10 Techno-Economic Advantages of Steel Tubes in Structural Applications (Buildings and Bridges)

The 10 techno-economic advantages of using steel tubes (hollow sections) in structural application when compared with traditional stand alone either open-section steel or concrete structures are given below near the end of article.

Design is an interactive process between the functional and architectural requirements and the strength and fabrication aspects. In a good design, all these aspects have to be considered in a balanced way. But before we come into that, see the following images where steel tubular sections were used in structural applications for hallmark buildings and bridge construction. Simply google to find more information about these structural engineering marvels where steel tubes were extensively used.

Centre Pompidou, Paris

Centre Pompidou, Paris: Rare Architectural marvel with all visible services on exterior face marked with distunguished coloures (mechanical, electrical, water suppy, plumbing, and steel superstructure with structural tubes). There are four basement floors which are of rcc construction.

Firth of Forth Bridge

Firth of Forth Bridge in Scotland: (1890) with a free span of 521 m, shown above. This bridge has been built up from tubular members made of rolled plates which have been riveted together, because other fabrication methods were at that time not available for these sizes.

Wushan Yangtze River Bridge, China

Wushan Yangtze River Bridge, China: Completed in 2005, span 460m, height 130m, total length 610m. CFST Arch bridge and ranked in top 10 longest arch span bridge. There are more that 450 similar CFST arch bridges in China who dominates the world in arch bridge engineering.

Fleet Place House, London

Fleet Place House, London: Concrete filled steel tubes(CFST) used for vertical load bearing memebers. Aesthetically pleasing because of slender CFST colums carrying massive loads.

Grandstand Roof, Olympic Stadium, Berlin

Kansai Airport Terminal, Japan

Mumbai Domestic Airport Terminal, India

Areas of Structural Application

• Architectural: Shopping Malls, Space Frames, Atriums, Long Span Truss Floor, Retail Store
• Infrastructural: Airport Terminal Buildings, CFST Arch Bridges, Road Signage Structure, Grandstand Roof over Stadia, Metro Station Shed, Bus Stand Terminals, Spheroidal Domes
• Industrial: Factories and warehouses, Trusses Columns Purlins, Material Storage Racks, Conveyor Galleries and Trestles, Pipe Racks, Drilling Rigs, Marine Structures & Platforms
• Others: Automobile Chesis, Transmission Shafting, Green House Structure, Amesement parks and Playground Equipment (Roller Coasters), Scaffolding, Truck and Bus Body members, Diagrid System in High-Rise Structures

Reliable Supplier in India and Europe

Tata Structura: Grade Yst 310 MPa, Circular Sections to IS:1161, Rectangular & Square to IS:4923

Tata Celcius Europe: Grade Yst 355 MPa conforming to EN10210 Part 1: S355J2H

Advantages of Steel Tubes as Structural Member

In a good design not only does the strength have to be considered, but also many other aspects, such as material selection, fabrication including welding and inspection, corossion protection, fire protection, erection, in service inspection and maintenance. Extensive studies and experiments in last 40 years show the excellent properties of the tubular shape with regard to loading in compression, torsion and bending in all directions. These excellent properties are combined with an attractive shape for architectural applications (Figures above). Furthermore, the closed shape without sharp corners reduces the area to be protected, extends the corrosion protection and fire protection life for CFST sections

Followings are the 10 major techno-economic advantages of structural steel tubes over conventional open steel sections and RCC structures:

