Steel frame house in Batumi: a frame of hot-rolled structural steel

What a steel house frame is made of

The steel frame of a house is assembled from hot-rolled sections: columns of I-beams or square hollow sections, floor beams of I-beams, and bracing and purlins of angles, channels and smaller hollow sections. The parts are cut, drilled and welded at the factory, and on the plot they are bolted together. We keep site welding for the occasional joint: a weld made in the field is harder to inspect, and every time the protection around it has to be restored.

On steel beams we usually make cast-in-place concrete floors on profiled steel decking. The decking serves as permanent formwork, the reinforcement and concrete go on top of it, and the slab carries the screed, tiles and partitions. The walls are filled in with aerated concrete on flexible ties with a gap at the columns, with a light frame or with glass, and in commercial buildings with sandwich panels. This frame should not be confused with thin galvanised sections: the difference is described in the comparison of structural steel and light gauge steel.

Drawings for the factory: the steel design is finished before the metal is ordered

From the structural engineer's calculations, detailed fabrication drawings (shop drawings) are produced: a separate sheet for each column and beam, with its length, holes, stiffeners and plates. The steel is cut and drilled to these drawings, and after that a part can no longer be adjusted on site. A hole that is a couple of centimetres out means a new plate, welding on site and protecting the joint all over again.

So before the steel is ordered, we bring together on one drawing everything that is fixed to the frame: the window openings, the staircase, ventilation shafts, drains passing through beams, and the fixing plates for the facade and the pergola. The vertical bracing between columns also ends up on the facade, so the architect can see straight away where a window won't fit. Anchor bolts are cast into the foundations using a steel template, and once the columns are aligned, non-shrink grout is poured under their base plates.

Corrosion protection by the sea: a paint system, coat by coat

On the coast, protection for steel is chosen according to the corrosivity category of the plot, and the closer to the water, the higher the category. What these categories are, and why hot-dip galvanising or a multi-coat paint system is used right by the shore, is explained in the comparison of concrete and steel frames. In practice our protection works like this:

• at the factory the steel is blast-cleaned down to bare metal, because primer does not hold on rust and mill scale;
• straight after blasting, a zinc-rich primer is applied, followed by the intermediate and top coats of the same system from the same manufacturer;
• the thickness of each coat is written into the estimate and checked with a coating thickness gauge before dispatch, and the readings go into the handover file;
• exposed members, such as terrace beams and canopy posts, are hot-dip galvanised and then painted, and closed hollow sections are given holes so that the zinc gets inside too;
• after erection we touch up the bolts, washers and every scratch left by the lifting slings.

A frame inside the warm, dry envelope needs only a basic system. For exposed steel we put an inspection schedule and the name of the paint system into the handover file: chips are later touched up with the same product, because a different paint may not bond to the old coats.

Fire protection for steel columns and beams

Steel doesn't burn, but in a fire it heats up quickly and loses strength, and an unprotected beam can sag before the fire brigade arrives. How many minutes the frame has to carry its load in a fire depends on what the building is used for and on its class. This requirement is set in the design by the structural engineer and the architect under the fire regulations.

There are three ways to protect the steel. Intumescent paint swells into a thick layer of foam when heated, and it suits beams that stay exposed. Fire boards or several layers of plasterboard enclose columns inside walls and beams above a suspended ceiling, and this is usually cheaper than paint. Concrete around a column gives the most reliable protection, but it adds weight. We apply intumescent paint only over a primer approved by its manufacturer, otherwise the coats come away together.

Thermal bridges and condensation on steel beams

A steel beam that runs from a warm living room out under a terrace or canopy conducts the cold into the house. In winter condensation forms on it, the ceiling around it darkens, and the steel rusts under the finishes, where nobody can see it. We build these joints with a thermal break: the external cantilever is fixed to the internal beam through a structural thermal break element or pads, as the calculation requires.

Columns in the plane of an external wall are covered on the outside with the same insulation as the wall, without a gap. Rooms with humid air get special attention: in a bathroom, a laundry room or by a pool, a steel beam under the ceiling is wrapped in insulation and a vapour barrier, otherwise it becomes the coldest surface in the room.

Foundations, erection and seismic design

A steel house is lighter than a concrete one, but its load is concentrated in the columns, and each of them needs its own foundation, or the house needs a common slab. The type is chosen by the structural engineer according to the ground on the plot. Erection needs a crane or a crane truck with space to stand, and we look for that space at the sketch stage. On a narrow street or a slope, long beams are split into pieces with bolted splices.

Steel is ductile: in a strong tremor the beams and bracing can deform noticeably and still hold up the floors. You can rely on this only if the joints are made as calculated. So we inspect the welds on factory parts before painting, tighten the bolts with a torque wrench to the value given in the design, and photograph the joints that carry seismic loads before the ceiling and linings cover them.

Where a steel frame pays off

Steel comes out ahead when a concrete beam runs up against limits of depth or weight. That means a villa with a column-free living room and a glass wall facing the sea, an extra storey on an old house that cannot take any more weight, a terrace or canopy on slender supports, or an external staircase. For an extra storey, we first survey the old house and decide whether the steel storey will sit on its walls or on its own columns with separate foundations. In commercial construction, a steel frame is almost always the first option for a warehouse, workshop, showroom or gym.

For an ordinary two-storey house with rooms 4–5 m across, we more often recommend a cast-in-place concrete frame. Steel with corrosion protection and fire protection costs more, and the time saved is eaten up by the drawings and the factory fabrication.

How much a steel frame house costs in Batumi

The shell of a steel frame house costs from 990 to 1,200 ₾ per m² of floor area. The price includes the foundations, rolled steel columns and beams with corrosion protection, the floors, wall infill and the roof. Windows, building services and finishing are priced separately, and so is the design. A turnkey steel frame house starts from 1,920 ₾ per m².

The frame itself, meaning the columns, beams and bracing with fabrication, priming and erection, costs from 380 to 540 ₾ per m² of the house's floor area. Heavy-duty coastal painting adds from 45 to 60 ₾ per m² of steel surface, and a cast-in-place floor slab from 305 ₾ per m². For comparison, a light gauge steel shell starts from 900 ₾ per m², and a cast-in-place concrete shell from 1,110 ₾.

The price goes up with long spans and cantilevers, hot-dip galvanising of exposed members, fire protection of exposed beams and crane work in a tight spot. It comes down with a repeating column grid and a frame hidden entirely inside the warm envelope. We send a preliminary estimate from a sketch within 24 hours on business days, and the final cost is set on the basis of the design, after a visit to the plot and once the estimate is agreed.