compression/Tension, Skeletal Geometry)
Advantages & disadvantages of steel as structural steel
(reference Design of steel structures (LSM) by SK Duggal, McGraw hill publication)ADVANTAGES
- Steel members have high strength per unit weight. Therefore, a steel member of a small section which has little self-weight is able to resist heavy loads. The high strength of steel results in smaller sections to be used & fewer columns in buildings.
- Steel being a ductile material hence does not fail suddenly, but gives visible evidence of impending failure by large deflections.
- Structural steels are tough, i.e. they have both strength and ductility. Thus, steel members subjected to large deformations during fabrication and erection will not fracture.
- Steel may be bent, hammered, sheared or even the bolt holes may be punched without any visible damage.
- Properly maintained steel structures have a long design life.
- Additions and alterations can be made easily to the steel structures.
- They can be erected at a faster rate.
- Steel can be reused after a structure is disassembled.
- Steel is the ultimate recyclable material.
- Steel structures, when placed in exposed conditions, are subjected to corrosion, hence requires anti-corrosive treatment which increases cost for maintenance.
- Steel structures need fireproof treatment which increases cost for maintenance.
- Steel is an excellent heat conductor and therefore steel members may transfer enough heat from a burning section or a room of a building to ignite materials with which they are in contact in the adjoining room.
- Fatigue of steel is one of the major drawbacks. Fatigue involves a reduction in the strength when steel is subjected to a large number of variations in tensile stress.
- At the places of stress concentrations in the steel sections, under certain conditions, the steel may lose its ductility.
Structural Steel
Grade Designation | Quality | Ladle analysis, Percentage, Max | Carbon Equivalent, Max | ||||
C | Mn | S | P | Si | |||
E165 | - | 0.25 | 1.25 | 0.045 | 0.045 | - | - |
E250 | A | 0.23 | 1.50 | 0.045 | 0.045 | 0.40 | 0.42 |
E250 | B | 0.22 | 1.50 | 0.045 | 0.045 | 0.40 | 0.41 |
E250 | C | 0.20 | 1.50 | 0.04 | 0.04 | 0.40 | 0.39 |
E300 | - | 0.20 | 1.30 | 0.045 | 0.045 | 0.45 | 0.4 |
E350 | - | 0.20 | 1.50 | 0.045 | 0.045 | 0.45 | 0.42 |
E410 | - | 0.20 | 1.60 | 0.045 | 0.045 | 0.45 | 0.44 |
E450 | D | 0.22 | 1.60 | 0.045 | 0.045 | 0.45 | 0.46 |
E450 | E | 0.22 | 1.80 | 0.045 | 0.045 | 0.45 | 0.45 |
Grade Designation | Quality | Tensile Strength (MPa) | Yield Strength (MPa) | Percentage Elongation | ||
t < 20 | 20 < t < 40 | t > 40 | ||||
E165 | - | 290 | 165 | 165 | 165 | 23 |
E250 | A | 410 | 250 | 240 | 230 | 23 |
E250 | B | 410 | 250 | 240 | 230 | 23 |
E250 | C | 410 | 250 | 240 | 230 | 23 |
E300 | - | 440 | 300 | 290 | 280 | 22 |
E350 | - | 490 | 350 | 330 | 320 | 22 |
E410 | - | 540 | 410 | 390 | 380 | 20 |
E450 | D | 570 | 450 | 430 | 420 | 20 |
E450 | E | 590 | 450 | 430 | 420 | 20 |
Stress-Strain Relationship of Steel
Product of Structural Steel (Indian)
Hot-Rolled Sections
- Indian Standard Column Sections (ISSC)
- Indian Standard Junior Beams (ISJB)
- Indian Standard Light Weight Beams (ISLB)
- Indian Standard Medium Weight Beams (ISMB)
- Indian Standard Heavy Weight Beams (ISHB)
- Indian Standard Wide Flange Beams (ISWB)
Channels:
- Indian Standard Junior Channel (ISJC)
- Indian Standard Light Weight Channels (ISLC)
- Indian Standard Medium Weight Channels (ISMC)
- Indian Standard Medium Weight Parallel Flange Channels (ISMCP)
Tees:
- Indian Standard Rolled Normal Tee Bars (ISNT)
- Indian Standard Rolled Deep Legged Tee Bars (ISDT)
- Indian Standard Slit Light Weight Tee Bars (ISLT)
- Indian Standard Slit Medium Weight Tee Bars (ISMT)
- Indian Standard Slit Tee Bars from H Sections (ISHT)
Angles:
Tubular Sections:
Plates, Sheets, Strips and Flats:
