Portal frame construction refers to the technique of designing and constructing structures, whereby two-dimensional rigid frames with basic characteristics of the rigid joint are between the beam and the column are utilized (Hiriyur,2010). The main aim of using this method of designing and construction is to reduce the bending moments in the beam in order to allow the frame to act as one structural unit. With that, the size of the structural element can be reduced or at the same time, and the span can be increased for the same size of the structural elements. Due to that, the portal frames are considered to be very efficient and reliable construction method to be used for the buildings with long span.
Portal frames are generally being used in the low-rise structures, which are comprised of beams, columns or pitched rafters that are connected by the moment resisting connections. The resistance to vertical and lateral actions is offered by a suitable haunch, or the deepening of the rafter sections This type of continuous frame structure is usually stable in its plane, and it offers clear span which is not obstructed by the bracing. In most cases, the portal frame construction is used in the construction of single-level structures, and it is usually seen in the construction of factories, warehouses, barns and other areas where large open spaces are needed at low cost and a pitched roof is also accepted.
A portal frame structure usually comprises a series of transverse frames which are braced longitudinally. The primary steelwork is made up of rafters and columns, which makes the form portal frames and bracing. The gable frame can be either a portal frame or a braced arrangement of columns and rafters.
The light gauge secondary steelwork is made up of the side rails which are used for the walls and the purlins that are used for the roof. The secondary steelwork offers support to the building envelope, but it also plays a very significant role in restraining the primary steelwork. The wall cladding and the roof separates the building envelope from the external environment as well as offering acoustic and thermal insulation. The function of cladding is to transfer the loads to the secondary steelwork and at the same time to restrain the flange of rails or purlins to which it is attached (Paolacci and Giannini 2012). The figure below shows the anatomy of a typical portal frame.
The key focus of this paper provides a discussion of the technical aspects of the portal frame construction. The area which is analyzed in this paper include; structural system, footing system, wall system, floor system, roof system, and services. The information which is contained in this paper is based on the deep investigation of Industrial unit 6D,1-3 Endeavour road. The photographs are included in the discussion part to over clarification. The areas which were mentioned in the assignment brief have been covered for simplicity.
The industrial unit is located within the center of the Sutherland Shire’s Industrial Precinct. It has a total area of approximately 850 m2. The ground floor is typically a warehouse of approximately 600m2 while the upper floor of 150m2 is for office. The warehouse has a parking capacity of 10 vehicles. The main structure is a fixed structure of steel portal frame with a clear span of 18m and a ceiling height of 7.0 m. The figures below are illustrations of the Industrial unit 6D,1-3 Endeavour road.
3.0 Footing system
Footing refers to the components of the building which transfers the load from the build to the foundation. The primary purpose of designing the footing is to ensure that the loads from the structure are safely transmitted to the subsoil, economically and at the same time ensuring that there is no unaccepted movement during the construction process and throughout the anticipated lifespan of the structure.
The factors which are usually put into consideration during the design of the footing system are; type of the structure, the soil conditions, economic factors, structural loadings, construction problems and the proposed construction period. Of all these factors structural loading and the soil condition are the most essential during the design process of the footings. The footing system which is properly designed can significantly help to eliminate or minimize the differential settlement which occurs when the weight of the structure stresses the soil. In the cases where no settlement occurs, it must occur equally under the building.
According to the geotechnical report of the industrial unit above the site, foundation materials are made up of sandy over the shale, having a high bearing capacity at a depth of about three meters. According to this data, it is reasonable to assume that there is some soil movement on the upper stratum of the foundation. Hence it is much safer to rest the footing at the shale level which offers sound bearing pressure with very minimal movement. The engineer used two types of footing during design.
A 150mm thick structural raft shaft having stiffened edge and integral beams over the piers were driven to the shale foundation. The main function of the raft is to distribute the load both the live and dead load evenly over a large base in order to reduce the load which is acting per unit area. The piers were used to bypass the reactive soil and at the same time transfer the superstructure load at the edge beam to the lower level of subsoil where suitable ABP exist. The figure below shows the arrangement of this structural members.
Another appropriate kind of footing possibly is pad footing. This is the most common footing style for the portal frame. A concrete pad footing is the easiest and cheapest type of footing used in vertical support and transmitting of structure loads to the subsoil. A pad footing of adequate size to stop uplift will be sufficient as long as it does not surpass an Acceptable Bearing Pressure of 100kPa. Occasionally deep pad footing has to be used to reach the soil lay providing sufficient APB, or a pedestal may be required to support the column base plate. The figure below shows how the pad footing is connected to the frame.
The images below which were obtained from the site shows a holding down bolt, a pad footing having HD bolt that are cast in situ and a universal column set into the concrete pad footing. It was observed that the holding down bolts were underneath the top layer of the additional concrete grout over the footing.
