Innovations in Designing Pre-Engineered Building Structures Part -II

Innovations in Designing Pre-Engineered Building Structures Part -II

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6.0. Metal Building Systems of PEB

The metal building systems industry was born to fulfill an urgent need-to house troops during World War II. The most famous example, the Quonset hut, was a half-round structure that could be easily constructed by unskilled labor using only hand tools and then disassembled and transported to a new location. After the war, the first standard metal building kits were mostly utilized for industrial and agricultural purposes. By the late 1960s, the industry had gained a reputation for its economy and speed of production and delivery and was used for wider applications. The systems, however, lacked any flexibility in design. So as demand grew from architects, engineers, and building owners for greater systems flexibility, manufacturers of metal building systems retooled their factories offering more systems options. Today, metal building systems dominate the one-to two-story nonresidential building market, comprising nearly 70 percent of the market.

 

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

The great opportunity for change, growth, and development paralleled the rise of computer technology and the use of CAD systems in architecture and engineering. In effect, the term “pre-engineered metal buildings” has become a misnomer. The standardization inherent in systems-based buildings makes it a natural application for computer-aided design and engineering. There are, in fact, no standard buildings anymore. Each is custom designed as a unique project.

Greater flexibility than ever before is now possible. Designers can select straight or tapered columns, variable or odd-sized bays or modules, single-slope or double-slope buildings, with centered or off-center ridges. These systems can be integrated with a wide variety of wall materials, such as block, brick, tilt-up concrete, curtain wall, or metal. Flat, conventional roof design has given way in popularity to a watershed design, in which a sloped roof element is a key part of the overall image of the structure. Size can vary tremendously: the average is 10,000 to 20,000 square feet, but can range to more than one million square feet.

Critical to the engineering/design process is determining the safe loads which the structure must be able to support such as dead load, roof live load, roof snow load, wind load, seismic load, and auxiliary load. The architect should consult local building code requirements. The MBMA publishes the Low-Rise Building Systems Manual that defines and recommends minimum design loads for metal building systems. In addition, formulas for calculating wind pressure and suction for various building geometries are given. Values of lateral, tractive, and impact loads for overhead cranes are also listed.

While computer technology has streamlined the design and engineering process, real economies still come through the fabrication of structural elements. Completion of a metal building systems structure is possible in roughly two-thirds the time of a conventional building. Since all elements are factory fabricated, precut and pre-punched under controlled factory conditions, where efficiency and quality can be exercised to a much greater degree than in the field, waste material is minimized. And it is generally easier to predict erection costs and erection time than with conventional construction. Typically, the buildings are shipped within six to eight weeks from the date an order is received.

The design professional reaps the benefits of single source responsibility, since the metal building systems company takes responsibility not only for engineering and fabrication, but also the performance of the entire building shell. It is important for the design professional to ensure that the design criteria, particularly lateral drift and deflection of the primary frame and/or secondary supports, are consistent with the chosen wall materials. Manufacturers can provide a “letter of certification” with each building that assures the metal building system has been designed in accordance to specified state and local codes and/or specified project design requirements. A variety of warranties are offered on metal building systems. These may include finish warranties on roof and wall systems and weather tight warranties on roof systems.

Pre-Engineered Steel Buildings use a combination of built-up sections, hot rolled sections and cold formed elements which provide the basic steel frame work with a choice of single skin sheeting with added insulation or insulated sandwich panels for roofing and wall cladding. The concept is designed to provide a complete building envelope system which is air tight, energy efficient, optimum in weight and cost and, above all, designed to fit user requirement like a well fitted glove.

These Pre-Engineered Steel Buildings can be fitted with different structural accessories including mezzanine floors, canopies, fascias, interior partitions, crane systems etc. The building is made water-tight by use of special mastic beads, filler strips and trims.  This is a very versatile building system and can be finished internally to serve any required function and accessorized externally to achieve attractive and distinctive architectural styles.  It is most suitable for any low-rise building and offers numerous benefits over conventional buildings.

