Portal frame
Encyclopedia
Portal frame construction is a method of building and designing structures, primarily using steel
or steel-reinforced precast concrete
although they can also be constructed using laminated timber such as glulam
. The connections between the column
s and the rafter
s are designed to be moment
-resistant, i.e. they can carry bending forces.
Because of these very strong and rigid joints some of the bending moment in the rafters is transferred to the columns. This means that the size of the rafters can be reduced or the span can be increased for the same size rafters. This makes portal frames a very efficient construction technique to use for wide span buildings.
Portal frame construction is therefore typically seen in warehouse
s, barn
s and other places where large, open spaces are required at low cost and a pitched roof is acceptable.
Generally portal frames are used for single story buildings but they can be used for low rise buildings with several floors where they can be economic if the floors do not span right across the building (in these circumstances a skeleton frame, with internal columns, would be a more economic choice). A typical configuration might be where there is office space built against one wall of a warehouse.
Portal frames can be clad with all sorts of material but the most popular solution, for reasons of economy and speed, is some form of lightweight insulated metal cladding with cavity masonry work to the bottom 2m of the wall to provide security and impact resistance. The lightweight cladding would be carried on sheeting rails spanning between the columns of the portal frames.
Portal Frames can be defined as two dimensional rigid frames which have the basic characteristics of a rigid joint between column and beam.
The main objective of this form of design is to reduce bending moment in the beam which allows the frame to act as one structural unit.
The transfer of stresses from the beam to the column results in rotational movement at the foundation which can be overcome by the introduction of a pin/hinge joint.
For warehouses and industrial buildings, sloping roof made of purlins and ac sheet roofing between portals is provided. For assembly halls, portals with R.C slab roof cast monolithically is used.
Portal Frames are designed for the following loads:
While designing, care should be taken for proper
If the joints at B, C, and D are not rigid, they will "open up" and the frame will be unstable
when subjected to loads. This is the pack of cards effect.
1.Vertical loading results in A and E pushed outwards. If the foundation can't resist horizontal push, outward movement will occur and the frame will lose strength.
2.Wind subjects the frame to uplift forces. Overturning forces on the sides & ends of the building. Drag forces on the roof and sides.
3.These destabilizing forces are resisted essentially by the weight of the building and in this regard, the foundations contribute significantly to this weight. The foundations are regarded as building's anchors.
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
or steel-reinforced precast concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
although they can also be constructed using laminated timber such as glulam
Glued laminated timber
Glued laminated timber, also called Glulam, is a type of structural timber product composed of several layers of dimensioned timber bonded together with durable, moisture-resistant adhesives. This material is called 'laminating stock' or lamstock for short.By laminating several smaller pieces of...
. The connections between the column
Column
A column or pillar in architecture and structural engineering is a vertical structural element that transmits, through compression, the weight of the structure above to other structural elements below. For the purpose of wind or earthquake engineering, columns may be designed to resist lateral forces...
s and the rafter
Rafter
A rafter is one of a series of sloped structural members , that extend from the ridge or hip to the downslope perimeter or eave, designed to support the roof deck and its associated loads.-Design:...
s are designed to be moment
Moment (physics)
In physics, the term moment can refer to many different concepts:*Moment of force is the tendency of a force to twist or rotate an object; see the article torque for details. This is an important, basic concept in engineering and physics. A moment is valued mathematically as the product of the...
-resistant, i.e. they can carry bending forces.
Because of these very strong and rigid joints some of the bending moment in the rafters is transferred to the columns. This means that the size of the rafters can be reduced or the span can be increased for the same size rafters. This makes portal frames a very efficient construction technique to use for wide span buildings.
Portal frame construction is therefore typically seen in warehouse
Warehouse
A warehouse is a commercial building for storage of goods. Warehouses are used by manufacturers, importers, exporters, wholesalers, transport businesses, customs, etc. They are usually large plain buildings in industrial areas of cities and towns. They usually have loading docks to load and unload...
s, barn
Barn
A barn is an agricultural building used for storage and as a covered workplace. It may sometimes be used to house livestock or to store farming vehicles and equipment...
s and other places where large, open spaces are required at low cost and a pitched roof is acceptable.
Generally portal frames are used for single story buildings but they can be used for low rise buildings with several floors where they can be economic if the floors do not span right across the building (in these circumstances a skeleton frame, with internal columns, would be a more economic choice). A typical configuration might be where there is office space built against one wall of a warehouse.
Portal frames can be clad with all sorts of material but the most popular solution, for reasons of economy and speed, is some form of lightweight insulated metal cladding with cavity masonry work to the bottom 2m of the wall to provide security and impact resistance. The lightweight cladding would be carried on sheeting rails spanning between the columns of the portal frames.
Portal Frames can be defined as two dimensional rigid frames which have the basic characteristics of a rigid joint between column and beam.
The main objective of this form of design is to reduce bending moment in the beam which allows the frame to act as one structural unit.
The transfer of stresses from the beam to the column results in rotational movement at the foundation which can be overcome by the introduction of a pin/hinge joint.
For warehouses and industrial buildings, sloping roof made of purlins and ac sheet roofing between portals is provided. For assembly halls, portals with R.C slab roof cast monolithically is used.
Portal Frames are designed for the following loads:
- roof load
- wind load
While designing, care should be taken for proper
- joints
- foundation
- bracing
If the joints at B, C, and D are not rigid, they will "open up" and the frame will be unstable
when subjected to loads. This is the pack of cards effect.
1.Vertical loading results in A and E pushed outwards. If the foundation can't resist horizontal push, outward movement will occur and the frame will lose strength.
2.Wind subjects the frame to uplift forces. Overturning forces on the sides & ends of the building. Drag forces on the roof and sides.
3.These destabilizing forces are resisted essentially by the weight of the building and in this regard, the foundations contribute significantly to this weight. The foundations are regarded as building's anchors.