Innovations in Designing Pre-Engineered Building Structures Part -I

Innovations in Designing Pre-Engineered Building Structures Part -I

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1.0. Pre-Engineered Building Concept

Pre-Engineered Building (PEB) is a combination of-precast- & -prefabricated-structures. Pre- engineered buildings are generally low rise buildings which are ideal for offices, houses, showrooms, shop fronts etc. The application of pre engineered buildings concept to low-rise buildings is very economical and speedy. Buildings can be constructed in less than half the normal time. Although PEB systems are extensively used in industrial and many other non residential constructions worldwide, it is relatively a new concept in India. Presently, large column free area is the utmost requirement for any type of industry and with the advent of computer software’s it is now easily possible. With the improvement in technology, computer software’s have contributed immensely to the enhancement of quality of life through new researches. Pre-engineered building (PEB) is one of such revolution. “Pre-engineered buildings” are fully fabricated in the factory after designing, then transported to the site in completely knocked down (CKD) condition and all components are assembled and erected with nut-bolts, thereby reducing the time of completion.

 

Dr. A.N. Sarkar
Ex-Senior Professor (International Business) & Dean (Research), Asia-Pacific Institute of
Management, New Delhi

 

The scientific-sounding term pre-engineered buildings came into being in the 1960s. The buildings were -pre-engineered, because, like their ancestors, they relied upon standard engineering designs for a limited number of off-the-shelf configurations. Several factors made this period significant for the history of metal buildings. First, the improving technology was constantly expanding the maximum clear-span capabilities of metal buildings. The first rigid-frame buildings introduced in the late 1940s could span only 40 ft. In a few years, 50-, 60-, and 70-ft buildings became possible. By the late 1950s, rigid frames with 100-ft spans were made; ribbed metal panels became available, allowing the buildings to look different from the old tired corrugated appearance. Third, collared panels were introduced by Strand-Steel Corp. in the early 1960s, permitting some design individuality. At about the same time, continuous span cold-formed Z purlins were invented (also by Strand-Steel), the first factory-insulated panels were developed by Butler, and the first UL-approved metal roof appeared on the market. All these factors combined to produce a new metal-building boom in the late 1950s and early 1960s. As long as the purchaser could be restricted to standard designs, the buildings could be properly called pre-engineered.

Pre-Engineered Building concept involves the steel building systems which are predesigned and prefabricated. The concept of PEB is the frame geometry which matches the shape of the internal stress (bending moment) diagram thus optimizing material usage and reducing the total weight of the structure. In recent years, the introduction of Pre Engineered Building (PEB) concept in the design of structures has helped in optimizing design. The adoptability of PEB in the place of Conventional Steel Building (CSB) design concept resulted in many advantages, including economy and easier fabrication.

Furthermore, Pre-engineered Building system focuses on using pre-designed connections and pre-determined material stock to design and fabricate the building structures, thus significantly reduces the time for design, fabrication and installation. The use of steel structures is not only economical but also eco-friendly at the time when there is a threat of global warming. Here, “economical” word is stated considering time and cost. Time being the most important aspect, steel structures (Pre-fabricated) is built in very short period and one such example is Pre-Engineered Buildings (PEB). Important features of PEB include:

– Pre Engineered Building System is computer assisted, and designs to create a building for a specific use.
– The complete building system is Pre Engineered to facilitate easy production & assembly on site.
– In PEB System a superior corrosion protection of a thin coating of resin applied on the top of the Aluminium Zinc coating make Galvalume sheet.
– Galvalume steel is a steel sheet product with a highly corrosion resistance steel, aluminium zinc alloy hard dip coating.
– A clear organic acrylic resin coating is then applied & then thermally cured.
– A colour coating is given on the top surface for a bright appearance with a colour of the customer’s choice.

The typical PEB frame of the structure is as shown in the Figure 1 and Figure 2.

 

 

2.0. Technical Components of PEB

A typical assembly of a simple metal building system is shown below to illustrate the Synergy between the various building components as described below:

– Primary components
– Secondary components
– Sheeting (or) cladding
– Accessories

2.1. Primary Components

Main framing

Main framing basically includes the rigid steel frames of the building. The PEB rigid frame comprises of tapered columns and tapered rafters (the fabricated tapered sections are referred to as built-up members). The tapered sections are fabricated using the state of art technology wherein the flanges are welded to the web. Splice plates are welded to the ends of the tapered sections. The frame is erected by bolting the splice plates of connecting sections together. All rigid frames shall be welded built-up “I” sections or hot-rolled sections. The columns and the rafters may be either uniform depth or tapered. Flanges shall be connected to webs by means of a continuous fillet weld on one side. All end wall roof beams and end wall columns shall be cold-formed “C” sections, mill-rolled sections, or built-up “I” sections depending on design requirements. Plates, Stiffeners, etc.

