Numerous factors contribute to the extent that a storm damages a dock system. Storm strength and type play a significant role. However, structural design, maintenance, and prior damages can significantly affect the amount of damage a dock sustains from a storm event.
Unlike construction codes applicable to upland structures, the requirements applicable to docks are generally not as detailed, which allows for significantly more variation in their construction. Even docks made of similar components, such as steel frame docks, often vary in design in ways that impact the strength of the dock system. For example, the amount and type of roof bracing correlates to the roof’s ability to withstand various types of loads – yet the specific roof bracing design is not dictated by construction code and thus may vary wildly from dock to dock. The following three figures are examples of the variations in roof bracing systems:
Figure 1 Roof Support with Double Horizontal and X-Bracing Support System
Figure 2 Roof Support with Thicker Single Horizontal Bracing but Limited Corner Z-Bracing
Figure 3 Roof Support with Double Horizontal and Wire X-Bracing Support System
Each of the above systems has its own particular benefits and weaknesses as compared to other designs. In general, the design should be evaluated on a case-by-case basis based on anticipated load events. For example, different systems may be required where lateral loads (i.e., wind events) versus horizontal loads (i.e., ice and snow events) are expected.
In addition to a dock’s structural design, regular maintenance or the lack thereof also plays a role in how much damage a dock may sustain from a storm event. As dock systems are constantly in motion and exposed to the elements, components often fatigue and fail over time. Such failures may not always be observable from above water. For example, on older docks it is not uncommon for underwater trusses to become detached over time. Also, cable moorings can deteriorate over time. If the cables are not routinely inspected and replaced, the mooring may fail in a significant wind event.
Figure 4 Potentially Compromised Cable
Prior damages that have not been correctly or completely repaired can also leave a dock susceptible to additional damage from a new storm event. Damages, especially those that are not viewed as a complete failure may appear minor and not be repaired; however, these damages can compromise the structural integrity of the system. Dock systems are designed such that all the components work in unison. If one component is not functioning correctly, an increased risk exists that during a storm event added stress will be placed on the remaining components, which can lead to further component failures.
Figure 5 Older Unrepaired Damaged Underwater Truss
In conclusion, to accurately underwrite marinas, individual assessments of all dock systems – both above water and underwater - should be performed to identify potential design, maintenance or prior uncorrected damages that could contribute to the severity of any damages the system may sustain in a new storm event.