Steel structural systems and erection stability

Structural steel is fast, light for its strength and largely prefabricated. Members arrive cut, drilled and ready to bolt or weld, and a skilled erection crew can frame several floors in a week. That speed comes with a particular risk: a steel frame is at its most vulnerable while it is being erected, before the floors, bracing and connections that make it stable are complete. Many of the worst steel failures have happened during erection, not in service. This lesson explains how steel buildings work, how they are connected and braced, how composite floors are built, and what constructors must do to keep a partly erected frame stable.

Steel as a structural material

Structural steel is strong and ductile: it yields and stretches a long way before it breaks, giving warning and redistributing load. It is equally strong in tension and compression, although slender compression members are limited by buckling. Its weaknesses are loss of strength at high temperature, which is why fire protection is needed, and corrosion, which is why exposed steel is painted or galvanized.

In the United States, structural steel design follows AISC 360 (the 2022 edition is current), and wide-flange shapes are typically ASTM A992 (yield strength 50 ksi). Shapes are designated by nominal depth in inches and weight in pounds per foot, such as W12x40.

How steel frames are put together

A typical steel building has columns at a regular grid, girders spanning between columns, and beams or joists spanning between girders, carrying a steel deck. The connections between these members decide how the frame behaves.

Simple (shear) connections, such as shear tabs, double angles or end plates bolted through the web only, transfer vertical shear but allow the beam end to rotate. They are cheap and quick, but a frame made only of simple connections has no lateral stiffness and needs bracing or walls for stability.

Moment connections, with flanges welded or bolted to the column, transfer bending as well as shear and stop the joint from rotating. Moment frames resist lateral load through bending of beams and columns but are more expensive and heavier.

Most steel buildings use simple connections for gravity framing and a separate lateral system: braced frames (diagonal steel braces in selected bays), moment frames in selected bays, or concrete cores and shear walls. The lateral system is only effective once it is complete and connected to the floors that deliver load to it. Until then, the frame relies on temporary bracing.

Bolting and welding

Most field connections are bolted, because bolting is fast and easy to inspect; welding is done mostly in the shop. High-strength bolts are used in two broad ways:

• Snug-tight bearing connections, where the bolts are tightened by an ironworker with a spud wrench and the joint carries load by the bolts bearing against the holes.
• Pretensioned or slip-critical connections, where bolts are tightened to a specified minimum tension, using turn-of-nut, calibrated wrench, twist-off tension-control bolts or direct tension indicators, so the plates clamp together. Slip-critical joints carry load by friction and are used where slip would be harmful, such as in braced frames subject to reversals or oversized holes.

In the United States, high-strength bolting follows the RCSC Specification for Structural Joints with ASTM F3125 bolts (grades A325 and A490), and welding follows AWS D1.1 with qualified welders.

Field welding needs qualified welders, appropriate weather protection, preheat in cold conditions, fire watch and inspection. Special inspection or third-party inspection of bolting and welding is normally required by the building code and specifications.

Composite floors

Most steel floors are composite: a concrete slab is poured on a steel deck, and shear studs welded through the deck to the beams make the slab and beam act together. The slab becomes the compression flange of a deeper, stiffer beam, allowing lighter steel. Composite action exists only after the concrete hardens. During the pour, the bare steel beam and deck carry the wet concrete, workers and equipment on their own.

Construction issues for composite floors include:

• Beam deflection under wet concrete. Unshored beams sag during the pour; designers may camber beams (curve them upward in the shop) or specify the slab be placed to a constant thickness, which adds concrete, or to a level top, which thins the slab at mid-span.
• Deck as formwork. Deck must be fastened (puddle welds, screws or powder-actuated pins) to supports as specified before it is used as a working platform, and its span limits for wet concrete must not be exceeded.
• Concentrated loads. Piling bundles of rebar or deck on unbraced beams or unfinished deck can overload them.
• Shear stud quality. Studs are bend-tested and inspected; wet or galvanized deck can cause poor welds.

Erection stability

A partly erected steel frame is a collection of members held together by a few bolts, with no floors, no permanent bracing and often only part of the lateral system. Wind, out-of-plumb columns, crane impacts and unbalanced loads can topple it. The erector is responsible for stability during erection, using temporary bracing, guys and an erection sequence planned so that each stage is stable.

In the United States, OSHA 29 CFR 1926 Subpart R (Steel Erection) sets specific requirements. Before steel erection starts, the controlling contractor must notify the erector in writing that the concrete in footings, piers and walls has reached 75 percent of its intended minimum compressive design strength or sufficient strength to support erection loads (1926.752). Columns need at least four anchor rods (1926.755). Each beam connection needs at least two bolts per connection, drawn up wrench-tight, before the hoisting line is released (1926.756). Open-web steel joists need bridging installed before workers or loads are placed on them (1926.757).

Common erection stability controls include:

• Anchor rods correctly located, embedded and grouted, with leveling nuts or shims as designed; columns plumbed and guyed until braced.
• Minimum bolting before releasing the crane hook, and full bolting of connections as erection proceeds.
• An erection sequence that closes braced bays early, so stable 'boxes' form and the rest of the frame can be tied to them.
• Temporary cables or braces sized for wind and construction loads, kept in place until permanent bracing, deck and diaphragm are complete.
• Wind limits for erection, and securing loose bundles of deck and joists at the end of each shift.
• Bridging on joists, and not placing loads on joists or deck until they are attached and bridged.

Light steel, joists and cold-formed framing

Open-web steel joists and joist girders are efficient but slender and unstable until bridging is installed. Workers walking on unbridged joists, or landing bundles of deck on them, have caused many falls and collapses. Joist manufacturers specify bridging rows, and some joists need erection bridging bolted in before the hoisting cable is released.

Cold-formed steel framing (light gauge studs and joists) is common for infill walls, mid-rise load-bearing walls and trusses. It relies on bracing, blocking and sheathing for stability, and its thin sections are easily damaged by careless handling or cutting. Load-bearing cold-formed systems are engineered and must be built to the shop drawings, including strap bracing and hold-downs.

Inspection and hold points for steel

Steel erection proceeds quickly, so inspection has to keep pace. A practical inspection and test plan for a steel frame sets hold points at the stages where errors become hard to correct: anchor rod survey before columns are set, column plumbness before connections are fully bolted, bolting and welding inspection before deck is laid over connections, and shear stud and deck fastening inspection before the slab is poured. Each hold point names who inspects, what records are kept and who releases the work.

Survey records matter as much as structural inspection. Columns out of plumb throw beams out of line, create eccentric loads and make cladding and curtain wall difficult to fit. Erection tolerances are set in the specifications, typically based on the AISC Code of Standard Practice in the United States and CSA S16 and the CISC Code of Standard Practice in Canada, and the erector checks plumb as each tier is completed, before connections are finally tightened.

Non-conformances, such as misaligned holes, short bearing, missing stiffeners or reamed or slotted holes made in the field, must be reported to the engineer rather than fixed by the crew with a torch. Field modifications such as enlarging holes, cutting flanges or adding welds can reduce capacity or create brittle conditions, so they need written engineering approval.