The construction of a pre-engineering building is a methodical step-by-step progression centred in design and function. Charu Bahri takes you through the process.
Here's a classic case of form following function. The construction of a pre-engineered building (PEB) commences with identifying the precise functions the structure will serve, so the loading of the building can be worked out and parameters influencing key structural components be charted and frozen. “Every PEB is constructed to serve a certain purpose and, hence, the user plays a key role in this process, more so as the built-up components of a PEB are manufactured under stringent quality norms in a factory leaving no room for change at a later date,” emphasises N Srinivas, Director - Technical, Lloyd Insulations (India) Ltd.
Design is the key
Companies forming part of the PEB industry typically use meticulous design software facilitating 3D modelling and detailing of the proposed structure. “At Lloyd Insulations, we prefer using proprietary software as this takes care of the standard code regulating Indian buildings and construction, which also applies to PEBs,” adds Srinivas. “Some international software options follow American standards, and thus do not serve our purpose.”
Zamil Industrial, the parent company of Zamil Steel Buildings India Pvt Ltd., has progressed from manually detailing engineering drawings to using sophisticated licensed CAD software for pre-engineered construction industry and, further, to using its in-house developed proprietary design software, including INTLEST (Intelligent Building Estimator), AGOSED (Automatic Generator of Shop and Erection Drawings), ASFAD (Advanced Structural Frame Analysis and Design) and Ez-Build. As a result, today, “100 per cent of engineering output is in digital format, facilitating the digital transportation of drawings to customers through FTP sites,” observes VenuGopal K, Marketing Manager, Zamil Steel Buildings India Pvt Ltd.
The foundation work entailed in constructing a PEB starts with the preparation of site, similar to the construction of conventional buildings. This involves two steps: civil foundation work, followed by the construction of the PEB superstructure on the foundation. Here, Mrinal Roy, Head - Marketing, Minaean Habitat India Pvt Ltd, cites plain cement concrete (PCC) as providing a strong and stable base for the foundation as it avoids direct contact of the foundation with soil and provides a flat base (vis-à-vis an undulating base) for the upcoming foundation. To a certain extent, PCC also reduces the stresses on the soil and, as the grade of concrete used in PCC usually differs from that used in the foundation, it provides the substructure in a wholesome differential to reduce the impact of ground bacteria on the foundation.
The actual foundation differs depending on the kind of soil. “We chose a relatively shallow, isolated foundation for our project involving the construction of residential PEBs in Valsad, Gujarat, as black cotton soil [sandy loam or clay-sand soil] is well suited to this method, involving the construction of a rectangular footing and a pedestal,' explains Roy. “The pedestal carries the column load to the footing, while the footing spreads the load onto the soil and the PCC. If the same project were to be taken up in a coastal area, we would probably have gone in for a pile foundation. However, the depth of the foundation would be significantly less than a conventional heavier structure. For instance, the construction of a ground-plus-two-storey brick and mortar housing structure would entail a 2.5 m pile foundation in a coastal region, whereas a PEB of the same height would only require a 1.25 m pile foundation. The steel frames of the PEB are anchored to bolts embedded into the pedestal at an appropriate depth depending on the load of the structure.”
For his part, Rohit Ranjan, General Manager - Marketing (Building Solutions), Tata BlueScope Steel Ltd, affirms, “As the unit weight of PEBs is considerably lower than conventional RCC buildings, similar functionality can be achieved with much smaller footings and foundations. This makes the structure economical by reducing the amounts of steel and concrete used for reinforcement. Anchor bolts are used to secure the steel structure to the foundation.”
Steel at the core
At the core of every PEB-whether it is a one-room kiosk or an expansive airport hangar, styled pillar-free warehouse or multi-storied building-is a steel frame manufactured to meet exacting specifications. However, the grade of steel used to make the frame differs depending on the type of structure. As Manish Garg, President, Steel Building Solutions, Everest Industries Ltd, explains, “A modular PEB, limited to being a ground-plus-one-storey structure, requires a light gauge steel frame of a lesser thickness, typically 0.9 mm to 1.2 mm but with a higher tensile strength of 550 mpa. In comparison, the steel framing for a large, pillar-free structure is made of thicker steel, usually 4 mm to 50 mm, with a lower tensile strength of 345 mpa.”
An integral component of a PEB, steel also finds expression in the roofing of single-storey and in the flooring and roofing of multi-storied PEBs. As Garg tells us, “Contrary to laying an RCC slab supported by RCC columns and beams, in a traditional construction, a multi-storey PEB mandates steel columns and beams connected by nuts and bolts, to support a steel corrugated decking sheet that is typically 1 mm thick. The steel deck becomes a permanent part of the structure, unlike the supportive shuttering used in conventional construction that is removed after the concrete has matured. The method specifies minimum reinforcement for the concrete that is poured on the form, that is, the steel deck, and which becomes the floor of the next storey.”
