Table of Contents
The aim of this study is to study the basic characteristics of portal frames, examine the use and the impact of steel portal frames in the construction processes. Additionally, the study aims at finding out or learn the main areas of their application the portal frames. Also, the paper seeks to study the basic components of the frames, their construction and bracing Perceptions and Perspectives on the use as construction equipments in their classrooms as well as analyzing ways in which to improve civil engineering in learning institutions practice in school settings. The paper also outlines through its study of the methods of portal design. The paper too wants to outline the typical details of the eaves and ridge joint through the observed results from the study indicated that steel portal frames study in learning institutions learning in civil engineering Classroom has greater impact on student performance in the civil engineering studies in regard to practical performance. Additionally, from the study the results implied that most architects have positive and favoaurable attitudes towards the use of steel portal frames. The paper therefore carries out a comprehensive research on this topic by analyzing several facts and data as well as reviewing several studies done earlier on this topic. Furthermore, the paper covers a wide analysis of facts in which the researcher finally makes a conclusion after reviewing the information collected of the research. The main objective of this study is to evaluate in depth the effectiveness of steel portal frames in learning institutions tools in civil engineering, the impact on the students skills or not and in the entire architectural field. The research study seeks to analyze the efficiency of the steel portal frames in learning institutions Learning tools to students skills in all aspects and how it actually help the students to improve their architectural skills in class. The paper too covers the theoretical structural layouts, detailing and design for the steel portal frames that are fabricated from hot rolled with the light-gauge steel cladding. This is generally the most economical form of construction for steel portal framed buildings. Many of the general guidelines are applicable to other forms of construction, e.g. hollow sections. Both elastic and plastic designs are considered and are too presented through explanations.
Steel portal frame description
Recently, most architects and engineers do use the steel portal frames because its strength and efficiency with no limitation in regard to other features that do include the modifications such as sculpture. The material do provide a wide range of opportunities that aid in design of the sophisticated modern building. This is believed to be attributed to by the metallurgy preparation of the steel strides, the analysis of the structures and the fabrication. The steel portal frame is too known for its enormous strength that is high with a relatively low weight giving it a remarkable ability in carrying load and spanning. In addition to this, the frames are known for its adaptability in that it easy to modify the frames and even alter them to the design needed by the architecture during construction. The steel is affordable in terms of purchasing, recyclable and other varied opportunities. Steel is basically a combination of iron and carbon although its combination can be improved through the addition of other elemental alloys and the process of manufacturing. Steel portal frames are very efficient and economical when used for single-storey buildings, provided that the design details are cost-effective and the design and analysis assumptions are well chosen.
The strength and the rigidity in the joints attributes to the transfer of the bending moments that is in the rafters to columns which imply that the rafter size can either be reduced or the span increased for the same rafter size making the portal frames more efficient in the technicalities of construction in side span structures. According to studies, most commonly used applied frames do rely on hot rolled steel sections that are prepared from the materials which have undergone heating and passed as a billet through the heavy rollers which do gradually cut down the cross section while increasing at the same time in its length. This process helps in the achievement of the gradual flow of the materials in the shape that is needed in the wide span column and the beam frame hence the structural members are arranged in the three dimensional matrix therefore the steel portal frames are designed based on the sections above. In the cases of the larger span, the hot rolled sections and the plates may also be fabricated to form in the making of particularly some more complex members like the roof beams that are on the Renault centre or even the steel arch of the Lehter Bahnhof. Other standard sections can too be curved after the process of manufacturing by the use of heavy bending equipments and even be converted to the perforated profiles of the web with the use of a number of approaches where some do split the beam into two sections that are reweld to make it adapt its and the ability in spanning is thus increased. The lighter steel portal frames can be prepared via the bending of a steel sheet to C, Z or the S sections through the application of a press or a folding type of machine for some specific sections, a cold rolling line is sometimes applied. According to architects, cold formed sections are believed to have more structural capacity compared to timber sections that do have the same structural profiles that do range from 75mm to 500mm. they are in particular suitable towards the close center frames like the wall the floor panel, the roof purlins which do support the cladding, the light portal frames, the beams and the columns and the lightly loaded and the nonstructural applications like the support to the internal wall and the partition. The advantages of steel portal frames are low cost rapid fabrication simple to clad wide range of possible exteriors, it has simple erection, it’s easy in maintenance, and it’s easily adaptable to future needs (additional bays, additional plant or services) with large clear spans for a small increase in cost. The low marginal cost of a large clear span can be attractive for some reasons; the flexibility of the internal layout, the adaptability to the changing use and its greater range of the possible purchasers in the cases of the sale of the building.