Each componponent of thye portal frame systen are discussed into details in this section.
4.1 Fly bracing
A fly brace refers toa trap, bar or angle which runs from the flange of the rafter, endwall column or central column to a wall or a roof batten and therefore refrains the section laterally. The top flange of beams will possibly be under compression due to loads of gravity. As the roof purlins try to restrain the top flange from buckling under the load, the system will then work efficiently. The primary function of the fly brace is to prevent a rafter or column from rotating or twisting when under load. The pictures below show some of the fly braces.
4.2 Column and rafter
The portal frame of the selected industrial unit is made from the 410UB54 column with 360UB56 rafters. The steel rafters were fabricated in a factory in that they are welded to the steel plate, and later they were bolted to the UB column rafters.At the ridge of the Industrial Unit, rafters are connected to each other with bolts. Steel fly bracing between steel purlins and purlins also can be seen. The system of this industrial unit can be considered to be a rigid portal frame system which gives excellent resistance to side wind load, at the same time support roof load, and it can be designed to carry the weight of the external wall cladding such as a precast concrete panel. The image below shows the columns and rafters.
The difference between the rafter and purlin
Rafters and purlins are the primary members of any given roof system. They are usually used to transmit the load from the load to the walls and later on to the foundation. They transfer the dead weight,live load, and wind load together with other loads which are acting on them. The rafters and purlins are like a two-way reinforcement of the roof. The purlins are ones which are parallel to the ridgeline or the ones which run along the span o0f the span of the roof while the rafters are elements which are perpendicular to the ridgeline of the roof truss.
The roof truss is usually supported at the columns, and the rafters are supported on the roof truss upon which the purlins are welded or bolted and upon the purlins lies the roof covering materials which may be asbestos sheets or any other roof covering materials.The figure below shows the difference between purlins and rafters.
4.3 Endwall column
An end wall column refers to the vertical member which is located at the end of any given wall which offers support to the girts. In the beam and column end frames. The end wall column also provides support for the beam which is also referred to as wind column. The figure below shows the detailing of the end wall beam connected to a rafter.
In the Industrial unit which was under investigation, the end frames have additional columns added to them. The end wall columns are also referred to as wind columns this is because they transfer the lateral load of the wind to the roof members and then later on to the wall members.With that, the weight is eventually distributed evenly in the entire structure. The extra columns make the end wall frames not to be strictly portal frames as the additional columns greatly help to share the load. Another function of the end wall columns is to offer to fix support to the wall members.
4.4 Girts and purlins
The Girts and purlins provide support for roof sheeting and cladding. The purlins are usually cold steel sections and in most case are connected to the rafter with the use cleat which can be bolted or welded on the rafter during the process of fabrication on site. The purlin is usually bolted to the cleat. Sarking and safety mesh is located between purlins and roof sheeting.girts refer to the cold steel sections that are bolted to the cleats of the columns of the portal rafters ensuring support for the wall cladding.The girts and purlins in the industrial unit are the ‘C’ sections.The ‘C’ sections are butt jointed.
he purlins depend on the cladding to offset the deflection and twisting due to the alignment of the purlins, but a single row of the standard bridging pieces is usually required.
Bridging is usually used between the sag rods and the purlins between the girts to overcome the rolling and the excessive deflection. One row of the bridge is typically common through two rows regularly are used and the bays where the girts or the purlins are transferring the lateral load to the inner braced frame.
4.5 Bracing
The portal frames are self-bracing in their plane, but the wind loads which are acting in right angle to the portal must be solved by the use of wall or roof bracing. This is often in the form of double diagonal bracing between the portal bays. Tubes, rods and the angles are very common for bracing. In this industrial unit 75 × 75 × 6 mm mild steel angle bracing is usually used to brace the roofing structure to help the structure to be stable when the lateral forces are applied on the roof
.5.0 Floor system
The selection of the floor system to be used in the building should involve an extensive consideration of all aspects of the building. Starting from the design of the structure, construction and the future uses of the building. Sometimes it is essential to compromise since the final selection of the floor system must meet some contradictory criteria.
Usually, the choice of the floor system has two meet two criteria. First, the finished flooring must be able to serve its intended purpose. That means that floor system after completion should behave well under the future service conditions, it should be free or minimal maintenance, be appealing of the finishes, easy to install among others. Secondary, the floor system must be structurally sound. It should have enough reserve of the strengthy against peak loadings and be able to behave acceptably under the standard conditions and loads.
When one compares the domestic buildings .the floor system of the industrial unit will encounter greater imposed loads such as forklifts and trucks which will be imposing rolling loads, and at the same time, the machinery in the factory will be imposing static loads. With that, the slab for the industrial unit must be able to support higher loads and tolerate less movement.