What's on the wall?
The extensive range of wall finishes for a steel-framed building includes granite, stone cladding, fibre cement insulated panels, conventional brickwork, structural glazing, insulated steel panels, and aluminium composite panels. The choice usually depends on the client's preference and type of the building.
Nevertheless, the PEB industry seems given to using insulated steel panels in commercial establishments, because of their strength and ability to function as an effective weather barrier. For instance, for the construction of a chain of cash 'n' carry mega-sized Walmart stores, Everest Industries is using 50-mm-thick insulated polyurethane foam puff steel panels. “The panels have twice the normal insulation and still the thickness of the walls does not exceed 2 inch, vis-à-vis a conventional 9-inch-thick brick wall,” adds Garg. “The roof of the stores also makes use of insulated steel panels to bring down the energy load of the structure.”
Lloyd Insulations is primarily engaged in the design and fabrication of specialised PEBs for industrial purposes, for which Srinivas endorses the use of metal wall and roofing panels that accommodate sway as opposed to more rigid options. In fact, Srinivas prefers steel panels even as he says that the focus in designing PEBs is always on optimising the use of steel to reduce its consumption.
Speaking of the scope of brickwork in PEBs, Venugopal notes that if the client prefers fully or partially blocked walls, some brickwork is entailed, otherwise the company adopts its own metal panel-based, wall building system that offers complete insulation as required.
High-density fibre cement boards are Minean Habitat's choice for external and internal layers of the walls of the multi-storied residential buildings it is constructing in Valsad. While the external board is 12 mm thick, the internal board is 10 mm and the space between is occupied by electrical and plumbing conduits and mineral wool insulating panels. “We opted to use high-density fibre cement boards as we wanted the exterior appearance of the buildings to resemble that of conventionally constructed structures,” reveals Roy. “So the boards will be plastered by the EIFS system [a composite wall and ceiling finish system consisting of base coat, reinforcing mesh and finish coat applied to cement board]. The cement board is attached to framing over a code-approved, water-resistive barrier, and painted over.”
High-density fibre cement boards or gypsum drywalls are equally useful as internal partitions and, as experts point out, these materials have no adverse impact on the surrounding environment. In fact, PEBs are increasingly being cited as fulfilling the three key features of green buildings: being made of reusable or recyclable components; being energy-efficient (using natural ventilation and natural lighting systems); and being environment-friendly. After all, 98 per cent of a completely pre-engineered structure is composed of aluminium or steel. Both materials are recyclable and reusable, on an as-is basis with minimum loss. PEB buildings that are designed to have adequate ventilation and lighting using natural ridge ventilators and FG/FRP skylights are also energy-efficient. What's more, PEBs have a long life and entail minimal maintenance costs. Indeed, it wouldn't be surprising if PEBs emerge as the green buildings of the future!
The Time Factor
Explaining the saving in the overall time taken to construct a PEB (vis-à-vis a conventional RCC structure), Rohit Ranjan, General Manager - Marketing (Building Solutions), Tata BlueScope Steel Ltd, says that every component is designed and fabricated in the plant. The components are only required to be assembled at site, which ensures precise and faster erection of the structure. In contrast, RCC buildings require a considerably higher level of coordination at site between various building material suppliers. Conventional buildings also require curing time for the concrete and mortar used, whereas PEBs are ready to occupy as soon as the last bolt is in place.
Industry experts point out that a medium-sized PEB intended to be used for a manufacturing setup (size 300 m long × 180 m wide × 6 m high) would take only six months to construct, whereas it would take 12 to 14 months to construct a conventional structure of the same size.
Scope of PEBs in India
According to VenuGopal K, Marketing Manager, Zamil Steel Buildings India Pvt Ltd, the concept of PEBs has progressed by leaps and bounds since it was introduced in India in the early 1990s, primarily as a rapid building solution for warehouses and kiosks. He estimates the present market potential for pre-engineered steel buildings exceeds 0.3 million tonne per annum while the current available manufacturing capacity is over 0.6 million tonne per annum. The industry is presently growing at a rate of 10 to 15 per cent, which is not surprising given the overall rule of thumb governing costs and time for construction - a PEB intended for general purpose is 30 per cent cheaper vis-à-vis a conventional RCC building, if you consider that it reduces the total construction time of the project by at least 30 to 40 per cent, not to mention the durability and salvage value of steel. The estimated use of PEBs will increase even further once heavy industries like power and steel are more lenient towards the concept. Companies like Zamil Steel have the required engineering and supply capability in these areas.