An Industrial Warehouse is a storage building usually characterized as a single storey steel structure with or without mezzanine floors. The enclosure of these structures may be of brick masonry, concrete walls or GI sheet coverings. The walls are generally non-bearing but sufficiently strong enough to withstand lateral forces caused by wind or earthquake. The designing of industrial warehouse includes designing of the structural elements including principal rafter and roof truss, column and column base, purlins, sag rods, tie rods, gantry girder and bracings. A combination of standard hot-rolled sections, cold-formed sections, profiled sheets and steel rods are used for the construction of industrial steel structures.
Industrial buildings can be categorized as Pre-Engineered Building (PEB) and Conventional Steel Building (CSB) according to the design system involved in the built form. Steel is a material which has high strength per unit mass and therefore commonly used in construction of structures with large column-free space – a criterion most of the industrial structures require.
Conventional Steel Building (CSB)
CSB are low rise steel structures with roofing systems of truss with roof coverings. Various types of roof trusses can be used for these structures depending upon the pitch of the truss. For large pitch, Fink type truss can be used; for medium pitch, Pratt type truss can be used and for small pitch, Howe type truss can be used. Skylight can be provided for daylighting and to add more natural light, north light type truss can be installed. The selection criterion of roof truss also includes the slope of the roof, fabrication and transportation methods, aesthetics and climatic conditions. Several compound and combination type of economical roof trusses can be selected depending upon the utility. Standard hot-rolled sections are usually used for the truss elements along with gusset plates, in passing.
Pre-Engineered Building (PEB)
PEB involves a steel building system which is predesigned and prefabricated. As the name indicates, this concept involves pre-engineering of structural elements using a predetermined registry of building materials and manufacturing techniques that can be proficiently complied with a wide range of structural and aesthetic design requirements. The basis of the PEB concept lies in providing the section at a location only according to the requirement at that spot. The sections can be varying throughout the length according to the bending moment diagram. This leads to the utilization of non-prismatic rigid frames with slender elements. Tapered I-sections made with built-up thin plates are used to achieve this configuration. Standard hot-rolled sections, cold-formed sections and profiled roofing sheets are also employed along with the tapered sections. The use of optimal least section leads to effective saving of steel and cost reduction.
The industrial warehouse structure located at Ernakulam is a container warehouse of Vallarpadam Container Terminal. The actual structure is proposed as a Pre-Engineered Building with four spans each of 30m width, 16 bays each of 12m length and an eave height of 12m. For the comparative study, a typical PEB frame of 30 meter span is taken into account and the design is carried out by considering wind load as the critical load for the structure. CSB frame is also designed for the same span considering an economical roof truss configuration. Both the designs are then compared to find out the economical output. The designs are carried out in accordance with the Indian Standards and with the help of structural analysis and design software Staad.Pro.
Type of structure: Single Storey Industrial Structure
Area of building:22979 m2 (247343.900 sq.ft.)
Eave height: 12.00 m
Total bay length: 192.00 m
Support condition: Pinned
PEB roof slope: 5 degree
CSB roof slope: 15 degree
The loads acting on the structure includes dead load, live load, wind load, earthquake load, crane load, erection load and accidental load. The load calculation for the structure is carried out in accordance with IS: 875 – 1987 and IS: 1893 – 2000. For this structure, wind load is critical than earthquake load. Hence, load combinations of dead load, live load, crane load and wind load are incorporated for design considerations.
Dead Load: Dead load comprises of self-weight of the structure, weights of roofing, G.I. sheets, gantry girder, crane girder, purlins, sag rods, bracings and other accessories. The dead load distributed over the roof is found to be 0.438 kN/m excluding the self weight. This load is applied as uniformly distributed load over the rafter while designing the structure by PEB concept. For CSB concept, the load is applied as equivalent point load of 0.657 kN at intermediate panel points and half the value at end panel points over the roof truss.
Live Load: According to IS: 875 (Part 2) – 1987, for roof with no access provided, the live load can be taken as 0.75 kN/m2 with a reduction of 0.02 kN/m2 for every one degree above 10 degrees of roof slope. Total uniformly live load acting on the rafter of the PEB structure is found to be 4.5 kN/m. Similar to dead load, live load is also applied as point loads at panel points for CSB structure and is found to be 6.75 kN at intermediate panel points and half this value at end points.
Crane Load: Cranes are used in warehouse for lifting heavy materials from one point to another. The cranes are supported by crane bridge end trucks bearing on rails that are supported on the top of the crane beams. The crane bridge itself moves over the rails on the gantry girder which is in turn supported on the column brackets. The crane load is calculated by positioning the moving load for maximum effects of shear force and bending moment. The dead load contribution of crane system along with the gantry girder is found out to be 7 kN acting over the column brackets.
