Steel Structure Design methods

Steel structure may be design three methods.

Elastic design

Plastic design

Limit sate design

Elastic design is the traditional method. Steel is almost perfectly elastic up to the yield point and elastic theory is a very good method on which ton base design. Structures are anayesed by elastic theory and sections are sized so that permissible stresses are not exceed. Design is in accordance with BS 449: plastic theory developed to take account of behavior past the yield point based on finding the load that cases the structure to collapse. Then working load is the collapse load divided by a load factor. This too is permitted under BS449.

Plastic design

In plastic analysis and design of a structure, the ultimate load of the  structure as a whole is regarded as the design criterion. The term  plastic has occurred due to the fact that the ultimate load is found from the strength of steel  in the plastic range. 

This method is rapid and provides a rational approach for the analysis of the structure. It also provides striking economy as regards the weight  of steel since the sections required by this method are smaller in size than those  required by the method of elastic analysis. Plastic analysis and design has its  -main application in the analysis and design of statically indeterminate framed  structures.

Limit sate design

A Civil Engineering Designer has to ensure that the structures and facilities he designs are (i) fit for their purpose (ii) safe and (iii) economical and durable. Thus safety is one of the paramount responsibilities of the designer. However, it is difficult to assess at the design stage how safe a proposed design will actually be. There is, in fact, a great deal of uncertainty about the many factors, which influence both safety and economy. The uncertainties affecting the safety of a structure are due to 

 · Uncertainty about loading

 · Uncertainty about material strength and 

 · Uncertainty about structural dimensions and behaviour.

These uncertainties together make it impossible for a designer to guarantee that a structure will be absolutely safe. All that  the designer can ensure  is that the risk of failure is extremely small, despite the uncertainties. 

Tension membrane structures

Tension membrane structures offer an aesthetic,  practical, and cost-effective way to provide roofing to either new or existing areas. These shade  supporting structures have been put into use for most  part of the twentieth century and have found wide  spread applications including, sports facilities,  convention centres, concert halls, shade roofs of large  commercial buildings, airport roofs, gas stations and terminals, to mention only a few.  The membrane part in the structure is pre-stressed and thus enables the structure to maintain its form.

 Two types of shapes are found in tension membrane  structures. The first type is the 'anticlastic structures' with two double curvatures in the  opposite direction while the second is a   'synclastic structures' with the double curvatures  in the same direction. Anticlastic structures can  take a variety of shapes and forms including,  the arched vault, hyper and cone. Technically speaking a tension membrane  structure is a combination of elements, which carry only tension and no compression or bending. This  is the reason why the use of compression rings or  beams, that form the bending or compression elements,  is used in most tensile structures.

Why Tension Membrane Structures?

The semi-translucent nature of fabric structures is  what makes them a favourite with engineers and architects looking for roofing systems to cover large  areas, such as sports stadia or terminals. Fabric  structures help in increasing the sustainability quotient of  a building in more ways than one. The fabric allows for  entry of natural light, while cutting down the  transmission of heat. The high reflectivity of the  membrane makes it an ideal alternative to glass as a  roof glazing system.  Tension membrane structures are usually reinforced  using either PVC /Polyester or PTFE based coatings. This makes the fabric structure perform well from the fire  performance perspective too. For example, a tension membrane structure with PTFE coating is rated non  combustible as per ASTM 136, making them completely safe. Additionally the inert nature of the fabric aids in  self-cleaning, a characteristic which makes them  perfectly suited for application over large areas.

The  dependency on artificial lighting is vastly reduced. The unique properties of light reflectance and  transmission also offer exciting possibilities for lighting  after dark. Directing lights under the canopy to reflect  off the underside is a great way to use uplighters, but  more even lighting can be achieved under the fabric by  shining lights down on the fabric from above.

The thermal insulation achieved with a single layer  of either PVC/Polyester or PTFE membrane with a typical weight of around 1200gm per sq metre and a  U value of approximately 4.5 W/m2K, is more or less  similar to that of glass. White is mostly the preferred  colour when it comes to tension membrane structures.  This is because with dark coloured membranes, the  absorption of heat is very high. Dark coloured  membranes can also re-radiate heat. White is  therefore the preferred choice in the case of tension  membrane fabrics.