Main Areas of Application
Steel portal frames are mainly seen in the construction of the bams, warehouses and other large scale structures where open spaces are required at an economical cost where the pitched roof is acceptable. In most occasions, the steel portal frames are used in the construction of the single storey buildings but they can as well be used in the low rise buildings that do contain a number of floors which is viewed as economical to most architects as they do not span right across the structure or the building. The lightest steel portal frames are occasionally designed with the use of plastic design methods and the use of purlins towards the stabilization of the rafters and the columns. The spacing that is placed in the purlins is generally less than as required by the sheeting to carry the expected loads. A variety of purlins have been designed to carry any diagonal bracing members emanating from the purlin to the inner flange of the portal so that the inner flange is given stability.
In the cases of heaviest steel frames, portal steel frames can be applied if there is less or no bracing to give stability to the inner flanges. This has always led to a need for the elastic analysis of the structure and to the large column and the rafter sections which will ensure the proper stability of the elements. Despite this, such a design can give room for the use of the deep deck roofing spanning which will be directly between the portals instead of purlins and the sheeting. This can on the other hand reduce the fabrication works and can too reduce the site-work. According to studies, it is assumed that this heavy frame design can cause the production of an economically completed building bearing in consideration the costs of labor that could be highly relative to the costs of the materials because such heavy frames do use less labor for manufacture than the light frames.
In the construction of the multi-span portal framed buildings, in many occasions there has been the use of valley beams which is used in the elimination of some internal columns. In most occasions, many alternate columns are left out and the valley of the frame is supported on the valley-beam spanning between the columns of the adjacent frames.
In another application of the steel portal frames; the roof pitch that is adopted does depend on the type of the roofing including the required specifications of the main structural frame. The pitch will hence affect the economy of the steel portal frame and its deflections. In most structures today, the lowest possible roof slope is needed in order tolimit the area of cladding and the enclosed volume of the building. The model of cladding dictates the minimum practical slope to watch on water tightness. Where the external face of the roof cladding is steel sheeting, the minimum slope is usually 1:10 to confirm that the laps that are between the lengths of the sheeting are easy to waterproof and there is no stagnation of the water on the roof. This is the most common form of roof cladding for use with purlins. Lower slopes are possible, but only with special verifications or measures to avoid ponding and with sheets with specially engineered joints, or single length sheets to avoid joints. Careful attention must also be paid to the fixings to avoid creating leaks. Where the external face is a waterproof membrane, the roof slope can be less than 1:10. This is the most common form of roofing where deep profile decking is used to span directly between the purlins. Systems using a steel external skin with purlins spanning between the portal frames are both visually attractive and economical, which is why they dominate the market in the UK, where steel portal frames are most common.
In another application of the steel portal frames which is the on the bases of the structures, Portal bases can be defined or rather categorized as the pinned or the fixed actual stiffness or the flexibility where the nominally pinned or nominally fixed bases are supposed to be considered for the design of the steel portal frame. The greatest advantage of the pinned bases is that the base or rather the foundation is simplified and thus should be less expensive. The fixed bases do have the advantage that this frame is stiffened, thus assured of improving the structure stability. Partial base fixity is believed to reduce the deflections greatly without necessarily affecting the foundation costs. In cases where there is doubt about the acceptability, it is safe to commercially assume a truly pinned base. In the cases with ground that is weak, the horizontal reactions that are generated at the bases might be resisted by the floor slab, and this is if suitable details are provided and put into consideration during the structure establishment. Portal frames too can be clad with other material but the most popular solution, for reasons of economy and speed, is that some form of lightweight insulated metal cladding with cavity masonry does work to the bottom 2m of the wall to give provision to the security and impact resistance. The lightweight cladding can be carried on the sheeting rails spanning between the columns of the portal frames.
