How do portal frames support loads

The length of the overhang can be determined according to the application requirements and should be 0. Generally, it uses a single color metal sheet or sandwich panel. Mainly bear wind load. It uses a single color metal sheet or sandwich panel. Type: Roof horizontal bracing, wall bracing. It consists of a roof cross bracing, tie beam, and fly bracing. Use: 1 Enhancing the spatial rigidity of the building structure 2 Guaranteed structural stability 3 Transmit wind load, crane brake load and seismic load to the load-bearing members.

External loads act directly on the envelope — vertical and lateral loads transmitted to the lateral portal frame of the primary structure through the secondary structure. The portal frame relies on its stiffness to resist external effects. Longitudinal wind loads transferred to the foundation through roof and wall bracing. The span and column spacing of the portal steel frame mainly determined according to the building requirements. The main issues to consider in the architectural design are the determination of the temperature interval and the layout of the bracing system.

Considering the temperature effect, the length of the longitudinal temperature section of the portal steel frame buildings should not exceed m, and the transverse temperature section not exceed m.

If the size exceeds the temperature section, it should arrange the temperature expansion joint. Temperature expansion joints can realize by setting double columns or adjusting the secondary framing. The horizontal roof bracing and the wall bracing arranged between the same column, it uses to ensure the formation of a geometrically unchanged system and improve the overall stiffness of the building structure; 3. If the roof bracing arranged between the second columns, rigid tie bars should arrange between the first columns.

When the angle of the single-layer bracing member is too large due to the high column, the double-layer or three-layer wall bracing should set;. Rigid tie bars shall provide at the turning points such as the column tops and roof ridges.

Longitudinal rigid tie bars shall provide at the structure longitudinally at the bracing truss nodes;. The rigid tie bar of the portal steel frame buildings can use the purlin at the corresponding position. The tie bar provided when the stiffness or bearing capacity is insufficient.

Install the steel column First, fix the anchor bolts, and the steel column is set on the foundation by connecting with the anchor bolts. Assemble the steel beam Steel beams should be combined with high-strength bolts on the ground and assembled. Installation sequence: start with the two rigid frames supported between columns near the gable. Install purlin, bracing, and fly bracing, etc. The Design software generates the self-weight of the portal steel frame buildings.

A load of roof, purlin, bracing, and other loads added to the steel frame was calculated according to the actual design. Corrugated single color sheet and sandwich panel could use as the roof or wall panel. The Insulation materials of the sandwich panel include polystyrene foam, polyurethane, rock wool, glass wool, etc. The design should combine with specific materials to determine the roof and wall load. Variable loads include roof live load, ash load, crane load, seismic action, wind load, etc.

So when calculating steel frame usually uses 0. Under the same load conditions, the column spacing arrangement has a great influence on steel consumption. Several statistical analyses show that the recommended column distance is m, and the span should not be greater than 36m. Purlins should be thin-walled C and Z type steel, while steel frames generally use H-shaped sections. Steel structure splicing includes splicing in the workshop and on-site. The splicing methods include welding and bolting.

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Curved portal frames are generally an architectural requirement. The rafters are modelled as straight members and guidance can be obtained from the Steel Construction Institute publication: Guidance on the stability of curved rafters in portal frames. As with cellular beam sections, they cannot develop plastic hinges, hence the advantage of plastic analysis commonly available to other forms of portal frames is not available to this, only elastic design can be carried out.

The conceptual design stage of any structure must take into cognizance a series of interconnected factors influencing on the overall design, a portal frame is not different.

The frame dimension is critical to determining the overall height and width of the frame to give adequate building space for the internal functions of the building. The frame designer must decide on the following at the conceptual design stage. The clear span and height required by the client as evident in the architectural drawings is the key to determining the dimensions to be considered for use in the analysis and design.

The architectural requirement for clear span is often the dimension between the internal flanges of the columns, the span used for design is the centre-centre dimension, hence this would be greater than the architectural clear span by the column section depth. The clear height of the frame is measured from the finished floor level to the underside of the haunch or suspended ceiling.

This will be determined using the specified internal floor height specified in the architectural drawing. A typical main frame in a portal frame structure should be characterized by the following 1 :. Preliminary sizing of portal frame design is carried out using the resistance of the steel sections to their cross-section resistance to flexure, shear and axial forces. At the detail design stage, the resistance of the chosen sections to buckling would need to be carried out, with restraints positioned at the critical areas.

Buckling is usually less critical in the rafters due to the restraints from purlins, although sometimes additional restraints might be required. However, the same cannot be said for the columns, as there is usually more inhibition to positioning rails to provide restraints due to requirement for openings within the elevations of the buildings.

Where intermediate restraints are impossible, the buckling resistance of the column will most likely determine the preliminary sizing of the columns. Thus, it is very essential at this early stage to understand if provision of continuous side rails is allowed on the frame elevations. Only continuous side rails can be guaranteed to effectively restrain the column.

Side rails occasionally interrupted by the requirement for doors and windows cannot be relied on as providing effective restraints 1. Having established the frame dimensions and use of restraint in resisting buckling, the frame designer must now select trial sections that would be used to check the analysis and design.

The steel sections typically used in portal frame design are usually grade S steel. For portal frames designed using plastic analysis, the classification of steel sections at locations where the formation of plastic hinges that rotate is likely must be Class 1, compact section can be used elsewhere.