All base plates splice plates, cap plates, and stiffeners shall be factory welded into place on the structural member’s .Built- up I section to build primary structural framing members (Columns and Rafters)

Columns

The main purpose of the columns is to transfer the vertical loads to the foundations. However apart of the horizontal actions (wind action) is also transferred through the columns. Basically in pre-engineered buildings columns are made up of I sections which are most economical than others. The width and breadth will go on increasing from bottom to top of the column. I section consists of flanges and web which are made from plates by welding.

Rafter

A rafter is one of a series of sloped structural members (beams) that extend from the ridge or hip to the wall-plate, down slope perimeter or eave, and that are designed to support the roof deck and its associated loads.

2.2. Secondary Components

Purlins and Girts

Purlins, Grits and Eave struts are secondary structural members used as support to walls and roof panels. Purloins are used on the roof; Grits are used on the walls and Eave struts are used at the intersection of the sidewall and the roof. They are supplied with minimum yield strength of 34.5KN/m. Secondary members’ act as struts that help in resisting part of the longitudinal loads that are applied on the building such as wind and earthquake loads and provide lateral bracing to the compression flanges of the main frame members for increasing frame capacity. Purloins, Grits and Eave struts are available in high grade steel conforming to ASTM 607 Grade 50 or equivalent, available in 1.5 mm, 1.75 mm. 2.0 mm, 2.25 mm, 2.5 mm and 3.0 mm thickness. They come with a pre-galvanized finish, or factory painted with a minimum of 35 microns (DFT) of corrosion protection primer. Purlins and girts shall be cold-formed “Z” sections with stiffened flanges. Flange stiffeners shall be sized to comply with the requirements of the latest edition of AISI.

Purlins and Girts shall be roll formed Z sections, 200 mm deep with 64 mm flanges shall have a16 mm stiffening lip formed at 45 to the flange. Purlins and Girts shall be cold-formed “Z” sections with stiffened flanges. Flange stiffeners shall be sized to comply with the requirements of the latest edition of AISC .Purlin and girt flanges shall be unequal in width to allow for easier nesting during erection. They shall be pre punched at the factory to provide for field bolting to the rigid frames. They shall be simple or continuous span as required by design. Connection bolts will install through the webs not flanges.

Eave Struts

Eave Struts shall be unequal flange cold-formed “C” sections. Eave struts are 200 mm deep with a 104 mm wide top flange, a 118 mm wide bottom flange; both are formed parallel to the roof slope. Each flange has a 24 mm stiffener lip.

Bracings

The Cable bracing is a primary member that ensures the stability of the building against forces in the longitudinal direction such as wind, cranes, and earthquakes. Diagonal bracing in the roof and sidewalls shall be used to remove longitudinal loads (wind, crane, etc.) from the structure. This bracing will be furnished to length and equipped with bevel washers and nuts at each end. It may consist of rods threaded each end or galvanized cable with suitable threaded end anchors.

2.3. Sheeting or Cladding

The sheets used in the construction of pre- engineered buildings are composed of the fallowing: Base metal of either Galvalume coated steel conforming to ASTM A 792 M grade 345B or aluminium conforming to ASTM B 209M . Galvalume coating is 55% Aluminium and about45% Zinc by weight. An exterior surface coating on painted sheets of 25 microns of epoxy primer with a highly durable polyester finish. An interior surface coating on painted sheets of 12 microns of epoxy primer and modified polyester or foam. The sheeting material is cold-rolled steel, high tensile 550 MPA yield stress, with hot dip metallic coating of Galvalume sheet.

2.4. Accessories

Anchor bolts

Bolts used to anchor the structural members to the concrete floor, foundation or other support. This usually refers to the bolts at the bottom of all columns. Anchor bolts are manufactured with circular steel rods having threading portion at the top for bolting and bent up at the bottom for Foundation.