Roofing and Cladding

With tension membrane structures, it is possible to  have both the roofing and cladding in one single structural element. Typically the seam and curve of the  fabric structure that reflects the tension is aesthetically  pleasing, while also being important for the structural  integrity. Due to the integration of the roofing and  cladding, the structures are also easy to clean and  maintain, when compared to glaze glass roofing. The  roof, in the case of tensile membrane structures is  factory welded and therefore easy to install, apart from  acting as a weatherproof skin that does not contain  expansion joints. All these factors enable rapid  construction and coverage of large areas.

Span Capabilities

An excellent span capability is another factor that  puts fabric ahead of other materials. While every other  possible roofing material requires rigid intermediate  support, it is not the case with fabric structures. The  fabric can span from one boundary to another in one  unbroken (sweep). This ensures that there are no sealing  related issues that need to be addressed. The amazing tensile capacity of fabric helps to  reduce the number of components that make up the  supporting framework to a minimum, thus enabling a  structure that is much more light-weighted when  compared to other types of construction. On the flip side  though, the structures incorporating the concept need  large foundations in order to prevent wind currents  lifting the canopy. This factor is offset by the fact that in  terms of cost foundations are cheaper to prepare, than  the visible above ground construction components that  are exposed to the vagaries of weather and therefore,  more prone to damage. 

Why Tension Membrane Structures?

The semi-translucent nature of fabric structures is what makes them a favourite with engineers and architects looking for roofing systems to cover large areas, such as sports stadia or terminals. Fabric structures help in increasing the sustainability quotient of a building in more ways than one. The fabric allows for entry of natural light, while cutting down the transmission of heat. The high reflectivity of the membrane makes it an ideal alternative to glass as a roof glazing system. 

Tension membrane structures are usually reinforced using either PVC /Polyester or PTFE based coatings. This makes the fabric structure perform well from the fire performance perspective too. For example, a tension membrane structure with PTFE coating is rated non combustible as per ASTM 136, making them completely safe. Additionally the inert nature of the fabric aids in self-cleaning, a characteristic which makes them perfectly suited for application over large areas. The dependency on artificial lighting is vastly reduced.

The unique properties of light reflectance and transmission also offer exciting possibilities for lighting after dark. Directing lights under the canopy to reflect off the underside is a great way to use uplighters, but more even lighting can be achieved under the fabric by shining lights down on the fabric from above.The thermal insulation achieved with a single layer of either PVC/Polyester or PTFE membrane with a typical weight of around 1200gm per sq metre and a U value of approximately 4.5 W/m2K, is more or less similar to that of glass. White is mostly the preferred colour when it comes to tension membrane structures. This is because with dark coloured membranes, the absorption of heat is very high. Dark coloured membranes can also re-radiate heat. White is therefore the preferred choice in the case of tension membrane fabrics. 

Structural Steel Fire Protection

Fire Resistance Rating for loadbearing (structural) elements is defined as: “the ability of the structural element to withstand the effects of a defined fire (e.g. hydrocarbon time/ temperature profile) for a specified time without loss of the loadbearing function of structural members.” Fire Resistance Rating for loadbearing elements is described by defining the following:

the structural element being considered

 duration of loadbearing ability

 fire load

 restricted critical core temperature

Therefore, every loadbearing member shall be suitably fire protected to meet the requirements of the fire resistance rating. 

Light Gauge Steel Construction System

Light gauge steel has been used in the construction industry in the developed countries for the last few decades, mainly for non-load bearing interior partition walls in high-rise buildings constructed in reinforced concrete or structural steel. It is also very widely used in commercial buildings interior partitions.

The use of LGS load-bearing studs and components are now becoming more and more common in low (single story) to medium rise building (four to eight storey), particularly in residential homes, apartments and condominiums. Light Gauge Steel Construction system is a pre-engineered & pre-fabricated quick build dry construction system. The key advantages are consistency in quality & controlled project implementation timing.