Elements of modern steel construction
Many engineers have put in practice the use a reinforced concrete cantilever that rises out of the foundation and the cast around the steel column which will be used to resist this loading. This concrete cantilever is strategically designed to give resistance to the impact thus ignoring the presence of the steel column while on the other hand too the steel column is designed to ignore the impact. The columns are usually in I sections rather than H sections, although the H sections are sometimes applied. The external column sections are usually heavier in a significant manner in regard to mass/unit length compared to the rafter sections in the structure, if in any case the rafters are haunched at the rafter or the column connections. The effective length of members for lateral buckling and the lateral torsional buckling can be reduced a bit by the diagonal stays to purlins and the side rails. Despite this, the application of the purlins for lateral restraint is supposed to be agreed with the checking engineer before proceeding. In some presiding engineers or the building code, this may not be applicable. In a number of countries it can only be given chance if the purlins are arranged with the nodes of the bracing truss on the roof and the forces from the bracing loads must be put into consideration, whenever doing the estimation of the resistance of the purlins in the structure.
With the column stability under the maximum gravity combination, the frame is in this case designed plastically or with the elastically, a torsional restraint should always be give on the underside of the haunch. More torsional restraints may be needed within the length of the column because the side rails are attached to the outer tension flange and not to the compression flange. a side rail that is not continuous for instance the one that has been interrupted by industrial doors cannot be relied upon to provide adequate restraint and therefore the column section will need to be increased if intermediate restraints cannot be provided.
The Restraint in the structure may be provided by the stays on the inside flange, to support or rather signify the stiffeners in the column, which are only typical at the level of the underside of the haunch where they act as compression stiffeners. At other places, the stiffeners in the structure are basically not needed. At the underside of the haunch level, it may be good to give a hot rolled member that is precisely a hollow section that will provide restraint. It is essential to join the bracing on the inner flange to the outer flange at some point in the length of the building. Columns are built with a typical moment distribution for permanent and variable actions and indicate the positions of restraints on a typical column. The presence of a plastic hinge will depend on loading, geometry and choice of the column and the rafter sections. In a similar way to the rafter, both out-of-plane and in-plane stability must be verified.
In the places where there is a plastic hinge in the construction at the underside of the haunch, the distance to the adjacent torsional restraint must be less than the limiting distance Ls as given by EN 1993-1-1 § BB.3.1.2 building code. According to Expression BB.7, it should be used when the moment is linear, and BB.8 when the moment is not linear. In addition, the spacing between intermediate lateral restraints should satisfy the requirements for Lm as given in BB.3.1.1. If the stability between torsional restraints cannot be verified, it may be necessary to introduce additional torsional restraints. If it is not possible to provide additional intermediate restraints, the size of the member must be increased. In all cases, a lateral restraint must be provided within Lm of a plastic hinge. If in this case there is no plastic hinge, the stability of the column should be checked in accordance with Expression Account can be taken of the benefits of tension flange restraint. The column web is subjected to a high compression at a level of the bottom flange of the haunch. To add on that needs that web stiffeners should be provided at the plastic hinge locations, if in any case the applied transverse force is more than 10% of the member’s shear resistance. For these reasons, full depth stiffeners are usually required to strengthen the web. When the frame is subject to uplift, the column moment will reverse. The bending moments will generally be significantly smaller compared to those under gravity loading combinations, and the column will remain elastic. Out-of-plane checks should be undertaken in accordance with Expressions. The Columns should be IPE or similar sections with Class 1 or Class 2 proportions under the combined moment and axial load. The column should in this case be able to give resistance to the high shears that are within the depth of the eaves connection with no shear stiffening.