Pre-engineered buildings are generally low rise buildings; however the maximum eave heights can go upto 25 to 30 metres.  Low rise buildings are ideal for offices, houses, showrooms, shop fronts etc. The application of pre-engineered concept to low rise buildings is very economical and speedy.  Buildings can be constructed in less than half the normal time especially when complimented with other engineered sub-systems.

The most common and economical type of low-rise building is a building with ground floor and two intermediate floors plus roof. The roof of a low rise building may be flat or sloped. Intermediate floors of low rise buildings are made of mezzanine systems.  Single-storeyed houses for living take minimum time for construction and can be built in any type of geographic location like extreme cold hilly areas, high rain prone areas, plain land, extreme hot climatic zones etc.

There are basically nine major components in a pre-engineered building such as:

– Main framing or vertical columns

– End wall framing

– Purlins, girts and eave struts

– Sheeting and insulation or prefab panels

– Crane system

– Mezzanine system

– Bracing system

– Paints and finishes

– Miscellaneous services

6.1. Metal Building Components

Metal building systems offer a completely integrated set of interdependent elements and assemblies, which, taken together, form the total building. Included are primary and secondary framing elements, covering components, and accessories. These building block parts come in many different configurations, as illustrated below.

Roof Systems 

Roof systems are made up of two components: purlins and roof panels.

Purlins: Two types of purl ins support the weight of the roof and any applied loads-cold-formed steel (either “Z” or “e” sections) and open web joist. The purlins work to transfer these loads to the primary structural system. The “Z” or “e” sections can either be simple spans, or more commonly used as continuous beams between frames. They can be used on spans of up to 30 feet. For spans greater than 30 feet, open web joists or deep “Z” sections may be used for puriins. Open web joists are utilized as simple spans.

Roof panels: Panels are fabricated from light gauge steel as a lap seam roof or a standing seam roof system. The panel of a lap seam roof is typically 1 inch to 1-112 inches deep, 26 or 24 gauge thick, and connected together by lapping. A sealant is installed between the panels at the sidelaps and end laps and fasteners are used to secure them. The selection of panel depth and thickness is affected by the roof load, purlins spacing, and insurance considerations.

Standing seam panels: The seam between two panels is made in the field with a device that produces a cold formed weather-tight joint at the side-lap of each panel. The panel is attached to the purlins with a clip concealed inside the seam, which allows a secure attachment while permitting thermal expansion or contraction. Since a majority of the through-thereof fasteners have been eliminated, a continuous single skin membrane results. Thermal spacer blocks can be placed between the panels and purlins in order to provide a consistent thermal barrier. The metal standing seam roof can be used to renovate and restore old, leaking flat roofs to better than original condition by adding a slope-in other words, turns a flat roof into a water-shedding one. Various finishes and colors are available. The products can be used in either a structural. low slope application or a highly visible architectural metal roof situation.

Bracing Requirements 

To complete the roof system, metal building manufacturers typically provide bracing for the purlins. Depending on the design assumption used, different types and arrangements of bracing may be utilized. Bracing systems include “straps,” channels, or sag angles. All of these systems span from purl in to purl in. For a standing seam roof system, the amount of lateral support provided by the panels to the purlins has to be determined through testing, if it is to be included in the design. That is why a standing seam roof system from one manufacturer may have more visible bracing than another.

Special Design Considerations

Frame shape and peak location. Both can be important components of the design. The majority of buildings supplied today are the traditional rectangular shape, yet many other shapes are possible-L, 1’s, U’s, and octagons. The majority of metal buildings supplied are single slopes or gable buildings with the ridge on center of the frame. The peak can be moved off center, however, to almost any location on the frame. A single slope building can be positioned with the high or low side facing the front, depending upon drainage or architectural requirements. When frames with multiple ridges or with a valley instead of a ridge are specified, an interior drainage system will be designed by the metal building manufacturer.

Bay sizes. While in the past only two or three bay sizes were commonly offered, any bay size is now possible. When the building has interior columns, the interior columns can be of different spacing patterns.