The industrial unit under investigation has a stiffened raft lab which is supported on the reinforcement concrete piers. It equivalents to the column-beam-two way slab system. The slab is supported on four sides and the load the then to a carried in two different directions. Hence this type of the design is adaptable to a wide range of different layouts and a wide range of layouts.Well organized grids result in the more economical use of materials. Incorporating the services within the floor slab is possible.The distribution of the suspended services can be taken through holes that are cast in the web of the beams within the mid-third of the span.The figure below indicates the layout of the floor slab, internal beams and piers(columns) of the selected industrial unit.
.1 Surface treatment
The selected industrial unit has a concrete floor which is treated chemically. Through the chemical reactions, the concrete floor hardened and a ‘case’ formed around the top of the concrete slab. With that, it creates a more solid more durable and robust finish which also is more appealing, resistant to the staining, deterioration and at the same time dust proof.
The product which was used in the industrial unit is Pentra-Sil (NL) which is a concrete sealer, and hardener formulated to improve the overall integrity, strength and the lifespan of the concrete surface. The treatment to the floor offers long-term protection against abrasion, dusting, efflorescence, waterborne contamination, alkalinity and harsh chemicals. The figure below shows the surface treatment of the industrial unit under investigation.
The product which was applied should only be applied when the temperature is above 20oC for not less than 4hours after the application. The Concrete should be adequately trowelled or finished by use of dry grinding, and be dry and at the same time to be clean.If the mentioned conditions are not met, the petra won’t be able to penetrate the surface equally. In case it is applied immediately after the finishing, or shortly after the joints have been cleaned and cut or any other time afterward.
An HVLP sprayer often needs to spray Pentra-Sil equally on the surface.Spread out any pools using smooth microfiber or a broom with soft bristles.The covers need to be maintained wet for at least 20minutes, adding more materials where there is a need. Surface will then be ready for use when it is already dry, but the complete reactions take up to two weeks to develop depending on the site temperature and the conditions.
There are usually four joints which are commonly found in the concrete slab construction. They include; contraction joints, construction joints, expansion joints and isolation joints. Nonetheless, it was very challenging to locate all the four joints in the chosen industrial unit which was under construction.
5.21 Contraction Joints
Usually, the concrete slab shrinks once it has dried or hardened, and it will never be the same length as it was when it was constructed. The shrinkage leads to cracking.The construction joints are weakened planes installed to predetermine the location of transverse cracks and at the same time to ensure that they occur in straight lines. In the plastic concrete, the weakened planes can be created by permanent or removable inserts or special tools. In the hardened concrete, they can be formed by the sawing diamonds or carbide-tipped blades as shown in the figure below.
5.22 Construction joints
Use of the chequerboard method poured the slab of the selected industrial Unit under investigation. There were individual bays of approximately 6metres that can be cast alternatively within the stop ends which forms the construction joints. Usually, the preplaced mesh reinforcement continuous through splits form on all the sides of each of the bay. The benefits of this technique are that they are very convenient construction method and they are easy to control contraction and shrinkage. In the massive slab construction, there is need to have dowels which should be placed across the construction joints. The dowel is required to be treated to allow horizontal movement which is not restricted as the joint closes and opens. The primary function of the construction joint is to facilitate easy and quick construction of large slabs.The figure below illustrates the longitudinal construction joints in the industrial unit selected.
5.23 Isolation joints
This type of the joints isolates the slab from the primary structure of the main structure of the building. where the slab meets with the primary structure of the building, for instance, a pedestal for the portal base, an isolation joint often required. The primary use of the isolation joint is to allow movement of the slab and at the same time isolate the slab movement from the fixed structure.
24 Expansion joint
This type of the joints is usually installed in the situations where the slabs are very large or in the cases where the structural system varies.hence need for the articulation of the structure. For instance, where part of the structure is on the good stable soil, and the other part is on poor expansive soil, then in such a situation there should be two different types of footing forcing the slab to be separated by use of a control joint. The joints are usually designed to allow movement of the slab generally in the plane of the slab. In most cases, there are no expansion joints that are required in the chosen industrial unit slab.
6.0 Wall sytems
All the external walls of the industrial unit that was selected for the study are made from vertical precast concrete panels. The figure below shows the external walls of the selected industrial unit.
6.1 Connctions and fixtures
The details of the panels to the footings
The panes are normally supported on the seating which is in the direct bearing. Its weight in most cases should be transmitted at one level by use of two seatings unless in the situations where the wall is supported longitudinally by the use of ground seatings. Different fixing techniques are, but the commonly used conncetion technique is the connection to the stepped footing.
load resistance
It is very essential that the walls of any given structure offer enough resistance mechanism from the applied loads. In most structures, the roof can function as a diaphragm to transmit the applied lateral loads on one set of the walls to the ones at right angles. Then later acts as a shear wall to resist all the applied loads.
7.0 Roof stsem for portal frame systems
The roof system of the portal frame systems is as shown below.