The rafters on the structure are subject to high bending moments in the plane of the frame, that vary from a maximum ‘hogging’ moment at the junction with the column to a minimum sagging moment close to the apex. They are also subject to overall compression from the frame action. They are not subject to any smaller axis moments. Although the member resistance is vital, the stiffness of the frame is also important so that it limits the impacts of the deformed geometry and to reduce the SLS deflections. For these reasons, high strength members are generally not used in portal frames, but the lower steel grades with higher inertias. Optimum design of the portal frame rafters is generally achieved by use of a cross-section with a high ratio of Iyy to Izz that complies with the requirements of Class 1 or Class 2 under combined major axis bending and axial compression. A haunch that extends from the column for approximately 10% of the frame span. This basically implies that the maximum hogging and sagging moments in a plain rafter length are the same.
Portal frame design is usually governed by the verification of members at ULS. Although SLS checks are important, orthodox frames are mostly sufficiently stiff to give satisfaction to the SLS deflection limits. The economy in the entire overall frame is achieved by the application of the plastic analysis; this needs Class 1 or 2 sections entirely and Class 1 where there is a hinge which is predicted to rotate.
The resistances in the critical cross-sections of the rafter in the structures must be verified in accordance with the Sections of the building code. Both in-plane and out-of-plane checks are needed. In the recent past, the out-of-plane checks are completed to ensure that the restraints are located at facilitated in the positions and the spacing. Purlins are placed at about 1,8m spacing but according to architects this spacing may need to be reduced in the high moment regions near the eaves. Three stability areas are noted which are known to in the following sections. The presence of plastic hinges in the rafter will depend upon the loading, geometry and choice of column and rafter sections. The selection of the suitable check is based on the availability of a plastic hinge, the shape of a bending moment diagram and geometry of the section three flanges or two flanges. The main aim of the checks is to provide enough restraints to make sure that the rafter is stable out-of-plane.
In the aplicational Zone A, the bottom flange of the haunch is seen in compression. The stability checks are complicated by the variation in geometry along the haunch. The junction of the inside column flange and the underside of the haunch is supposed to be always restrained. Sharp ends of the haunch do have the restraint to the bottom flange, from the purlin that is located at this position, and forming the torsional restraint at this point. If at all the plastic hinge is predicted at such a position, a restraint is then supposed to be located within h/2 of the hinge position, where in this case h is the depth of the rafter. In a hinge is predicted at some of the point, and a restraint to the bottom flange has been provided. Zone B in the rafter construction generally extends from the sharp end of the haunch to beyond the point of contraflexure point. The bottom flange is partially or wholly in compression over this length. Depending on the overall analysis, this zone may or may not contain a plastic hinge at the sharp end of the haunch. In this zone, torsional and lateral restraint will be provided at the sharp end of the haunch. At the upper end, restraint will be provided by a purlin beyond the point of contraflexure. Some national authorities allow the point of contraflexure to be considered as a restraint, provided the following conditions below are satisfied. The rafter is a rolled section at least two bolts are provided in the purlin to rafter connections, The depth of the purlin is not less than 0,25 times the depth of the rafter.
If a plastic hinge is predicted at the sharp end of the haunch, a torsional restraint must be provided within a limiting distance in accordance with BB.3.1.2. The limiting distance can be calculated with the assumption of the constant moment using Expression BB.6 . A non linear moment gradient does use the Expression BB.8.
In addition to this, the purlins are assumed to provide for the lateral restraint to the top compression flange as long us they are placed in some overall restraint system. In many places of construction, it is simply assumed that the diaphragm action of the roof sheeting is sufficient to carry restraint forces to the bracing system; in many places with architecture any purlins providing restraint must be connected directly to the bracing system. The out-of-plane checks need the verification of the member in accordance with the studies carried out. Normally, if the purlins are regularly spaced, it is sufficient to check the rafter between restraints assuming the maximum bending moment and maximum axial load. If a plastic hinge is predicted to form adjacent to the apex, it must be restrained. In addition, the usual requirements for stability near a plastic hinge must be satisfied: The distance between the restraint at the plastic hinge and the next lateral restraint must not exceed the limiting distance Lm. The distance to the next torsional restraint each side of the hinge must not exceed the limiting distance Lk, or Ls, with the spacing of intermediate restraints satisfying the requirements for Lm, all as described for zone B. This type of bending moment diagram will generally occur under internal pressure and wind uplift. Normally, the bending moments are smaller than the gravity load combinations and the members will remain elastic. The stability checks recommended below assume that plastic hinges will not occur in this uplift condition.