LGS construction system can be classified under two board categories:Modular Construction: Modular consists of actually pre-fabricating the walls and floor components into sections, usually in the factory under controlled conditions.
The modules are built in sections with doors and windows openings already built into the wall panels. The finished modules are than put on to a trailer and transported to site. The major benefit of this method is the speed of erection as the modules are simply 'craned' into place on site as the wall panels is already pre-assembled.

On Site Construction: There are several innovative systems available today, which consist of ready-made panels, or pre-cut and pre-drilled framing members, which only require easy assembly at site. This system has been used particularly for Industrial and Commercial buildings where large spans and standard building sizes are the norm.

Advantages of LGS Systems




Stable supply of steel means little fluctuation in price. With steel framing, walls remain straight and true. That means fewer callbacks due to nail pops and shrinkage cracks. Because materials can be pre-cut to desired lengths, there is almost no waste.

Steel is a non-combustible material and does not contribute fuel to a fire, or be susceptible to fire.

Cut-to-length material reduces in-field cutting and waste up to 75%. Framing members are manufactured with pre-punched holes for running piping and electrical wiring, minimizing preparation work for other trades. Steel's light weight also makes for easier on-site handling.
MPROVED CONSTRUCTION QUALITY
Galvanized steel studs are corrosion resistant and dimensionally stable, eliminating nail popping and reducing cracking so often associated with wood shrinkage and expansion. Steel studs will not warp, split, rot, settle, shrink, or attract termites.

Steel is inert and does not emit fumes, gases, vapour or support the growth of moulds & fungi (which can potentially caused some health problems).


Steel has one of the highest strength to weight ratios of any construction material. Steel framing can weigh only one-third as much as traditional construction produces, reducing seismic loads. Lower weight means easier maneuverability and lower foundation costs.
It is dimensionally stable - it does not expand or contract with moisture content.

Because studs are attached with screws, they can be easily moved to assure accurate attachment of wall board and other construction components. Steel framing also makes remodeling easier because complex shapes are easy to fabricate. Changed are easy. Simply put your screw gun in reverse and back out the screw.
DESIGN FLEXIBILITY
Multiple product dimensions and thickness of steel components gives the builder greater flexibility in determining cost-effective framing solutions. Steel can also provide greater strength for loner floor spans and higher walls. Steel frames can be finished with virtually any type of material and can be easily remodeled.

crew labor time and costs can be reduced compared to wood framing.
ECYCLABLE
All steel products are 100% recyclable. The amount of steel recycled over the past decade has extended the life of nation's landfills by more than three years.

Basic Structural Symbols and abbreviations part i

Aggregate ------------------------------------agg
Bitumen --------------------------------------bit
Block work -----------------------------------blk
Brick work -----------------------------------bwk
Building -------------------------------------bldg
Column ---------------------------------------col
Concrete -------------------------------------conc
Dam proof course/membrane --------- dpc/dpm

Drawing------------------------------- drg
Elevation ---------------------------------- EL
Foundation ----------------------------- fds
Full size ------------------------- FS
Setting out point ---------------------- SOP
Setting out line -------------------------- SOL
Near face------------------------------- NF
Far face ------------------------------- FF
Each face ------------------------------- EF
Each way ------------------------------- EW
Centers --------------------------------- crs
Centre to centre -------------------- c/c

Finishing floor level ---------------------- FFL
Structural floor level ----------------------- SFL
Average ----------------------------------- av
External ---------------------------- ext
Figure ---------------------------- FIG or fig
Internal ------------------------------------ int
Holes -------------------------------- hls or HOLES
Radius -------------------------------- rad
Inside ------------------------------------ id
Outside ---------------------------- od
Sheet ---------------------------------- sh
Horizontal ---------------------------------- hor
Vertical -------------------------- vert
Not to scale -------------------------- NTS or nts
Bottom ---------------------- B or b
Top ------------------------------- T or t