Turbo ventilators

A Turbo Ventilator is a free spinning roof ventilator that works on free wind energy. When there is a difference in thermal or wind pressure between the inside and outside of the building, the air is forced to move through the opening of the Turbo Ventilator in order to maintain an equilibrium condition. The benefits of using turbo ventilators are that it improves air circulation and cuts off the suffocation. Eco friendly turbo ventilator involves no operating cost, are free from maintenance and are has trouble free operations.

Sky lights (or) wall lights

Sky lights may consists of poly carbonate sheets which is translucent sheet that allows maximum light and minimum heat. High strength translucent panels are glass fiber reinforced polyester, high strength and may be either and it provides with an estimated light transmitting capacity of 60%. High strength translucent panels match standard panel profiles, are 1/16 thick, weigh 8ounces per square foot, and are white with a granitized top surface. Insulated translucent panels are available in type 1, “R” panel and standing seam profiles only. Damper, Standard size is 3000 mm long with a throat opening of 300 mm.

Louvers

Standard Louvers shall have a 26 gauge galvanized steel frame, painted, with 26 gauge blades. Heavy Duty Louver frames shall be 18 gauges galvanized steel frame, painted, with 20 gauge blades. Both Standard and Heavy Duty louvers shall be self-framing and self flashing. They shall be equipped with adjustable or fixed blades as specified.

Fasteners

Standard fasteners shall be self drilling screws with metal and neoprene washers. All screws shall have hex heads and are zinc plated.

Gantry Girder

In mills and heavy industrial buildings such as factories and workshops, gantry girders supported by columns and carrying cranes are used to handle and transport heavy goods, equipment, etc. there are several types of canes; overhead travelling, under-slung, jib, gantry, and monorail are among the most common. A building may have one or several of these, either singly or in combinations. Hand operated overhead cranes have lifting capacities of up to 50 KN and electrically operated overhead cranes, called EOT cranes, can have capacities in the range of 10-3000 KN. The overhead travelling crane runway system consists of the following:

1. The crane, comprising the crane girder (crane frame), crab or trolley, hoist, power transmitting devices, and a cab which houses the control and operator
2. The crane rails and their attachments
3. The gantry girder
4. The gantry girder supporting columns or brackets
5. The crane stops

Steel is such a versatile material that every object we see in our daily life has used steel directly or indirectly. There is no viable substitute to steel in construction activities. Steel remains and will continue to remain logical and wide choice for construction purpose, environmentally also, as much of the steel used is recycled .Steel building offers more design and architectural flexibility for unique or conventional styling .Its strength and large clear spans mean the design is not constrained by the need for intermediate support walls. As your requirements changes over the years, you can reuse, relocate, & modify the structure. Pre-engineered Metal building concept forms an unique position in the construction industry in view of their being ideally suited to the needs of modern Engineering Industry. It would be the only solution for large industrial enclosures having thermal and acoustical features. The major advantage of metal building is the high speed of design and construction for buildings of various categories.

3.0. Advantages and Applications of Pre-Engineered Buildings

A pre-engineered metal building is a pre-fabricated building that is constructed using a structural steel framing system specifically engineered to fit your project requirements. All of the pre-determined components for the structure are factory fabricated off-site under precise factory conditions. Once the pieces are pre-cut, pre-punched, and pre-fabricated, per your designer’s specifications, they are delivered directly to the project where they are assembled on-site by a qualified construction company. Unlike the name “pre-engineered metal buildings” may lead one to believe, pre-engineered metal buildings are actually quite unique and extremely versatile. Architects and Engineers combine and select from standardized components such as framing systems, wall systems, roof systems and other special design options to create building design and structure. The following are the advantages of PEB:

a) Construction Time: Buildings are generally constructed in just 6 to 8 weeks after approval of drawings. PEB will thus reduce total construction time of the project by at least 40%. This allows faster occupancy and earlier realization of revenue. This is one of the main advantages of using Pre-engineered building.
b) Lower Cost: Because of systems approach, considerable saving is achieved in design, manufacturing and erection cost.
c) Flexibility of Expansion: These can be easily expanded in length by adding additional bays. Also expansion in width and height is possible by pre designing for future expansion.
d) Large Clear Spans: Buildings can be supplied to around 90m clear spans. This is one of the most important advantages of PEB giving column free space.
e) Quality Control: Buildings are manufactured completely in the factory under controlled conditions, and hence the quality can be assured.

f) Low Maintenance: PEB Buildings have high quality paint systems for cladding and steel to suit ambient conditions at the site, which in turn gives long durability and low maintenance coats.
g) Energy Efficient Roofing: Buildings are supplied with polyurethane insulated panels or fiberglass blankets insulation to achieve required “U” values (overall heat transfer coefficient).

