MAINTENANCE OF STEEL STRUCTURES
Structural engineers should be consulted to ensure that the repair method will restore the steel structures to the desired strength and that the most effective method of repair has been chosen. Load-carrying members are usually replaced when 30% or more of the steel has been lost by corrosion or when they are deformed. If adjacent members show signs of serious deterioration, it may be more economical to replace whole frames or bents. A stressed member should not be removed before the stress has been relieved by transfer of load to adjoining members or by new temporary members and adequate bracing. In the replacing of piles, the load should be shifted temporarily to other piles by struts or beams using jacks. The replacement of wales on quaywalls may require excavation of fill to relieve lateral loads. In some cases, it may be more economical or practical to strengthen existing members than to replace them. This is especially true where corrosion is serious in only a limited area.
Steel piling may require coating protection or there may be some Those requiring cathodic protection should. The cathodic protection systems themselves must be inspected for depleted anodes, corroded or loose connections, electrical continuity, etc., and maintained on a yearly basis to assure continuous protection of the steel.
Reinforcement of H-piling by welding steel plates onto flanges and web may be appropriate in localized areas of corrosion, such as the tidal zone. The reinforcing plates should be of sufficient thickness to restore the original strength to the piling and of sufficient area to encompass and extend beyond the extremities of the corroded area . The old steel must be cleaned and cut back to a point where the metal thickness will ensure a strong weld. All cut edges should be feathered, and the weld should be made completely around the plate to eliminate crevices. Another method of reinforcement utilizes encapsulation in reinforced concrete. In this method, reinforcing rods are welded along the main axis of the repaired member, across the damaged area. Ties are welded or tied at all intersections with reinforcing steel, a form is placed around the piling, and concrete is placed inside . When replacement is necessary, the new piling must be accurately fabricated to match the old, making sure that bolt and rivet holes are properly located. When replacing bearing piling, the new pile is generally driven alongside the old one at a slight angle. It is then cut off at the proper elevation, capped (usually by welding on a steel plate), and pulled into position with a block and tackle. If the old pile is removed before the new one is driven, the load must be temporarily transferred until the new pile can assume it.
Repair of corroded steel pile.
Sheet piling usually serve as a bulkhead to retain fill. Thus, extreme care must be taken during replacement of one or more piles to prevent failure and passage of fill through the opened spaces into the water. More frequently, small holes are patched by welding steel plates over them, and badly deteriorated piling are generally replaced or protected by having new piles driven in front of them. In the latter case new wales, tie rods, and deadmen should be installed, and the space between the old and new piles should be filled with well-tamped earth, sand, gravel, or concrete. An alternate method of repairing badly deteriorated piling is to install a concrete facing. The old steel must be cleaned of rust, marine fouling, and other contaminants before a concrete cover of at least 6-inch thickness is installed. A bolted wooden form is generally used for this purpose. When the back of the bulkhead is accessible, the entire steel bulkhead can be encased in concrete with a minimum thickness of 3 inches on each side .Whenever backfill is replaced, it should be added in layers (preferably granular material) and be well compacted. To replace deteriorated tie rods, a trench is dug from the sheet piling to the deadman, and the new rods with new turnbuckles are installed one at a time .They should be covered with a bituminous coating, a fabric tape, and a final bituminous coating. The deadman should be inspected, and necessary repairs made before the trench is backfilled.
Concrete-protected steel sheet piling.
Repairing tie rods.
Pipe piling repair is generally similar to that of H-piling repair Because of their cylindrical shape they are more easily protected by wraps than are other pilings.
Steel supporting components (wales, braces, etc.) should be repaired or replaced, as necessary. As far as possible, they should be located above the high water line where corrosion is less severe.
The basic objective is to maintain the distribution systems for the utilities as economically as feasible and still be consistent with operating requirements, sound engineering practice, and proper protection to life, health, and property. All necessary repairs should be made as required by the periodic inspection. These repairs may require replacing items, tightening loose connections, tightening or repacking valve gland and conduit seal glands, or welding defective parts or sections. The cathodic protection systems should be maintained in accordance with manufacturer’s instructions. In gas distribution systems, leaking pipes are repaired by shutting off gas, tightening connections, and rechecking leaks with soapsuds. When working on electrical power transmission and distribution systems, an assistant must always be available to render assistance or first aid. Extensive replacements of defective systems shall be made in accordance with current criteria for new
Maintenance of mooring fittings (bitts, bollards, cleats, chocks, etc.) includes tightening or replacing bolts; replacement of cracked, broken or badly corroded fittings; and reinforcement or replacement of foundations. Boltheads exposed to the atmosphere should be protected from corrosion by potting the bolt holes with poured lead or with an epoxy putty. New fittings should be of cast steel and be at least the same size and capacity as those they replace. They should be painted with coal tar.
Repair of holes in the sides of floating structures, such as floating, lifts, and camels, should be made by welding on steel plates. The plates should be rounded and the welding be as smooth as possible to avoid conditions which accelerate corrosion. Temporary patching can be made by bolting plates over the holes or with epoxy putty if welding of plates would require dry docking. Cathodic protection will protect the underwater steel from corrosion, and protective coatings should be used above water. Because of their resistance to impact and abrasion damage and to corrosion, zinc inorganic coatings are recommended for steel work decks on barges and cranes.