Jetty design guidelines published by engineering institutions are written for engineers, and they assume the reader already has the site data and the brief. A public client commissioning a landing structure has neither, and the gap between what the guidance assumes and what the tender actually contains is where most projects lose time.
The useful division is not between good and bad design. It is between the decisions that only the client can make and the decisions that belong to whoever does the calculations. Almost every delay on a small marine project comes from the first group being left open.
The site data nobody else can supply
Six pieces of information define the structure before a single section is sized, and all six are properties of the location rather than of the design. The bed profile and what the bed is made of decide whether piling is realistic and how deep it has to go. The water levels, highest and lowest across the whole year, set the deck level and the access arrangement. The fetch and the resulting wave climate set the loads on the structure and on anything moored to it. The current sets the loads on piles and the behaviour of vessels approaching. Ice conditions, where they apply, change the mooring and the geometry. Finally, any contamination in the bed changes what can be disturbed and therefore what the consent will permit.
A tender issued without these produces priced guesses. Commissioning a bathymetric and geotechnical survey before the tender, rather than passing the risk to the bidders, is almost always cheaper than the contingency every bidder will otherwise carry.
The design vessel is the single most consequential line
What will come alongside decides more about the structure than any other requirement. It sets the deck level relative to the water, the length of the berth, the depth that has to be maintained, and above all the loads the structure has to absorb.
Berthing energy is the load that municipal briefs almost never mention and that dominates the design of the deck edge and the fixings. A vessel arriving at even a modest speed carries kinetic energy that has to be absorbed by fenders and transmitted into the structure, and the figure scales with displacement. A jetty specified for small craft and then used by a workboat has not been overloaded in any everyday sense; it has been asked to absorb an impact an order of magnitude larger than it was designed for.
Mooring loads follow the same logic. A vessel tied to the structure in wind and current pulls on cleats and bollards continuously, and those pull-out loads have to be carried into the frame rather than into the decking.
Fixed or floating is a site decision, not a preference
The deck can hold a constant level above the bed or a constant height above the water, and which of those suits depends almost entirely on how much the water moves. Where the annual range is small, a fixed structure gives a rigid deck, straightforward vehicle access and minimal maintenance. Where the range is significant, a fixed deck means the step down to a vessel changes through the season, and a floating arrangement keeps that step constant while the gangway absorbs the variation.
Bed conditions push the same decision from the other direction. Soft, deep or contaminated beds make foundations expensive and consents difficult, and the practical difference between a floating and a fixed structure is often decided there rather than by any preference about how the finished thing should feel.
Loads on the deck itself
The structural deck carries people, and on a public jetty it carries them in numbers. The imposed load category chosen for the deck sets the beam depth and, on a floating structure, the buoyancy that has to be provided underneath. Service vehicles are the item most often forgotten: if a small van has to reach the end of the structure for maintenance or for emergency access, that single wheel load will frequently govern the deck design and it costs far more to add later than to include at the start.
Where the public gathers to watch something, the eccentric case matters as much as the total. The category and value chosen for crowd loading should be stated in the brief rather than left to the designer to infer from the word public.
Accessibility is set at the extremes, not the average
An access gangway between the shore and a floating deck has a gradient that changes with water level, and the number that matters is the one at the worst case rather than at the mean. A ramp that is comfortable at average level and unusable at extreme low water has not met an accessibility requirement; it has met it for part of the year.
The fix is a longer gangway or a different connection geometry, and both are cheap decisions at design stage and expensive ones afterwards. Stating the maximum acceptable gradient together with the water level it applies at turns this from an argument into a calculation.
Service life and what it implies
A design life of twenty-five years and one of fifty produce different structures, different materials and different maintenance commitments, and the client is the only party who can choose. The choice is not only about durability: a structure intended to last fifty years in a place where the use may change within fifteen is arguably the wrong investment, and a demountable arrangement may serve better.
Whichever is chosen, the maintenance regime implied by it belongs in the tender. A low capital cost with an annual coating commitment and a higher capital cost with an inspection-only regime can be compared properly only if both are stated.
What to hand the designer
Give the survey data. Name the design vessel and the berthing speed assumed. State the imposed load category and whether a service vehicle is required. Give the water level range across the year. State the design life and the accessibility criterion with the level it applies at. Say whether the installation must be reversible.
Seven items, most of them a single line, and together they remove the ambiguity that otherwise turns into variations after award. Our engineers will assemble this brief with a client before anything goes to market, and will say which of the aluminium floating and landing structures in our range the site supports and which would have to be designed from scratch.