The rafter must be verified between torsional restraints. A torsional restraint will generally be provided adjacent to the apex. The rafter may be stable between this point and the virtual restraint at the point of contraflexure. If the rafter is not stable over this length, additional torsional restraints may be introduced, and each length of the rafter verified. The beneficial effects of the restraints to the tension flange (the top flange, in this combination) may be accounted for using a modification factor Cm, taken from § BB.3.3.1(1)B for linear moment gradients and from § BB.3.3.2(1)B for non-linear moment gradients. If this benefit is utilised, the spacing of the intermediate restraints should also satisfy the requirements for Lm, found in § BB.3.1.1. Rafters are supposed to be IPE or similar sections with Class 1 or Class 2 proportions under combined moment and axial load. Sections containing plastic hinges must be Class 1. Cross-sections are supposed to be checked to Section 6 of EN 1993-1-1. Detailed checks can be carried out to ensure adequate out-of-plane stability under both gravity and uplift conditions.
The major role of the eaves beam is basically to support the roof cladding, side walls, and guttering along the eaves, but studies say that it may also be used to provide lateral restraint at the top of the outer flange of the column. If vertical side wall bracing capable of resisting tension and compression is given at both ends of the structure, an eaves strut is not needed other than in the end bays. However, it is a good thing to give a member between the columns to act as a tie during erection and provide additional robustness to the structure. In the cases where a section of circular hollow is applied in restraining the plastic hinge at the bottom of the eaves, this can play the function of the longitudinal strut including the restraining of the plastic hinge.
A typical connection of the eaves works towards the improvement of the resistance moment of any rafter connected, the availability of the haunch strengthen the lever arms of the bolts that are in the tension zone, which is good if at the connection bears a large bending moment. Because the portal frame members are chosen for bending resistance, deep members with relatively thin webs are common in portal frames. A compression stiffener in the column is usually required. The web panel of the column may also need reinforcing, either with a diagonal stiffener, or an additions web plate referred to as a supplementary web plate. The end plate and column may be extended above the top of the rafter, with an additional pair of bolts. The end plate on the rafter is unlikely to require stiffening as it can simply be made thicker, but it is common to find that the column flange requires strengthening locally to the tension bolts. Stiffeners are expensive, so good connection design would reduce the need for stiffeners by judicious choice of connection geometry. Under a reversed bending moment, it may be necessary to provide a stiffener to the column web at the top of the column, aligned with the top flange of the rafter.
Bases are defined as a combined arrangement of the base plate which holds down the bolts including concrete foundation. The terms like the nominally pinnedand the nominally rigidare usually used in relation to the performance of the base in regard to the stiffness of the structure. Base plate can be referred to as the steel plate that is commonly at the base a column, that is connected to steel portal column by fillet welds. Holding down bolts are bolts through the base plates that is anchored into the concrete foundation. The foundation is the concrete footing required to resist the compression, uplift, and, where necessary, the over-turning moments. The anchor plates refer to the plates or the angles that are used to anchor the holding down bolts into the foundation. They are required to be of this size so that they can provide a sufficient factor in terms of safety in regard to the bearing failure of the concrete. In almost cases, a nominally pinned base has been given due to the difficulty and the expense of supplying a nominally rigid base that is moment resisting. Not only is the steel base connection significantly more expensive, the foundation must also resist the moment, which increases costs significantly. In the cases where the crane girders are supported by the column, moment resisting bases may be required to reduce deflections to acceptable limits. In a nominally pinned base for larger columns, the bolts can be located entirely within the column profile. For smaller columns less than approximately 400 mm, the base plate is made larger so that the bolts can be moved outside the flanges. A nominally rigid, moment resisting base is achieved by providing a bigger lever arm for the bolts and a stiffer base plate by increasing the plate thickness. Additional gusset plates may be required for heavy moment connections.