Column shapes. Tapered perimeter columns are often the most economical choice, but straight columns can be used at the perimeters. The perimeter columns are normally fabricated H-shape and can be parallel or tapered. Interior columns can be supplied in many different shapes, including hollow structural shapes, hot rolled H-shapes, and fabricated H-shapes.

Column heights. Heights can vary to provide a step in roof elevation or in floor elevation.

Accessories. A variety of structural and nonstructural accessories can be included: insulation, gutters, downspouts, roof ventilators, roof openings, interior liner panels, wall vents, wall openings, windows, pedestrian doors, overhead doors, canopies, skylights, fascia, and trim. These elements can add aesthetic variety.

Expandability. Metal building systems are easily expanded. This usually involves the removal of an end or sidewall, the erection of additional structural frames, and the addition of matching wall and roof coverings. Manufacturers routinely perform assessments of adding on to a metal building system, including structural and roof drainage analysis.

6.2. Typical Pre-Engineered Steel Structures 

The Pre-engineered steel structures shown in Figure 6 are design for resistant to moisture, adverse weather conditions, earthquakes, termites and fire that provide you with lifelong durability, safety and very low cost-maintenance. Pre-engineered steel building is very simple and economical with the necessary Architectural, Engineering and Construction with pre-engineered steel buildings. Assuming that a metal building system is selected for the project at hand, the next milestone is choosing among the available types of pre-engineered primary framing. Proper selection of primary framing, the backbone of metal buildings, goes a long way toward a successful implementation of the design steps to follow. Some of the factors that influence the choice of main framing include:

– Dimensions of the building: width, length, and height

– Roof slope

– Required column-free clear spans

– Occupancy of the building and acceptability of exposed steel columns

– Proposed roof and wall materials

 

Figure 6: Pre-Engineered Building with Steel Frame Structure

The following inherent quality of the PEB themselves is a huge contributory factor for getting favourable response and desired results:

– Reduced construction time.

– Flexibility of Expansion.

– Large Clear Spans

– Low maintenance

– Energy Efficient Roofing and Wall systems

– Architectural Versatility

6.3. PEB Steel Construction and Sustainability Aspects

The preoccupations of the sustainable development are of particular concern for the construction sector, which is responsible for 25% of greenhouse gas emissions and for 40% of the primary energy consumption. They constitute a major stake for all the involved professionals. Steel is an excellent solution for conserving raw materials, thanks to its recyclability. It can be infinitely recycled without losing its properties and strength. Today, the production of steel consists of 50% recycled metal, reducing the need for ore; for certain products intended for construction, this rate can reach up to 98%. This re-use of the material is in particular made possible by its magnetic properties facilitating the sorting the control of energy and the reduction of carbon dioxide emissions during production have led to vast improvements in developing new steel materials and taking into account life cycle of materials and products.

Steel is the mainspring in our quest to improve the quality of our buildings and their impact on our living environment. General principles are established according to three main considerations: ecological, economical and socio-cultural, although the methods for determining their impact have not yet been agreed on an international scale. The sustainability of buildings concerns a range of issues related to choice of materials, construction process, occupation and end of life. These issues may be expressed in terms of specific criteria, such as energy materials use, waste minimization, reduction of primary energy use (and CO2 emissions), pollution and other global impacts.

7.0. Structural Analysis and Design using STAAD Pro

STAAD Pro may be utilized for analyzing and designing practically for the Pre-Engineered Buildings. STAAD Pro can be used to advantage to implements the Bending Moment, Axial Forces, Shear Forces, Torsion, and Beam Stresses.

 

Figure 7: PEB Steel Typical Projects

Static Analysis Features of STAAD Pro include:

– 2D/3D Analysis based on state-of-the-art Matrix method to handle extremely large job.

– Rafter, Column, Tapered Sections, Rigid Frames, Purlins, Eave Hight.

– Full/Partial Moment Releases.

