Steel portal frame
The steel portal frame refers to the steel frame
which is normally the first part of the
superstructure to go up, it consists of a strong
steel frame that will support the main structure of
the building. Building the super structure will
normally require some form of crane to move the
steel and a cherry picker for workers to bolt the
section together.
The frame will be made off site and bolted
together on site, any errors that need to be
rectified in site will be very costly as drilling steel takes a long time and cutting steel will often not
be possible on site.
Substructure
Raft foundations are often used as the buildings
normally cover a large area, before this is poured
pads or piers are carefully poured in concrete with L
bolts inside them to support the base of the portal
frame. The raft foundation is then poured between
them.
Roof
The roof will normally be some form of metal sheeting,
historically this has been transported to site in and
craned into place, this can often lead to the sheets
getting damaged. A more recent method of roofing is to take a large roll of aluminium to site and
shape it in site. The aluminium is shaped to provide more rigidity and stop the sheet from flexing.
Steel portal frames have the advantage of the roof being completed very early in the construction
phase, the raft foundation can be poured after the roof has been completed providing a level of
protection to the workers and excavations from the weather. This is a popular way to build portal
frames in colder countries.
As steel is a very strong material it allows for a shallow pitch of roof to be
achieve, this allow for more usable storage inside the building. Whilst portal
frame buildings are generally large, the building would be be higher still if
they had to use wood and tiled roofs due to the steep pitch required
Steel
The steel used is normally called I section
steel which refers to the shape of the cross
section, this provides stability for steel and
prevents twisting and flexing, whilst
minimising the amount of steel used.
Walls
The walls are generally of a cavity wall construction, as the purpose of the building is normally
functional, aesthetics will be a secondary consideration to cost. Walls will often be built from just
block work with a central insulation layer, but precast concrete can also be used.
Internal finish
Portal frame buildings are generally designed for the storage of goods or manufacturing, the internal
finish is normally functional rather than based around good looks, conventional plaster board
techniques can be used but it more common for the inside of the external block walls to be painted
directly. Internal stud walls can be built when required to provide staff rooms and small offices,
these are often clad with dry wall and the joints taped and filled, this is a quicker method than
plastering the walls.
If a second floor is installed this will often be supported by its own steel work or load bearing wall. If
these do not form part of the external structure they can be removed at a later date if required by
the client without affecting the external envelopes stability or ability to keep out weather.