The steelwork contractor will usually be responsible for detailing the base plate and holding down bolts. However, it is supposed to be made clear in the contract documentation where the responsibility lies for the design of the foundation details, as special reinforcement spacing or details may be required. Base plates will usually be in grade S235 or S275 steel. The diameter of the bolt will generally be determined by consideration of the uplift and shear forces applied to the bolts, but will not normally be less than 20 mm. There is usually generous over-provision, to allow for the incalculable effects of incorrect location of bolts and combined shear force and bending on the bolt where grouting is incomplete. The length of the bolt should be determined by the properties of the concrete, the spacing of the bolts, and the tensile force. A simple method of determining the embedment length is to assume that the bolt force is resisted by a conical surface of concrete. Where greater uplift resistance is required, angles or plates may be used to join the bolts together in pairs as an alternative to individual anchor plates. Calculations should be carried out by the designer at the final design stage to check the viability of the proposed bolt spacing.
The most efficient purlins and rails are cold formed from thin galvanised steel. They are often attached to the frames by use of cleats and other fittings. There are many products available, including several complete systems that encompass all the necessary fittings. These cold formed members are far more economical than the alternative hot rolled angles, channels or I sections. They are applied in all modes of the steel portal structures in a period of time and have give results that have proved thy are worth in the UK and other places of construction.
Some specifications still require the minimum thicknesses of any component to be greater than the normal thicknesses of cold formed members. These requirements should be discussed with the prospective client, because they are usually irrelevant for galvanized members fitted internally. Different manufacturers choose different levels of complexity to increase the efficiency of the weight of steel. For a given size and resistance of section, the number of bends should be increased as the thickness of the steel is decreased. Many architects believe it is important to choose the shape of purlins and rails before doing the detailed drawing of the steelwork, as some of the more complicated shapes do not attach to the simplest cleats. Cold formed purlins spanning up to 15 m are available.
The safe span of a given purlin can be affected directly by the continuity developed at the end of each span. The possible conditions are; the Continuity of the elements single span double span multiple span continuity at the connections, the simple ends, the sleeved ends, the overlapping ends. Different purlin and rail systems are designed for different types of span and the connection details. In the case of the continuous or semi-continuous purlin structural systems, the purlins placed at the ends of buildings structure may require to be a heavier section, when the frame centers are more similar throughout the structure. Despite this, this can be ignored through the limitation of the load capacity for this case or through the use of the heavier or the longer sleeves or the overlaps at the end bay in the structure.
The word portal bracing refers to a system of bracing that comprises the portals other than the cross bracing used to give the restraint that is normal to the main frames in the structure. It is commonly applied to give a lateral stability on top of the applied internal columns on the structure, this is because the application of the cross bracing may lead in an unacceptable restriction thus affecting the freedom of use. It is also used at exterior walls, where cross bracing might obstruct windows, doors, etc. These bracing portals are classified as bracing systems.
Bracing is required to resist lateral loads, principally wind loads, and the destabilising effects of the imperfections defined. This bracing must be correctly positioned and have adequate strength and stiffness to justify the assumptions made in the analysis and member checks. The construction allows the imperfections to be described either as geometrical imperfections or as equivalent horizontal forces. The equivalent horizontal forces, which cause the forces in the bracing, do not increase the total load on the wholestructure, because they form a self equilibrating load case.
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Bracing is provided to hold the columns upright and resist loads, such as wind loading, occurring at right angles to the frame. Columns are assumed to be built slightly out of vertical, and the simplest method to account for this effect is by means of the equivalent horizontal forces and considered further in studies of Plastic design of single storey pitched-roof portal frames to Eurocode. The forces in this case do result in any given direction, but are assumed to exert pressure in one direction at a time. bracing is basically designed to give resistance to the wind loads including any equivalent horizontal forces. These equivalent forces are thus given and the amount to be approximately 0,5% of the vertical forces that do cause the axial compression in the structure joinings. Precautions has always been taken into consideration whenever noting the bracing because the details that do limit the stiffness through deflection from the local bending of the plates, the rings or the angles may add the imperfection loads as a result of the second-order effects. If the columns carry a net tensile force, as in an uplift load case due to wind, this loading does not destabilize the structure, so may be neglected in calculating the equivalent forces.