Wind Load: Wind load is calculated as per IS: 875 (Part 3) – 1987. The basic wind speed for the location of the building is found to be 39 m/s from the code. The wind load over the roof can be provided as uniformly distributed load acting outward over the PEB rafterand as point loads acting outward over the CSB panel points. For side walls, the wind load is applied as uniformly distributed loads acting inward or outward to the walls according to the wind case.
The Staad.Pro software package is a structural analysis and design software which helps in modeling, analyzing and designing the structure. The software supports standards of several countries including Indian standard. The procedure includes modeling the structure, applying properties, specifications, loads and load combinations, analyzing and designing the structure. This software is an effective and user-friendly tool for three dimensional model generation, analysis and explicit multi-material designs. A typical frame is selected for the structure and is analysed and designed according to the PEB concept as well as the CSB concept. On comparing the results of both the analysis, the following results were obtained:
Material Take off: PEB structures are lighter than CSB structures. From the software analysis, it was found that the PEB roof structure is almost 30% lighter than the CSB structure. Regarding the secondary members, light weight Z purlins are used for PEB structure whereas heavier hot-rolled sections are used for CSB structure.
Design: PEB design is rapid and efficient when compared to CSB design. Software analysis and optimization of materials is possible in PEB, increasing the quality of design. CSB design is done with fewer design aids and each project needs to develop the designs which require more time. Connection design is also lesser for PEB when measured up to CSB
Foundation: Support reaction for PEB is much lesser than CSB as per the analysis. Hence, light weight foundation can be adopted for PEB which leads to simplicity in design and reduction in cost of construction of foundation. Heavy foundation will be required for CSB structure.
Delivery of materials: For PEB, delivery is done in around six to eight weeks and for CSB it is 20 to 26 weeks.
Erection: Erection procedure is standard for all the projects and it is done free of cost by the manufacturer which results in faster and cost effective erection for PEB. Erection of CSB differs from project to project and separate labour has to be allocated, leading to 20 percent more expense than PEB.
Earthquake resistance: Low weight flexible frames of PEB offer higher resistance to earthquake loads than rigid heavy frames of CSB.
Cost: PEB costs 30% lesser than cost for CSB. Outstanding architecture can be achieved at low cost for PEB. Single sourcing and co-ordination of PEB is highly cost effective than multiple sourcing system of CSB. Building accessories are mass produced for PEB which also leads to economy.
Change of order: Due to standardized design, PEB manufacturers are able to stock large amount of elements and accessories which can be flexibly used in many types of PEB construction. Hence change of order can be fulfilled easily at any stage of construction. Cost for change of order is also lesser in this case. In case of CSB, change of order is expensive and time consuming as substitute sections are infrequently rolled by mills.
Future expansion: Single sourcing of PEB is advantageous for future expansion whereas multiple sourcing of CSB poses difficulty. Future expansion is easy and simple for PEB whereas it is most tedious and costly for CSB.
Performance: All components of the PEB system are specially designed to act together as a system for highest efficiency. PEB designs are revised regularly with respect to the actual field conditions and in accordance with various country codes, which resulted in improved standardized designs leading to high performance of the structure, as in . CSB system components are conventionally designed for a specific project and the performance depends on how the individual project is designed.
Pre-Engineered Building (PEB) methodology is versatile not only due to its quality pre-designing and prefabrication, but also due to its light weight and economical construction. The concept includes the technique of providing the best possible section according to the optimum requirement and has many advantages over the Conventional Steel Building (CSB) concept of buildings with roof truss. PEB structures can be easily designed in accordance with any country standards and have wide applications including warehouses, factories, offices, workshops, gas stations, showrooms, vehicle parking sheds, aircraft hangars, metro stations, schools, recreational buildings, indoor stadium roofs, outdoor stadium canopies, railway platform shelters, bridges, auditoriums, etc. PEB structures can also be designed as re-locatable structures.
Advantages of PEB
The concept of Pre-Engineered Buildings is extensively used for the construction single storey industrial steel buildings. PEB systems have numerous advantages including cost effectiveness, quality control, speed in construction, ease in expansion, achievement of large span, long durability, exceptional architecture, standardization of materials, standardization of design, single sourcing and co-ordination and speed in delivery. By understanding the preliminary design concepts, it is easy to achieve the design of PEB system.
Er. C. M. Meera
Researcher in pre-engineered building designs
Former lecturer at Government Engineering College, Thrissur