The following are some of the advantages of PEB over Conventional Systems:

– PEB System is zero maintenance & superior in strength.
– It is corrosion resistance & features an attractive appearance.
– Steel arriving at job site is dry with no residual oil on the surface.
– PEB System has protection against non uniform weathering.
– Excellent resistant in transit to corrosion & storage strain.
– This system reduces energy loads on buildings due to long term bright surface that helps to retain heat reflectivity.
– It is a higher level technology & innovation & better product over conventional material.
– Applications of PEB Technologies include:

Field of Applications of PEB includes the following:

– warehouses
– factories
– workshops
– offices
– gas stations
– vehicle parking sheds
– showrooms
– aircraft hangars
– metro stations
– schools
– recreational
– indoor stadium roofs
– outdoor stadium canopies
– bridges
– railway platform shelters

4.0. Various Systems in PEB

4.1. Primary System

Primary system consists of tapered or parallel columns and tapered beams which are called as rafters. The base of the columns can be either pinned or fixed based on the load requirements. Lengths of these members are generally restricted to 12m for ease of transports. Joints are connected with high tensile bolts (Figure 3).

 

 

4.2. Secondary System

Secondary structural system consists of Purlins (roof), Girts (side cladding) and Eave struts (at eaves) stiffened with sag rods. This also includes the flange stiffeners which connects the untied flanges of the PEB primary structure to secondary system. Some of the commonly used secondary systems are Lipped C or Lipped Z purlins, MS rods and Lipped Angles. These are generally cold formed sections conforming to IS: 801.

4.3. Wind Bracing System

There are two types of wind bracing systems. The first one is Rod bracing system and the second one is portal system. Each type of system is chosen based on the design and functional requirement of the structure (Figure 4).

 

 

4.4. Roofing & Cladding System

Roofing and cladding system can be with single skin zinc and aluminum coated steel sheets, GI sheets, both of which could be bare or color coated; It could with double skin sheets with or without insulation material in between; It could be sandwich panels with steel sheets outside and PUF/PIR/Mineral Wool/Glass-wool core inside. Roofing and cladding could have skylights with Poly Carbonate/ PVC or FRP sheets.

4.5 Accessories

Accessories include: Turbo Ventilators, Ridge vents, Flashings, trims, Gutters, Down pipes, ladders, Louvers etc. (Figure 5)

 

 

4.6. Finishing

Primary structural members are generally finished with shot peening, Sand blasting, and two coats of anti corrosive primers, followed by two coats of paints to specifications. Secondary members or either painted after sand blasting or Galvanized to 275 gsm or above based on the requirement.

5.0. Technical Parameters of PEB

Pre Engineered Buildings are custom designed to meet client’s requirements. PEBs are defined for definite measurements. The produced members fit to the designed dimensions. Measurements are taken accurately for the requirements. The basic parameters that can define a PEB are:

5.1. Width or Span of Building

The centre to centre length from one end wall column to the other end wall column of a frame is considered breadth or span of the building. The width between two columns can be measured as span. The span length for different buildings varies. The design is done on span length given by customer. The basic span length starts from 10 to150 meters or above with intermediate columns. Aircraft hangars, manufacturing industries, Stadiums posses major span width. No modifications or extending span be done.

5.2. Length of Building

The length of PEB is the total length extending from one frontend to the rear end of the building.
The length of PEB can be extendable in future.

5.3. Building Height

Building height is the eave height which usually is the distance from the bottom of the main frame column base plate to the top outer point of the eave strut. When columns are recessed or elevated from finished floor, eave height is the distance from finished floor level to top of eave strut.

5.4. Roof Slope

This is the angle of the roof with respect to the horizontal. The most common roof slopes are 1/10 and 1/20 for tropical countries like India. The roof slope in snowfall locations can go up to 1/30 to 1/60. Any practical roof slope is possible as per customers’ requirement.