The procedural design and the given guidance do rely on the application of on the particular or respective countries or regions building for instance the Eurocode. These regulations are implemented in EU and EFTA countries through respective state’s National Application Document. The use of ENV 1993–1–1 allows a common design philosophy, although the actual design calculations will vary slightly from country to country, according to the requirements of the particular National Application Document. Until the complete set of Eurocodes and supporting EN standards is available, reference must be made to certain national standards for example for loading specifications.
Elastic and plastic analysis
Elastic analysis has in the recent past been the most common technique used in the analysis of the structures in general although it has been noted for giving little economical portal structures than the plastic analysis. This has often allowed the cross-sectional resistance in the plastic for instance plastic moment, to be applied with the results of the elastic analysis. In addition to this, it allows some redistribution of moments although the percentage may be decreased to 10% in the EN. To make full use of this in portal design, it is important to consider the spirit of the Clause, which was written with continuous horizontal beams of uniform depth in mind. Thus, in a haunched portal rafter, up to 15% (by the ENV) of the bending moment at the shallow end of the haunch could be redistributed, if the moment exceeded the plastic resistance of the rafter and the moments and forces resulting from redistribution could be carried by the rest of the frame. Alternatively, if the moment at the midspan of the portal exceeded the plastic resistance, this moment could be reduced by up to 15% (by the ENV) redistribution, provided that the remainder of the structure could carry the moments and forces resulting from the redistribution. It is important to understand that the redistribution cannot reduce the moment to below the plastic resistance. To give room for the reduction below any of the plastic resistance would not make sense as it would result in dangerous assumptions that will affect the calculation of the buckling resistance of the member.
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Plastic analysis is used for more than 90% of portal structures in the UK and has been in use for 40 years. It is a well established and well proven method of design, even though it is not yet used extensively in continental Europe. The three common methods of analysis are: graphical, virtual work analysis of rigid-plastic mechanisms and elastic-perfectly plastic. The graphical method is explained by Morris & Randall (1978) and included in other standard texts. In the graphical method, the bending moment diagrams are drawn along the members, with the maximum and minimum values of bending moment limited by the plastic resistance of the member at each position. Alternatively, the members may be chosen to suit any statically admissible bending moment diagram. Therefore, the graphical method lends itself to analysis of very simple structures and the nitial design of any structure. The graphical method will always find the upper bound of bending moment or lower bound of the load factor, so it is always safe, assuming it is done correctly. However, for all but simple cases, it becomes unwieldy for final analysis.
Sway stability generally extends from the sharp end of the haunch to beyond the point of contraflexure. The bottom flange is partially or wholly in compression over this length. Depending on the overall analysis, this zone may or may not contain a plastic hinge at the sharp end of the haunch. In this zone, torsional and lateral restraint will be provided at the sharp end of the haunch. At the upper end, restraint will be provided by a purlin beyond the point of contraflexure. Some national authorities allow the point of contraflexure to be considered as a restraint, provided the following conditions below are satisfied.
For relatively slender frames, it is normally wise to check the deflections at the SLS before checking the member buckling resistance at the ULS, i.e. after step 6 in Sections 17.2 and 17.3. The following procedure is suggested; to determine SLS load combinations and R factors, to perform first-order analysis, to check that VSd / Vcr < 0,1 to allow second-order effects to be ignored, to check if VSd / Vcr > 0,1, re-analyse including magnification factor, to check if an elastic analysis method is used, chick that the structure remains elastic and to check that the deflections do not impair the function of building, including the cladding and doors.
In conclusion, the paper reviews the advantages of steel portal frame construction and clarifies that the scope of this publication is limited to portal frames without ties between eaves. Most of the guidance is related to single steel span frames, with limited guidance for multi-span steel frames. The importance of second order effects in portal frames, the use of the elastic and plastic analysis, the design at the ultimate and serviceability limit States, the element design, the cross-section resistance and sway stability. The paper does discus the Secondary structure: gable columns, bracing, putlins and eaves members in steel portal frames.