1. Low Drag Coefficient: Hollow sections, especially circular hollow sections, have a striking advantage for use in structures exposed to fluid currents, i.e. air or water.. Suitable for tall buildings, long span arch bridge, offshore marine structures
2. Enhanced Axial Compression: High strength to weight ratio and small l/r(min) ratio result in upto 30% saving in steel compared to I section columns of same cross sectional area. Good in bi-axial bending of moderate magnitude. Not much benefit in uni-axial bending like I sections.
3. Less cost for corrosion protection: Structures made of hollow sections offer advantages with regard to corrosion protection. Hollow sections have rounded corners which result in a better protection than sections with sharp corners and approx 40 to 50% less perimeter/surface area for coatings. This is especially true for the joints in circular hollow sections where there is a smooth transition from one section to another. This better protection increases the protection period of coatings against corrosion adding years of life to structure.
4. High Torsional Resistance: Architect's favourite section because hollow tube sections are having tortional rigidity almost 150 to 200 times compared to open I section of same cross section. Modern high rise buildings are based on the phenomena of creating "Iconic" buildings which are distinguished by their form, shape and scale; mostly possible due to tubular sections enormous torsional strength and compressive capacities. Some marvel examples are: Swiss Re, London; Westhafen Tower, Frankfurt; Tornado Tower, Doha; Mode Gekuen Spiral Tower, Nagoya; Hearst Tower, New York.
5. Less cost for Fire Protection: One of the modern methods for fire protection of buildings is to use water-filled hollow section columns. The columns are interconnected with a water storage tank. Under fire conditions, the water circulates by convection, keeping the steel temperature below the critical value of 450°C. This system has economical advantages when applied to buildings with more than about 8 storeys. If the water flow is adequate, the resulting fire resistance time is virtually unlimited. In order to prevent freezing, potassium carbonate (K2CO3) is added to the water. Potassium nitrate is used as an inhibitor against corrosion.
6. Long Span Structures: Sometimes hollow section chords of lattice girder bridges are used for conveying fluids (pipe bridge). Sometimes in buildings the rain water downpipes go through the hollow section columns or in other cases electrical wiring is located in the columns. The internal space can also be used for prestressing a hollow section.
7. Use of Inter Void in Tubes: The inner voids of hollow sections are sometimes used for air and water circulation for heating and ventilation of buildings. Many examples in offices and schools in European countries show the excellent combination of the strength function of hollow section columns with the integration of the heating or ventilation system. This system offers maximization of floor area through elimination of heat exchangers, a uniform provision of warmth and a combined protection against fire.
8. Aesthetics: A rational use of hollow sections leads in general to structures which are cleaner and more spacious. Hollow sections can provide slender aesthetic columns, with variable section properties but flush external dimensions. Due to their torsional rigidity, hollow sections have specific advantages in folded structures, Virendeel-type girders, etc. Lattice construction, which is often made of hollow sections directly connected to one another without any stiffener or gusset plate, is often preferred by architects for structures with visible steel elements. However, it is difficult to express aesthetic features in economic comparisons. Sometimes hollow sections are used only because of aesthetic appeal, whilst at other times appearance is less important.
9. Less Connection Materials: Though challenging, but connections are more direct on contact surfaces with precise profile cutting before welding of truss members. Less use of connecting plates and standard connections are developed by CIDECT
10. Concrete Filled Steel Tube (CFST) as Axial Member: If the commonly-available  tube wall thicknesses are not sufficient to meet the required load bearing resistance,the hollow tube section can be filled with structural concrete. For example, it may be preferable in buildings to have the same external dimensions for the columns on every floor. At the top floor, the smallest tube wall thickness can be chosen, and this wall thickness can be increased with increasing load for lower floors. If the hollow section with the largest available tube wall thickness is not sufficient for the ground floor, the hollow section can be filled with concrete to increase the load bearing resistance. Useful in high rise buildings and Arch Bridges where compression dominates. CFST has many advantages compared with ordinary steel (Yst 250MPa) or RCC system: 

• Better interaction between steel tube and concrete because of circumferential bond, friction, concrete confinement, and high tensile grade used for steel. The axial capacity increased by about two times the sum of capacities of ordinary steel tube and RCC column added together.
• Drying shrinkage and creep of confined concrete is much smaller than that of ordinary RCC
• Forms are omitted and concrete casting with reinforcements is done by pumping/tremie method
• Construction site remains clean with no westage, less labour, no dust pollution
• Better cost performance is obtained with CFST structure for more column free space 
• Concrete filling of hollow sections contributes not only to an increase in load bearing resistance, but it also improves the fire resistance duration. The extensive test projects and studies carried out have shown that reinforced concrete-filled hollow section columns without any external fire protection like plaster, vermiculite panels or intumescent paint, can attain a fire life of even 2 hours depending on the cross-section ratio of the steel and concrete, reinforcement percentage (>2%) of the concrete and the applied load. Just concrete fill gives 30 minute fire protection.

Recommendation

The above advantages often lead to a saving in cost over stand alone ordinary steel or RCC structural systems. Studies found that CFST solution is techno-commercially supirior to stand alone steel or RCC sysstem. Hope this information would serve as an aid to structural engineers and decision makers to explore and utilise steel tubes and CFST more and more in structural applications.

Disclaimer

The contents of this article is for informational purpose only and proper professional guidence must be sought before implementation of design and construction of steel tubes in structural/composite applications.