5.5. Design Loads

Unless otherwise specified per-engineered buildings are designed for the following minimum loads. The designed loads play a crucial role in case of PEB. The failure of the structures occurs if not properly designed for loads. The determination of the loads acting on a structure is a complex problem. The nature of the loads varies essentially with the architectural design, the materials, and the location of the structure. Loading conditions on the same structure may change from time to time, or may change rapidly with time. Loads are usually classified into two broad groups as dead loads and live loads. Dead loads (DL) are essentially constant during the life of the structure and normally consist of the weight of the structural elements. On the other hand, live loads (LL) usually vary greatly. The weight of occupants, snow and vehicles, and the forces induced by wind or earthquakes are examples of live loads. The magnitudes of these loads are not known with great accuracy and the design values must depend on the intended use of the structure.

5.5.1. Dead Load

The structure first of all carries the dead load, which includes its own weight, the weight of any permanent non-structural partitions, built-in cupboards, floor surfacing materials and other finishes. It can be worked out precisely from the known weights of the materials and the dimensions on the working drawings.

5.5.2. Live Load

All the movable objects in a building such as people, desks, cupboards and filing cabinets produce an imposed load on the structure. This loading may come and go with the result that its intensity will vary considerably. At one moment a room may be empty, yet at another packed with people. Imagine the `extra’ live load at a lively party.

5.5.3. Wind Loads

Wind has become a very important load in recent years due to the extensive use of lighter materials and more efficient building techniques. A building built with heavy masonry, timber tiled roof may not be affected by the wind load, but on the other hand the structural design of a modern light gauge steel framed building is dominated by the wind load, which will affect its strength, stability and serviceability. The wind acts both on the main structure and on the individual cladding units. The structure has to be braced to resist the horizontal load and anchored to the ground to prevent the whole building from being blown away, if the dead weight of the building is not sufficient to hold it down. The cladding has to be securely fixed to prevent the wind from ripping it away from the structure.

5.5.4. Roof Load

Live loads produced by maintenance activities, rain, erection activities, and other movable or moving loads by not including wind, snow, seismic, crane, or dead loads.

5.5.5. Roof Snow Load

Gravity load induced by the forces of wind blowing from any horizontal direction.

5.5.6. Auxiliary Loads

Dynamic loads induced by cranes, conveyers, or other material handling systems.

5.5.7. Seismic Loads

They are the Horizontal loads acting in any direction structural systems due to action of an earthquake.

5.5.8. Floor Live Loads

Loads induced on a floor system by occupants of a building and their furniture, equipment, etc

5.6. Bay Spacing

The distance between the two adjacent frames of a building is called as a Bay spacing. The spacing between two frames is a bay. End Bay length is the distance from outside of the outer flange of end wall columns of centre line of the first interior frame columns. Interior bay length is the distance between the centre lines of two adjacent interior main frames Columns. The most economical bay spacing is 7.5m to 8.0m. However bay length up to 10m is possible.

5.7. Types of Frame

A frame is a combination of Columns and inclined beams (rafters). There is various types of frames.

5.7.1. Clear Span (Cs)

It’s the span length between two columns without any obstruction. It has split Beams with ridge line at the peak or centre of the building. The maximum practical width or span is up to 90 meters, but it can also be extended up to 150 meters in case of Aircraft Hangars.

5.7.2. Arched Clear Span

The column is an RF column while the Rafter is curved. It has no ridge line and peak. The curved roof rafter is used in for aesthetic look. The maximum practical is up to 90meters, but can be extended to 120 meters.

5.8. Multi Span (Ms1)

The Multi spans (MS1) are those which have more than 1 span. The intermediate column is used for the clear span in which width of each span is called width module.

5.8.1. Arched Multi Span (Ams1)

Arched multi span has RF column and a curved Rafter with one intermediate column. It has width module for the entire span. The multi spans can be extended up to AMS1, AMS2 and AMS3 etc.

5.8.2. Multi Span 2 (Ms2)

The Multi Span (MS2) has more than one intermediate span. It has three width modules with one ridge line.

5.9. Single Slope

It has two columns with different heights having Roof sloping on both the column.

5.10. Multi Gable

Multi gable has two or more spans where no intermediate columns are used. The columns are added to the extended width and columns are not placed at the ridge lines.

5.11. Roof Systems

It has straight columns with Roof having supports are not by TPCA.

5.12. Lean to Slopes

Lean to slopes is used extremely for an extending to a building on either side with short span. The rafters rest on column designed for lean to on one side and rests on the main column of the building.

5.13. Canopy

Canopies are used in case of open ends where there is an easy access. There are columns in straight path having roof extended to a large length.

The Part II will be continued in the next edition

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