– Member Offset Specification.

– Fixed, Pinned and Spring Supports with Releases; also with inclined Supports.

– Automatic Spring Support Generator.

– Linear, P-Delta Analysis, Non-Linear Analysis with automatic load and stiffness correction. Multiple Analyses within same run.

– Active/Inactive Members for Load-Dependent structures.

– Tension-only members and compression-only members, Multi-linear spring supports.

– CIMSTEEL Interface.

Dynamic / Seismic Analysis

– Mass modeling, Extraction of Frequency and Mode shapes.

– Response Spectrum, Time History Analysis.

– Modal Damping Ratio for Individual Models.

– Harmonic Load Generator.

– Combination of Dynamic forces with Static loading for subsequent design.

Secondary Analysis

– Forces and Displacements at sections between nodes.

– Maximum and Minimum force Envelopes.

– Load Types and Load Generation:

– Loading for Joints, Members/Elements including Concentrated, Uniform, Linear, Trapezoidal, Temperature, Strain, Support Displacement, Prestressed and Fixed-end Loads.

– Global, Local and Projected Loading Directions.

– Uniform or varying Element Pressure Loading on entire or selected portion of elements.

– Floor/Area Load converts load-per-area to member loads based on one-way or two-way actions.

– Automatic Moving Load Generation as per standard AASHTO or user-defined loading.

– UBC 1997.AIJ/IS 1893/Cypriot Seismic Load Generation.

– Automatic Wind Load Generation.

 

Figure 8: Detailing for Haunch

Planning of the PEB buildings (low rise metal buildings and arranging different building components is a very important step for the designer before proceeding with the design of each component. The Following building configurations are significantly affecting the building Stability and Cost:

1. Main Frame configuration (orientation, type, roof slope, eave height)

2. Roof purlins spacing

3. Wall girts (connection & spacing)

4. End wall system

5. Expansion joints

6. Bay spacing

7. Bracing systems arrangement

8. Mezzanine floor beams/columns (orientation & spacing)

9. Crane systems

Some of the 3Pre-Engineered Building are outlinedas: Columns, Rafters, Frames, (Hot Rolled/Built up Sections) Secondary Members – Bracings, Purlins, Girts, (Cold Formed Sections) Roof & Wall Cladding –Roofing, Cladding, Sand witched Panels, Flashings (Ridge, Gutter etc,) (Figure 8).

Main Frame Configuration 

The various types of Main frame for the basic supporting component in the PEB systems; main frames provide the vertical support for the whole building plus providing the lateral stability for the building in its direction while lateral stability in the other direction is usually achieved by a bracing system. The width of the building is defined as the out-to-out dimensions of girts/eave struts and these extents define the side wall steel lines. Eave height is the height measured from bottom of the column base plate to top of the eave strut. Rigid frame members are tapered using built-up sections following the shape of the bending moment diagram. Columns with fixed base are straight. Also the interior columns are always maintained straight.

 

Figue 9: Pre Engineered Building Types

Main Frame Orientation

Building should be oriented in such a way that the length is greater than the width. This will result in more number of lighter frames rather than less number of heavy frames, this also will reduce the wind bracing forces results in lighter bracing systems.

 

Figure 10.Steel Building Frame

 

Main Frame Types

There are several types of main frames used in PEB buildings, The choice of the type of main frame to be used is dependent on:-

1. Total width of the building.

2. The permitted spacing between columns in the transversal direction according to customer requirements and the function of the building.

3. The existence of sub structure (RC or masonry)

4. The architectural requirements of the customer specially the shape of the gable.

5. The type of rain drainage (internal drainage availability).

6. Any customer special requirements.

 

Figure 14: Design of Pre-Engineered Tubular Structures
Figure 14: Long-Span Steel Frame Pre-Fabricated Roofs

Building Type: Primary Framing System

– Depth built-up -Il section, with the large depths in areas of higher stress according to the Bending Moment Diagram (Figure 9)

– Secondary structural members (roof purlins, eave struts and wall girts) which are light weight cold-formed -Zl and -Cl shaped members or open web steel joists;

• Roll formed profiled sheeting (roof and wall panels).

