Typical damage to steel structures and repair methods: cracks in the metal, deformations, loose connections

Where to start and what must not be made worse

Repair starts from a list of defects and damage drawn up from the survey results. For each defect it is determined whether it is dangerous now and whether it can develop. The defect categories and what to do with a category A defect are covered in the article Condition assessment of steel structures.

Some requirements apply to any repair. The repair must not reduce strength and spatial stiffness, so members cannot be removed in a way that changes the layout. The transfer of forces from the main members to the strengthened ones must be smooth. Strengthening or replacement must not increase the eccentricity in joints. The possibility of dismantling a member for replacement is checked by calculation. The surfaces being repaired are cleaned of dirt, oil and corrosion.

The working drawings for the repair are KMD drawings, and the designer of the project decides on removing, strengthening or replacing damaged structures. Welding of critical members is given to welders with a valid certification. The metal for the repair is not inferior to the existing one in mechanical properties, toughness and weldability; if the grade is unknown, it is first determined by chemical analysis and tests (see clarifying steel properties).

When the structure cannot be unloaded, the initial stress level is checked before welding: the ratio of the highest stress at the time of repair to the design strength of the member. The guideline limits are: up to 0.2 for especially severe conditions (crane runway beams under heavy duty cranes, members of hopper and unloading trestles), up to 0.4 for members carrying moving, dynamic or vibrating loads, up to 0.8 for the rest. If the limit is exceeded, the structure is first unloaded, or measures are taken to limit deformations, including welding ones.

Impact is forbidden on welded structures during erection: for steels with a yield strength up to 390 MPa below −25 °C, for steels above 390 MPa below 0 °C.

Overview: damage and typical methods

The table gathers what is covered below: which repair method is typical, and where repair ends and replacement begins.

Cracks in the parent metal

A crack in the parent metal is a category A defect: it can turn into a rupture. So it is removed in a strict order.

1. The crack is found along its whole length and its start and end are located. For this the surface is wetted with kerosene, wiped dry and coated with chalk, or etched, or magnetic particle inspection is used.

2. The ends of the crack are limited with through holes 5–8 mm in diameter. The centre of the hole is set at a distance of half the drill diameter from the visible end of the crack.

3. The edges along the crack are prepared as required by the standards for the structural elements of welded joints. The limiting holes are countersunk.

4. The adjoining zone is cleaned for 20–25 mm on both sides and welded in several passes so as not to overheat the parent metal. The limiting holes must be welded up.

5. Completeness of removal is checked: by examination after etching with a 10 % solution of nitric acid and, where needed, by dye penetrant, fluorescent or magnetic inspection.

Incomplete welding, where a part with broken continuity is left in the member, is not permitted. For part-depth cracks in massive parts the edges are bevelled to the full depth of the crack, and while the groove is prepared the depth is checked by drilling and, where needed, by surface inspection.

In a rigid contour, for example a closed cut-out one side of which is less than 60 plate thicknesses, measures to reduce welding stresses are needed, together with welding consumables giving a highly ductile weld metal. If the crack is long and dangerous by position, say across the direction of the force, the member is either replaced, or welded with radiographic inspection of the whole length of the weld, or an insert is butt-welded in.

Members that cannot be repaired and are replaced include those with two or more fatigue cracks, cracks running out to fixing holes, repeat cracks beside or at a former weld repair, and sharp bends at the places of deformation. Fatigue cracks in crane runway beams under heavy duty cranes (7K and 8K) may be repaired only as a temporary measure until production can be stopped for replacement. For cranes of other duties the repair is designed for long service.

Cracks in building structures in general, including concrete and masonry, are covered in the article Cracks in Building Structures.

Weld defects and strengthening of welds

Defective welds are removed by gas cutting or air-arc gouging and cleaned to bright metal. Welds with cracks, lack of fusion and other unacceptable defects are cut out over the length of the defective place plus 15 mm each side and welded again. Undercuts in the parent metal above the permitted size are ground and welded with a smooth transition to the deposited metal. The same weld is redone not more than three times.

When a weld falls short of the design size it is strengthened by increasing its length or thickness. Length is better, because short welds create stress concentration. The parameters are found from the difference between the design force after strengthening and the capacity of the existing joint. Deposition of new layers is used when there is no room for new welds, and as little heat as possible is put into the part.

In structures strengthened under load, welds across the acting forces are avoided, and where they cannot be avoided, special measures are worked out. New welds are placed in the least stressed sections, far from changes of section, cut-outs and stiffeners, without bunching and without crossing at a sharp angle. The distance between parallel butt welds, and between butt and fillet welds, is not less than 10 thicknesses of the member, and the angle between two butt welds is not less than 60°. Thin long welds are better than thick short ones. For dynamically loaded structures intermittent welds are not permitted.

Strengthening of members by welding goes in this order: the strengthening members are clamped along their whole length, tack-welded with tacks 20–30 mm long at 300–500 mm pitch, then the end sections that bring the strengthening into work are welded, and only after that the connecting welds are laid. In statically indeterminate systems the strengthening is first set on all members and welded starting from the least loaded one. Methods of strengthening are covered in the article Strengthening steel structures.

Deformations and bends

Deformed structures that are fit for further use are straightened. Cold straightening is used only on smoothly deformed members; it is done on rolls, presses or other devices that apply the load smoothly, without dents or gouges. At sub-zero temperatures cold straightening is not used. Straightening by laying weld beads is forbidden.