Fig 21:roof components for portal frame structures
7.1 Roof cladding
The roof of the Industrial unit which was selected for study is cladded by use of concealed fixed Lysaght Klip-lok 406. This is durable, robust and versatile wall and roof cladding materials. This type of materials usually combines the smart fluted pans, the strength of steel and the lock action rib which work together with the fastening, enables its use on the applications from low pitched which is as low as 1-degree roofs to the horizontal and vertical ribbed walling. The figure below shows the roof coverings that are used in the industrial unit under investigation
lysaght Klip lok 406 specificationsLysaght Klip lok 406 usually exists in two grades. The first one is typically the zinc/ aluminum alloy-coated steel. The second grade of Lysaght Klip lok 406 is the colour bond prepainted steel.Both of this grades have a metal thickness of 0.48mm.
7.2 Gutter and flashing details
A gutter refers to the surface or rainwater collection channel system for a building. The water from the pitched roofs usually flows into gutters whereby the water is further directed to the downpipe which discharges the water into the storage tanks or the stormwater drainage systems. In the selected industrial unit which was under study, the had a gutter which was connected to the downpipe which was directing water to the stormwater systems.
8.0 Service systems
8.1 Electricity and telecommunications
The primary electricity distribution and other services are located next to the main entrance of this industrial unit. Most of the electricity and services cables are laid underground, and in most cases, they are within the slab of the industrial unit structure/. Most of the wiring cables in the structure are surface mounted by use of bracket fix to the concrete panels directly.
8.2 Sewage and water
The industrial unit is supplied with the water and the sewage services. The water is obtained from the city council and is well distributed with the industrial unit. In short water in the building is sufficient to enable one to carry ou any industrial activity that may require plenty of water in the selected building. The sewage is also well connected in the structure which makes it easy to disposal any waste wastewater within the building. The images below show the sewage and water within the building.
8.3 Fire protection
In the selected industrial unit some measures have been put in place to ensure that incidents of fire are minimized for instance there are passive fire protection measures, active fire protection measures and fire alarm and detection system.
The passive fire protection systems are the measures that have been put in place to ensure that the spread of smoke and fire is controlled. That is achieved in most cases by the use of barriers which have characteristics which do not allow further spread of fire.
The active fire protection systems are the ones whereby the firefighting equipment are put in place to ensure that in case of incidents of fire. The fire can be quickly put off. In the selected industrial building there were fire extinguishers, sprinklers and another mechanism which help to fight the fire. The fire detection and alarm system is an active fire protection system which automatically detects fire and alerts the occupant’s use of alarms.
9.0 Conclusion
In conclusion, A portal frame structure usually comprises a series of transverse frames which are braced longitudinally. The primary steelwork is made up of rafters and columns, which makes the form portal frames and bracing. The gable frame can be either a portal frame or a braced arrangement of columns and rafters.
The factors which are usually put into consideration during the design of the footing system are; type of the structure, the soil conditions, economic factors, structural loadings, construction problems and the proposed construction period. Of all these factors structural loading and the soil condition are the most essential during the design process of the footings. The footing system which is properly designed can significantly help to eliminate or minimize the differential settlement which occurs when the weight of the structure stresses the soil.
The light gauge secondary steelwork is made up of the side rails which are used for the walls and the purlins that are used for the roof. The secondary steelwork offers support to the building envelope, but it also plays a very significant role in restraining the primary steelwork. The wall cladding and the roof separates the building envelope from the external environment as well as offering acoustic and thermal insulation.
Rafters and purlins are the essential members of any given roof system. They are usually used to transmit the load from the load to the walls and later on to the foundation. They transfer the dead load, live load, and wind load together with other loads which are acting on them. The rafters and purlins are like a two-way reinforcement of the roof. The purlins are ones which are parallel to the ridgeline or the ones which run along the span o0f the span of the roof while the rafters are elements .
which are An end wall column refers to the vertical member which is located at the end of any given wall which offers support to the girts. In the beam and column end frames. The end wall column also provides support for the beam which is also referred to as wind column. A fly brace refers toa trap, bar or angle which runs from the flange of the rafter, endwall column or central column to a wall or a roof batten and therefore refrains the section laterally. The top flange of rafters will possibly be under compression due to loads of gravity. As the roof purlins try to restrain the top flange from buckling under the load, the system will then work efficiently.
Reference
Hiriyur, B. K. (2010). U.S. Patent Application No. 12/178,078.
Knudson, G. A., & Flood, P. D. (2010). U.S. Patent No. 7,752,812. Washington, DC: U.S. Patent and Trademark Office
Paolacci, F., & Giannini, R. (2012). An experimental and numerical investigation on the cyclic response of a portal frame pier belonging to an old reinforced concrete viaduct. Earthquake Engineering & Structural Dynamics, 41(6), 1109-1127.
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