The entire primary framing members and secondary structural members are pre-sheared, pre-punched, pre-drilled, pre-welded and pre-formed in the factories before shipping to site for erection are shown in Fig: 3. Quality of building part is assured as buildings are manufactured completely in the factory under controlled conditions. At the job site, the pre-fabricated components are then fixed and jointed with bolt connections. Saving of material on low stress area of the primary framing members makes Pre-engineered Buildings more economical than conventional steel buildings especially for low rise buildings spanning up to 60.0 meters with eave heights up to 30.0 meters. 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.

Pre-engineered Buildings can be fitted with different structural accessories including mezzanine floors, crane runway beams, roof platform, catwalk and aesthetic features such as canopies, fascia’s, interior partitions etc. The buildings are made water proof by use of standing seam roof system, roof drainage components and trims. This is a very versatile building system and can be furnished internally to serve any functions, and accessorized externally to achieve unique and aesthetically pleasing architecture designs, making it ideal for application such as factories, warehouses, workshops, showrooms, supermarket etc.

The following are some of the representative and illustrative Pre-Engineered Steel-Frame, PEB Products and Building Structures (Figures 10-14).

8.0. Designing  of Pre Engineered Buildings (PEB)

The main framing of PEB systems is analyzed by the stiffness matrix method. The design is based on allowable stress design (ASD) as per the American institute of Steel Construction specification or the IS 800. The design program provides an economic and efficient design of the main frames and allows the user to utilize the program in different modes to produce the frame design geometry and loading and the desired load combinations as specified by the building code opted by the user. The program operates through the maximum number of cycles specified to arrive at an acceptable design. The program uses the stiffness matrix method to arrive at an acceptable design. The program uses the stiffness matrix method to arrive at the solution of displacements and forces. The strain energy method is adopted to calculate the fixed end moments, stiffness and carry over factors. Numerical integration is used.

Design Cycle

The design cycle consists of the following steps:

1. Set up section sizes and brace locations based on the geometry and loading specified for the frame design.

2. Calculate moment, shear, and axial force at each analysis point for each load combination.

3. Compute allowable shear, allowable axial and allowable bending stress in compression and tension at each analysis point.

4. Compute the corresponding stress ratios for shear, axial and bending based on the actual and allowable stresses and calculate the combined stress ratios.

5. Design the optimum splice location and check to see whether the predicted sizes confirm to manufacturing constraints.

6. Using the web optimization mode, arrive at the optimum web depths for the next cycle and update the member data file.

7. At the end of all design cycles, an analysis is run to achieve flange brace optimization.

Frame Geometry

The programme relating to Frame Geometry has the capability to handle different types of frame geometry as follows:

– Frames of different types, viz. rigid frames, frames with multiple internal columns, single slope frames, lean to frames etc.

– Frames with varying spans, varying heights and varying slopes etc.

– Frames with different types of supports viz. pinned supports, fixed supports, sinking supports, supports with some degrees of freedom released.

– Unsymmetrical frames with off centric, unequal modules, varying slopes etc.

– User specified purlin and girt spacing and flange brace location.

Frame design can handle different types of loadings as described below:

– All the building dead loads due to sheeting, purlins, etc. and the self weight of the frame.

– Imposed live load on the frame with tributary reductions as well.

– Collateral load such as false ceiling, light fixtures, AC ducting loads, sprinkler systems and many other suspended loads of similar nature.

Wind loads input such as basic wind speed or basic wind pressure that will be converted to deign wind pressure as per the building code specified by the user and shall be applied to the different members of the building according to the coefficients mentioned in the codes prescribed by the user. For the Pre Engineered Buildings standard building codes like MBMA, UBC, ANSI, IS: 875 parts 3 etc are used for this purpose. Crane and non crane loading can be specified by the user and the program has the capability to handle these special loads and combine them with the other loads as required. Seismic loads corresponding to the different zone categories of various international codes can also be defined and combined with other load cases as required. Temperature loads can also be specified in the form of different differential temperature value on centigrade and specifying the appropriate coefficient for the thermal expansion. Load combinations with appropriate load factors can be specified by the user as desired.

