Steel portal frame or concrete? Choosing a warehouse structure in Kenya
If you are planning a warehouse, factory or storage building in Kenya, one of the first decisions is the structural system. The two most common options are a steel portal frame and a reinforced concrete frame. Both can produce a good building. They suit different needs, and the right choice depends on how the building will be used.
What is a portal frame?
A portal frame is a series of steel frames, each made of two columns and two sloping rafters, joined rigidly at the eaves and the apex. The frames are spaced along the building at regular intervals, called bays. They are tied together with purlins on the roof, girts on the walls and bracing for stability, and then clad with roof and wall sheeting.
The rigid joints let a portal frame span a wide space without internal columns. That is why it is the standard form for warehouses and industrial sheds around the world.
Clear span and usable floor space
For a warehouse, open floor space is usually the priority. Racking, forklifts and trucks all work better without columns in the way.
Steel portal frames are well suited to wide clear spans. A reinforced concrete frame covering the same floor area typically needs more internal columns, or much deeper beams, to carry the roof. If you need a large, uninterrupted floor, steel usually has the advantage.
Speed of construction
Steel members are fabricated in a workshop while the foundations are being built on site. When the foundations are ready, the frame can be erected quickly, because the members arrive ready to bolt together.
A concrete frame is mostly built in place. Formwork has to be erected, reinforcement fixed, concrete poured and then left to cure before the next stage can proceed. For a large single-storey building, this usually takes longer. The actual time for either system depends on the size of the building, the site and the programme.
What drives the cost
It is not possible to say that one system is always cheaper. Costs depend on span, height, loads, site conditions and material prices at the time. What we can say is what drives each one.
- Steel frames: the biggest single factor is the weight of steel, which depends on span, eaves height, bay spacing and loads. Fabrication, erection, roofing and cladding add to that.
- Concrete frames: costs are driven by the volume of concrete, the quantity of reinforcement, formwork and the labour and time needed on site.
For wide-span, single-storey buildings, a well-engineered steel portal frame is often the more economical option. For smaller spans or multi-storey buildings, the comparison can go either way. The way to know for your project is to price both on the same brief.
Foundations
A steel frame is lighter than a comparable concrete frame, so the loads reaching the ground are generally lower. That can mean smaller foundations. Portal frames also push outwards at the base of the columns, and the foundations must be designed to resist that. Either way, the foundation design should be based on the actual soil conditions on your site.
Durability and maintenance
Concrete is naturally robust and needs little routine maintenance when it is well built. Steel needs protection from corrosion. That means a suitable coating system, and regular inspection of exposed steel, particularly in coastal areas such as Mombasa or in buildings with a humid or chemical environment. With the right protection and basic maintenance, a steel structure has a long service life.
Flexibility for the future
Businesses grow and change. A steel portal frame building can often be extended by adding bays at the end, and existing frames can be strengthened to carry new loads such as mezzanines or equipment. Modifying a concrete frame is generally more disruptive and more expensive.
So which should you choose?
A steel portal frame is usually a strong choice when you need:
- A wide, column-free floor for storage, manufacturing or logistics
- A shorter construction programme
- The option to extend or modify the building later
Concrete may suit better where spans are small, where the building is multi-storey with heavy floor loads, or where the design calls for masonry and concrete throughout.
Many buildings combine the two: a concrete ground slab, plinth walls and foundations, with a steel frame and roof above. Handled by one team, the two parts fit together without interface problems.
Get a figure to work with
If a steel portal frame looks like the right fit, our Request a Quote tool gives you an instant preliminary estimate from your building's length, width and eaves height. Our engineers will then review it with you and look at the options in more detail.