Plates are designated as ISPL followed by figures Length x Width x Thickness in mm, for example 600 x 450 x 12. Sheets are designated as IISH followed by figures Length x Width x Thickness in mm, for example 450 x 450 x 10. Strips are designated as ISST followed by figures denoting Width x Thickness in mm, for example 400 x 8 mm. Flats are designated by the width followed by letters ISF and the thickness in mm, 400ISF8. The properties of these sections are published in standard book IS 1730:1989
Cold-Formed Light Gauge Sections
The intention of reducing the weight of structures design engineers has focused of light weight section. These sections are produced from steel strips of 8mm thickness or less. The mass production of these sections are done with the help of press shop. They are available in the form of equal angles, unequal angles, channels, hats sections, and z section. these sections are designated by numbers denoting depth (mm) x width (mm) x thickness (mm). The properties of these sections are published in standard book IS 811:1987
The design practices is based on specification and stipulation of relevant codes. In India, several development and experimentation has been taken place for improving the material property of structural steel. Bureau of Indian standard has taken major effort in this process by publication of IS 800:1984 Code of Practice for General Construction in Steel.The earlier version of IS 800: 1984 was based on allowable Stress Design (Working), which has been revised incorporating the Load and Resistance Factor Design (LRFD) and Limit State Method (LSM) in 2007.
Structural Loads
A structure is designed to carry certain loads so as to serve the intended purpose. The primarily load considered are; Dead loads, Imposed(Live) loads, Wind loads, Seismic loads, Snow load, Erection load, and effects of temperature etc.
1. Dead Loads
- Roof geometry,
- Size of the structure,
- Insulation of the structure,
- Wind frequency,
- Snow duration, and
- Geographical location of the structure.
Impact load is caused by vibration or impact or acceleration. Thus, impact load is equal to imposed load incremented by some percentage called impact factor or impact allowance depending upon the intensity of impact.
Wind load is primarily horizontal load caused by the movement of air relative to earth. Wind load is required to be considered in design especially when the heath of the building exceeds two times the dimensions transverse to the exposed wind surface. The amount of wind load is dependent on the following IS 875 Part 3;
- Geographical location
- The height of structure
- Type of surrounding physical environment
- The shape of structure Size of the building
1. Tension Forces
1. Working Stress Method
With the development of linear elastic theories in 19th century it became possible to understand the stress-strain behavior of wrought iron and mild steel accurately. With this theories designers are able to analyze indeterminate structures. The response quantities are yield stress, ultimate stress, bending stresses, and shear stresses. The first attainment of yield stress of steel was taken s onset of failure. The limitation due to non-linearity and buckling are neglected.
The criteria for the design are expressed a;
Allowable stress (Working stress) = Yield stress/ Factor of safety
Stresses due to Dead load + Live load < Allowable stress
In practice there are severe limitations to this approach. These are consequences of material non-linearity, post-buckled state of elements, redistribution of loads.
2. Limit State Method
To overcome the limitations of working stress method, limit state method was proposed using the plastic range of material for the design of structural members and load factor to account for variability in loading configurations. This rational factor of safety in different structural performance enables to use steel efficiently and economically in different structural systems to withstand tension, compression, flexural and torsional loads.