Design Codes used include:

Following are the main design codes generally used:

– AISC: American institute of steel construction manual

– AISI: American iron and steel institute specifications

– MBMA: Metal building manufacturer’s code

– ANSI: American national standards institute specifications

– ASCE: American society of civil engineers

– UBC: Uniform building code

– IS: Indian standards

– Design Criteria

DESIGN METHOD: Allowable stress design method is used as per the AISC specifications.

DEFLECTIONS: Unless otherwise specified, the deflections will go to MBMA, AISC criteria and standard industry practices.

PRIMARY FRAMING: Moment resisting frames with pinned or fixed bases.

SECONDARY FRAMING: Cold formed Z sections or C sections for purlins or girts designed as continuous beams spanning over rafters and columns with laps.

LONGITUDANAL STABILITY: Wind load on building end walls is transferred through roof purlins to braced bays and carried to the foundations through diagonal bracing.

DESIGN SOFTWARE

The latest software that is used for design is STAAD 2007.

DESIGN PROCESS

The frame data is assembled based on number of frame members, number of joints, number of degrees of freedom, the conditions of restraint and the elastic properties of the members. Based on this, the data is stored and member section properties are computed. The overall joint stiffness matrix is obtained based on the above frame data by summation of individual stiffness matrices considering all possible displacements. The load vector is then generated based on the loading data and the unknown displacements are obtained by inverting the overall joint stiffness matrix and multiplying with the load vector.

Structural Framing

For the purpose of structural analysis and design, industrial buildings are classified as:

– Braced frames

– Unbraced frames

 

In braced buildings, the trusses rest on columns with hinge type of connections and the stability is provided by the following bracings in the three mutually perpendicular planes:

(a) Bracings in vertical plane in the end bays in the longitudinal direction

(b) Bracings in horizontal plane at bottom chord level of roof truss

(c) Bracings in the plane of upper chords of roof truss

(d) Bracings in vertical plane in the end cross sections usually at the gable end.

The function of bracing is to transfer horizontal loads from the frames (such as those due to wind or earthquake or horizontal surge due to acceleration and breaking of traveling cranes) to the foundation. The longitudinal bracing on each longitudinal ends provide stability in the longitudinal direction. The gable bracings provide stability in the lateral direction. The tie bracings at the bottom chord level transfer lateral loads (due to wind or earthquake) of trusses to the end gable bracings. Similarly stability in the horizontal plane is provided by

– Rafter bracing in the end bays which provide stability to trusses in their planes

– A bracing system in the level of bottom chords of trusses, which provide stability to the bottom chords of the trusses.

Braced frames are efficient in resisting the loads and do not sway. However, the braces introduce obstructions in some bays and may cause higher forces or uplift forces in some places. Hence unbraced frames are now-a-days preferred. Such unbraced frames are often designed, prefabricated, and supplied and erected at site by firms and are called pre-engineered buildings or metal building systems. Many companies are there India through whom we can order these buildings. For illustration few favourite Pre-Engineered Metal-Frame Pre-Engineered construction projects are cited below (Figures 15 and 16).

The Barlow, located in the heart of downtown Sebastopol, showcases both production and retail for artisan producers. FDC led and took charge of the entitling, permitting, and execution of the complete project over a two year period. It includes seventeen modern metal buildings, multiple 500-16,000 SF suites each with a glass roll up store front and up to 30 ft. ceilings, new city streets, a beautiful 12.5 acre landscaped campus, and parking.

FDC was integral in value engineering a design for the local non-profit grocery and community market in Sebastopol, CA. Our team created something very functional and cost effective, knowing that the less money spent on capital improvements and rent would allow for greater success as a community contributor.

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