Maintaining steel structures is crucial for ensuring their safety, longevity, and efficient performance. Over time, steel structures, including piles, beams, and other supporting components, face degradation due to factors such as corrosion, physical stress, and environmental exposure. Structural engineers play a vital role in selecting the most effective repair methods and ensuring that repairs restore the desired strength of the structure. In this article, we will explore various considerations for maintaining steel structures, including replacement, reinforcement, and maintenance practices for different components.
Replacement vs. Strengthening of Steel Members
One of the first decisions engineers make when addressing the deterioration of steel structures is whether to replace or strengthen the affected components. Load-carrying members, such as beams and columns, are typically replaced when corrosion exceeds 30% of the steel mass or when they show signs of significant deformation. However, in cases where adjacent members also show signs of deterioration, it may be more economical to replace entire frames or bents, rather than just isolated members.
Before removing a stressed member, it is crucial to relieve the stress by transferring the load to neighboring members or by introducing temporary members with adequate bracing. For example, when replacing piles, the load should be temporarily shifted to other piles using struts, beams, and jacks to prevent structural collapse.
In some cases, strengthening existing members may be a more cost-effective solution than full replacement. This is especially true when corrosion is confined to a localized area, allowing for targeted repairs that restore the member’s strength without the need for a complete overhaul.
Maintenance of Steel Piles
Steel piles are essential components in many structures, particularly in marine environments where they are exposed to harsh conditions such as tidal movements and saltwater. To ensure the longevity of steel piles, coating protection is often required to prevent corrosion. Additionally, piles that require cathodic protection must be regularly inspected and maintained. This includes checking the anodes for depletion, ensuring that electrical connections are secure, and verifying that electrical continuity is maintained.
When localized corrosion occurs in steel piles, engineers may opt to reinforce the piles rather than replace them. One method of reinforcement involves welding steel plates onto the flanges and webs of H-piles in areas affected by corrosion, particularly in the tidal zone. These plates must be thick enough to restore the pile’s original strength and cover the corroded areas completely.
Another method of reinforcement involves encapsulating the pile in reinforced concrete. This process involves welding reinforcing rods along the pile’s main axis and across the damaged area. After welding ties at the intersections, a concrete form is placed around the pile, and concrete is poured inside to create a durable protective shell.
When pile replacement is necessary, it is crucial to ensure that the new piles are accurately fabricated to match the old ones, including proper alignment for bolt and rivet holes. In most cases, new piles are driven alongside the old pile at a slight angle, then cut to the correct elevation, capped, and pulled into position. If the old pile must be removed before the new one is in place, the load must be temporarily shifted to prevent structural instability.
Maintenance of Sheet Piling
Sheet piling, often used as bulkheads to retain fill in marine and waterfront environments, is particularly susceptible to corrosion and damage from environmental factors. Repairing sheet piling requires special attention to prevent failure and the loss of fill material through gaps. In some cases, welding steel plates over small holes can provide an effective solution. However, when the deterioration is more extensive, entire sections of sheet piling may need to be replaced or reinforced.
In many cases, new piles are driven in front of the old piles, creating a new support system. These new piles should be supported by new wales, tie rods, and deadmen. After driving the new piles, the space between the old and new piles is filled with compacted earth, sand, gravel, or concrete to restore the structural integrity of the bulkhead.
For badly deteriorated sheet piling, encasing the steel in concrete is another effective repair method. After cleaning the old steel of rust and marine fouling, a concrete cover of at least six inches is installed around the pile, often using a bolted wooden form. If the back of the bulkhead is accessible, the entire steel bulkhead can be encased in concrete, with a minimum of three inches of thickness on each side.
Tie rods, which help secure the bulkhead to the surrounding structure, may also need to be replaced. This involves digging a trench from the sheet piling to the deadman, removing the old rods, and installing new ones with turnbuckles. The new rods should be coated with bituminous materials to prevent corrosion.
Maintenance of Pipe Piling
Pipe piling repair is similar to H-pile repair, but the cylindrical shape of pipe piles makes them easier to protect with wraps. These wraps offer an additional layer of corrosion protection, particularly when the pile is exposed to harsh environmental conditions. The process of replacing or reinforcing pipe piles follows similar principles to H-pile repairs, ensuring that the new piles are installed securely and aligned with the existing structure.
Supporting Components (Wales, Braces, etc.)
Steel supporting components such as wales, braces, and other structural elements must be regularly inspected for corrosion and deterioration. As far as possible, these components should be located above the high water line, where corrosion is less likely to occur. If deterioration is found, repairs or replacements should be made to maintain the structural integrity of the entire framework.
Utility Lines Maintenance
Maintaining the utility distribution systems within steel structures is essential for safety and efficiency. Utility lines, including gas and electrical systems, must undergo regular inspection and necessary repairs. This could involve replacing damaged components, tightening loose connections, and repairing defective parts or sections.
For gas distribution systems, leaking pipes must be repaired promptly, typically by shutting off the gas, tightening connections, and rechecking for leaks with soapsuds. Electrical systems require additional safety precautions, including the presence of an assistant to provide assistance or first aid in case of an emergency. Any extensive replacements should be carried out following current criteria for new construction.
Mooring Fittings Maintenance
Mooring fittings such as bitts, bollards, cleats, and chocks are essential for securing ships and other vessels to steel structures. Regular maintenance of these fittings includes tightening or replacing bolts, reinforcing or replacing the foundations, and ensuring the fittings are not corroded. Bolt heads exposed to the atmosphere should be protected from corrosion by filling bolt holes with lead or epoxy putty. New mooring fittings should be made of cast steel and be at least the same size and capacity as the ones they replace. Additionally, all new fittings should be coated with coal tar for extra protection.
Floating Structures Maintenance
Floating structures, including lifts and barges, are prone to corrosion due to constant exposure to water. Repairing holes in the sides of floating structures is essential to prevent further damage. This can be done by welding steel plates over the holes or using epoxy putty for temporary fixes when welding requires dry docking. Protective coatings, including cathodic protection, should be used to prevent underwater corrosion, while zinc inorganic coatings are recommended for steel work decks due to their resistance to corrosion and abrasion.
Conclusion
The maintenance of steel structures is vital to their continued functionality and safety. Regular inspections and timely repairs are essential to address issues such as corrosion, deformation, and other forms of deterioration. By choosing the appropriate repair methods—whether replacing, reinforcing, or maintaining individual components—engineers can ensure that steel structures continue to perform effectively throughout their lifespan. Regular attention to elements such as piles, sheet piling, supporting components, utility lines, and mooring fittings can prevent costly and dangerous failures, ensuring the long-term stability of the structure.