See: Assessing the Stability of Frames. The analysis and design of portal frame structures will be presented in subsequent articles, please stay glued! Your email address will not be published. Generally fabricated from UB sections with a substantial eaves haunch section, which may be cut from a rolled section or fabricated from plate.

Office accommodation is often provided within a portal frame structure using a partial width mezzanine floor. The assessment of frame stability must include the effect of the mezzanine; guidance is given in SCI P Where a travelling crane of relatively low capacity up to say 20 tonnes is required, brackets can be fixed to the columns to support the crane rails.

Use of a tie member or rigid column bases may be necessary to reduce the eaves deflection. The spread of the frame at crane rail level may be of critical importance to the functioning of the crane; requirements should be agreed with the client and with the crane manufacturer. In a tied portal frame the horizontal movement of the eaves and the bending moments in the columns and rafters are reduced. A tie may be useful to limit spread in a crane-supporting structure.

The high axial forces introduced in the frame when a tie is used necessitate the use of second-order software when analysing this form of frame. A mono pitch portal frame is usually chosen for small spans or because of its proximity to other buildings. It is a simple variation of the pitched roof portal frame, and tends to be used for smaller buildings up to 15 m span.

Where the span of a portal frame is large and there is no requirement to provide a clear span, a propped portal frame can be used to reduce the rafter size and also the horizontal shear at the foundations.

A mansard portal frame may be used where a large clear height at mid-span is required but the eaves height of the building has to be minimised. Portal frames may be constructed using curved rafters, mainly for architectural reasons. Because of transport limitations rafters longer than 20 m may require splices, which should be carefully detailed for architectural reasons. The curved member is often modelled for analysis as a series of straight elements. Guidance on the stability of curved rafters in portal frames is given in SCI P Alternatively, the rafter can be fabricated as a series of straight elements.

It will be necessary to provide purlin cleats of varying height to achieve the curved external profile. Rafters may be fabricated from cellular beams for aesthetic reasons or when providing long spans. Where transport limitations impose requirement for splices, they should be carefully detailed, to preserve the architectural features.

The sections used cannot develop plastic hinges at a cross-section, so only elastic design is used. In the design and construction of any structure, a large number of inter-related design requirements should be considered at each stage in the design process. The following discussion of the design process and its constituent parts is intended to give the designer an understanding of the inter-relationship of the various elements of the structure with its final construction , so that the decisions required at each stage can be made with an understanding of their implications.

Steel sections used in portal frame structures are usually specified in grade S steel. In plastically designed portal frames, Class 1 plastic sections must be used at hinge positions that rotate, Class 2 compact sections can be used elsewhere.

A critical decision at the conceptual design stage is the overall height and width of the frame, to give adequate clear internal dimensions and adequate clearance for the internal functions of the building. The clear span and height required by the client are key to determining the dimensions to be used in the design, and should be established early in the design process.

The client requirement is likely to be the clear distance between the flanges of the two columns — the span will therefore be larger, by the section depth.

Any requirement for brickwork or blockwork around the columns should be established as this may affect the design span. Where a clear internal height is specified, this will usually be measured from the finished floor level to the underside of the haunch or suspended ceiling if present.

The main portal frames are generally fabricated from UB sections with a substantial eaves haunch section, which may be cut from a rolled section or fabricated from plate. A typical frame is characterised by:. The use of a haunch at the eaves reduces the required depth of rafter by increasing the moment resistance of the member where the applied moments are highest. The haunch also adds stiffness to the frame, reducing deflections, and facilitates an efficient bolted moment connection.

The eaves haunch is typically cut from the same size rolled section as the rafter, or one slightly larger, and is welded to the underside of the rafter. The haunch length generally means that the hogging moment at the end of the haunch is approximately equal to the largest sagging moment close to the apex. The apex haunch may be cut from a rolled section — often from the same size as the rafter, or fabricated from plate.

The apex haunch is not usually modelled in the frame analysis and is only used to facilitate a bolted connection. During initial design the rafter members are normally selected according to their cross sectional resistance to bending moment and axial force.

In later design stages stability against buckling needs to be verified and restraints positioned judiciously. The buckling resistance is likely to be more significant in the selection of a column size, as there is usually less freedom to position rails to suit the design requirements; rail position may be dictated by doors or windows in the elevation. If introducing intermediate lateral restraints to the column is not possible, the buckling resistance will determine the initial section size selection.

It is therefore essential to recognise at this early stage if the side rails may be used to provide restraint to the columns. Only continuous side rails are effective in providing restraint. Side rails interrupted by for example roller shutter doors, cannot be relied on as providing adequate restraint.

Where the compression flange of the rafter or column is not restrained by purlins and side rails , restraint can be provided at specified locations by column and rafter stays to the inside flange. Permanent actions are the self weight of the structure, secondary steelwork and cladding.

Where information is not available, these may be determined from the data in BS EN [3]. Service loads will vary greatly depending on the use of the building. In portal frames heavy point loads may occur from suspended walkways, air handling units etc. It is necessary to consider carefully where additional provision is needed, as particular items of plant must be treated individually.

Depending on the use of the building and whether sprinklers are required, it is normal to assume a service loading of 0. A point load, Q k is given, which is used for local checking of roof materials and fixings, and a uniformly distributed load, q k , to be applied vertically. The loading for roofs not accessible except for normal maintenance and repair is given in the table on the right.

It should be noted that imposed loads on roofs should not be combined with either snow or wind. Snow loads may sometimes be the dominant gravity loading. Any drift condition must be allowed for not only in the design of the frame itself, but also in the design of the purlins that support the roof cladding.