Limit state method takes into account of variance by defining limit states addressing strength and serviceability. In LSM a structure or part of it is considered unfit for use when it exceeds the limit states, beyond which it infringes one of the criteria governing its performance or use. The limit states are classified as strength based limit state and serviceability based limit state. Strength based limit state are defined by yielding, plastic strength, fatigue, buckling etc. Serviceability limit state are defined by deflection, vibration, drift, etc.
In LSM, the factored loads, in different loading combinations are applied to the structure to determine the load effects. The later are compared with the design strength of individual element.
Let;
R* is the calculated factored resistance of the element being checked, and is function of the nominal size of the material yield strength.
Numerical 1
An industrial building is to be designed in Aurangabad. Compute the design wind speed and wind pressure at a height of 15m. the building has maximum dimension of 30m.
Answer:
For Aurangabad the basic wind speed Vb = 39 m/s (Appendix A: IS 875 [Part 4])
Design Life Span of Industrial Building is assumed to be 50 Years k1 =1.0
The terrain is in industrial area and hence it belongs to category 3 and Class B. Hence from Table 2 (IS 875: Part 4) k2 = 0.97 for height 15 m. The ground is assumed to be plain and uniform k3=1.
The plan and elevation of a three storey school building is shown in figure given below. The building is located in Aurangabad (Zone III). The type of soil encountered is medium soil and it is proposed to to design the building with special moment resisting frames . The intensity of dead load is 10 kN/m^2 and the floors are subjected to live load of 3kN/m^2. Determine the lateral loads on the various floor levels of the structure by static analysis.
Dead Load on Structure
= Intensity of loading x Plan area
= 10 x 3 x 3 = 90 kN
Live Load on Structure
= Intensity of loading x Plan area
= 3 x 3 x 3 = 27 kN
Seismic Load of one floor:
= Dead Load + 0.25 (Live Load) [ as per Table 8 of IS 1893(Part 1):2002]
= 90 + 0.25*27
= 96.75 kN
Total Seismic Load = 5 x 96.75 = 483.75
Zone factor (z) = 0.16 [as per Table 2 of IS 1893(Part 1):2002]
Importance Factor (I) = 1.0 [as per Table 6 of IS 1893(Part 1):2002]
Response Reduction factor (R) = 5 [SMRF as per Table 7 of IS 1893(Part 1):2002]
Base Shear coefficient (Ah) = (ZI/2R)*(Sa/g) [as per clause 6.4.2 of IS 1893(Part 1):2002]
(Sa/g) is read from Figure 2 of IS 1893(Part 1):2002 for medium soil type for natural period obtained from the relation Ta = 0.075h^0.75 = 0.075(15)^0.75 = 0.571 worked out to be 2.5.
Ah = (ZI/2R)*(Sa/g) = (0.16 x1/2 x 5) x 2.5 = 0.04
Base Shear (Vb) = Ah x Total seismic weight all floors
= 0.04 x 483.75 = 19.35 kN
Lateral load on each floor (Fi) = Vb x [(wihi^2)/ summation of (wihi^2)]
Types of Structural Steel
(Reference Design of steel structures-I by Dr. RV Gehlot, Scientific Publishers, India)
ADVANTAGES
- The process is silent, compare to the riveted connection where hammering is done.
- There is no risk of fire, as compared to red-hot riveting and welding.
- Less time consuming in compared to the riveted connection.
- The overall cost of bolted construction is cheaper than that of riveted construction because of the reduced labour and equipment costs and a smaller number of bolts required for resisting the same load.
- It facilitates the erection because of the ease with which these connections can be done.
- If bolted connections become loose, their strength reduces considerably.
- The unfinished bolts are not uniform in diameter and they have less strength.
- The bolted connection has less strength when they are subjected to axial tension because area at the root of the thread is less.
- Generally, the diameter of the hole is kept 2 mm more than the nominal diameter of the black bolt. The bolt does not fill the hole and there remains a clearance in bolted connections.
- Cost of material is high, about double that of rivets.