Strengthening steel structures: methods and how to choose

Why structures are strengthened

There are two reasons. First, the load capacity has dropped because of defects and damage, and has to be brought back to the design value. Second, loads have grown during reconstruction, and the structure must carry more than the design allowed for. Often both apply at once, because the structure being strengthened is already worn.

In the extreme case strengthening means replacing the member. Whether strengthening or replacement is cheaper is an economic question, and the main item in it is production downtime during the work. So the options are compared with downtime in mind, not by steel cost.

Before choosing a method you need the survey results: the actual condition, the steel properties and the loads acting. These have their own articles: diagnostics and condition assessment and clarifying steel properties.

Two groups of methods

All the solutions come down to two groups. In the first, the structural form and the design scheme stay the same and metal is simply added to the section. In the second, the forces in the existing members are adjusted: supports are added, the design scheme is changed, or prestressed elements are introduced.

Adding section

This is the most common method, and it is the closest to designing a new structure, with one major difference. New elements are placed not where it would be best, but where they can be fitted and fixed in a working shop with minimum dismantling. So the shape of the strengthened structure is usually not the best one for a new design.

The existing structure cannot be moved, and that dictates the details. Weld positions and the possibility of drilling holes and fitting bolts have to be considered in advance. Manual welding remains the main way of attaching, and the design requirement is as few overhead and vertical welds as possible. High-strength bolts are used less often, because holes are needed in the existing structure and that noticeably increases labour.

The existing section carries the dead load, or part of it, and the service loads are then carried together: existing metal plus the strengthening. That is why this method does not work without unloading.

Extra supports and changing the scheme

The second route is not to add metal but to reduce the forces. An extra support shortens the span. Trussing adds extra members and makes the system statically indeterminate. Closing the hinges at the supports turns a simply supported beam into a continuous one: forces are redistributed and the mid-span moment drops.

These methods even out the force diagrams, use the metal of the existing structure better and concentrate the strengthening in a few places. Like adding section, they need partial unloading.

Prestressing

This method is considered the most promising for steel structures. There are three reasons: high-strength tie material is brought into the design section, the existing metal is used more fully through adjusting the stresses, and there is a minimum of work to attach the strengthening.

There is a price. Prestressed elements need effort to maintain the specified force throughout service. One variant only increases the beam stiffness without raising the load capacity: it limits the working deflection and raises the fatigue strength of the top zone.

Example: crane runway beams

A crane runway beam is convenient for comparison: every method works effectively on it. The table gathers which spans and cranes suit each option best.

What goes into the calculation

Strengthening is calculated by limit states under the current standards. For the first limit state the initial load that acted before strengthening and the additional load applied afterwards are taken. For the second, the characteristic values of the same two parts.

From there the logic is as follows. In tension and centrally compressed members the strengthening parts carry only the increase in force from loads added after strengthening. Compressed members have one more condition: buckling happens in the member with its new section, so the slenderness after strengthening goes into the calculation. For a first estimate of the strengthening area the buckling coefficient is taken at the slenderness before strengthening and then refined at the actual slenderness of the new section.

Eccentrically compressed members are strengthened while the edge stresses have not exceeded a certain share of the design resistance. The strengthening parts are fixed to the main section with continuous small welds, usually 3, 4, 5 or 6 mm. For tension members and joints, continuous welds 3–6 mm high are also recommended.

Beams have their own features. For a working shop the working condition factor is taken as 0.9, allowing for the service condition. For asymmetric strengthening an asymmetry factor of 1.1–1.5 is used and the chosen flanges are checked by section modulus. The theoretical cut-off point of the strengthening elements is found from the moment condition, and the elements are carried past that point by a length enough to transfer the force. At the cut-off the strength is also checked by the reduced stresses.

In trussed beams the design scheme is statically indeterminate, so the stiffnesses of the strengthening elements are assumed first. If after the calculation they differ from the assumed ones by more than 30 %, the calculation is repeated. A prestressed beam is calculated like an ordinary prestressed one.

What this means for fabrication

Strengthening elements are mostly plate and strip parts, cover plates, trusses and tie rods. They are made to shop drawings (KMD). The special point is that the existing structure cannot be moved, so the drawings must allow for the weld positions, the hole positions and the welder’s access on site.

In the workshop we fabricate steel structures to the customer’s drawings and prepare KM and KMD drawings when needed: KM and KMD design, steel fabrication. A survey before strengthening is a separate service. Erection on site is done by our partner construction company.

Standards and validity

The condition of the structure before strengthening is assessed under the current survey procedure and method (see the survey articles above), and the strengthening is calculated under the current steel design standards: DBN V.2.6-198:2014 «Steel structures. Design standards» and DBN V.1.2-14:2018 «General principles of reliability and structural safety of buildings and structures». Decisions are checked against the current editions of these documents.