Hot straightening involves heating: carbon steel is heated to 700–800 °C, low-alloy steel to 900–1100 °C, and finished not below 700 °C. Overheating, general or local, is not permitted. The metal is not worked in the range 300–350 °C (the «blue» heat colour). Cooling must be such that no hardening, warping or cracks appear. After straightening, the gap between a straightedge or taut string and a flange or web must not exceed 0.001 of the member length, and not more than 10 mm.

A bulge in a web or flange means loss of local stability. If, as the loads grow, the web of a beam lacks shear strength or stability, transverse, longitudinal or inclined stiffeners are added. It is better to place them on one side and weld with the minimum leg size.

Members with sharp bends at the places of deformation, and also with dents, holes and cracks that cannot be repaired, are not straightened: they are replaced in part or in full with new ones. Not every bend needs straightening at all: small bowing can be entered into the verification calculation, as described in the article on condition assessment.

Loose riveted and bolted connections

Signs of loosening: a bolt or nut turns, a rivet head shakes or moves when struck with a control hammer weighing 300–400 g, rivets or bolts are missing, there is a gap between the head and the pack. A torn-off head or a sheared bolt or rivet is a category A defect.

Replacing rivets can be full, when all the rivets in a joint are changed, or partial. A defective rivet is removed by gas-cutting off the head or by drilling out, without damaging the metal. Then the hole is checked: if there is misfit, ovality or skew beyond the permitted, it is reamed to the next larger diameter, and the weakening of the walls and the permitted distance between rivets and to the edge of the member are checked by calculation.

In a joint of up to ten rivets not more than one is removed at a time, in joints with more rivets not more than 10 % of the total; removing more is possible only by calculation. Re-riveting is done when the pack is clamped with erection bolts and the load on the joint is at a minimum. Caulking a defective rivet is forbidden. Laying welds on a loosened riveted joint and near the rivet heads is also forbidden.

Rivets are replaced with high-strength bolts or with fitted bolts in holes reamed for them (H9 accuracy) with a plain shank not shorter than the thickness of the pack. The strength class of the bolts is not lower than that of the rivets removed. Combined joints, where part of the force is carried by welding and part by rivets or bolts, are not permitted; the only exception is welding designed for the whole force in the joint without counting the rivets and bolts.

High-strength bolts are checked on a sample: in a joint of up to 5 bolts all are checked, from 6 to 20 five, and from 21 not less than a quarter. If fewer than 10 % of the bolts are missing, new ones are fitted and tightened with a torque wrench to the design force. Bolts that turn are replaced. If a 0.3 mm feeler goes into the gap between members, the tension in the joint is checked, brought to the design value and the joint is painted round its contour. Cracks in bolts, nuts or washers mean replacement; a crack in a connected member means strengthening or replacement as decided by the designer. More than 10 % of loose bolts in a joint is already an emergency case.

Overstressed anchors are supplemented with additional foundation bolts fixed in the existing foundation or in an added concrete layer. Insufficient capacity of a base plate is dealt with by stiffeners that reduce the size of the plate panels.

Cut-outs and missing members

Cut-outs for services, removed bracing and cut ribs are among the most common damage. A cut-out in a member is checked on the net area, with the eccentricity from the shift of the centroid. If the capacity is insufficient, the section is increased with plates or the member is replaced. When the section is increased, joint action of the plate and the main member is achieved, including the local stability of the overhangs.

A missing member is restored, usually repeating the earlier structural solution. Replacement is used when the member cannot be strengthened at all.

Acceptance and inspection after repair

All welds are examined and measured, critical ones are inspected by radiography or ultrasound, and the results are recorded. Rivets and bolts are checked by examination, feeler gauge, template and tapping both heads with a hammer weighing 300–400 g. The new and adjoining ones are accepted; weak ones are replaced and the check is repeated.

The repaired structure is accepted by a commission before priming and painting. Presented are the working drawings, documents for the metal, test reports for steel without a certificate, documents for the electrodes, laboratory conclusions on weld inspection, welder qualification records and reports of control sample tests. Deviations of dimensions must not exceed the tolerances. In especially critical cases the project provides for a sample test of the strengthened structures by trial loading.

Priming and painting come after acceptance; the contact surfaces of joints on high-strength bolts are not primed. If the structure had fire protection, its integrity and thickness are checked and brought to the standard. Information about the repair is entered into the documentation of the object. How corrosion affects repair and strengthening is covered in the article Corrosion wear and load-bearing capacity.

What we can do

The list of defects is compiled from the results of technical inspection. POLYGONAL prepares KM and KMD drawings for repair and strengthening (design) and fabricates replacement and strengthening members to the customer’s drawings: cutting, bending, welding, hot-dip galvanising, powder coating (fabrication). Work on site, including erection of the repair members, is done by our partner construction company. How we inspect welds and dimensions in production is covered in the article Quality Control of Steel Structures.

Standards and validity

The condition of structures and the list of defects are determined under the current procedure for technical inspection and the Methodology for conducting an inspection and recording its results (approved by the order of 06.08.2022 No. 144) and DSTU 9273:2024. Repair and strengthening are calculated under 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» (both with Amendment No. 1).

The requirements for welding and erection work, for inspection of welded joints and for repair of crane structures are set by separate current standards: the contractor confirms their titles and editions before work starts. The values given in the article (straightening temperatures, distances between welds, initial stress levels) are for reference, and some of them come from the practice of repairing crane steelwork; for a specific object they are set by the repair project. The validity and current edition of all the documents mentioned need to be checked.