Highly elevated boardwalks are pedestrian walkways that stand at least 15 feet above the ground or water level. These structures allow people to traverse areas that would otherwise be hard to access, such as steep terrain, wetlands, or forest canopies.
The 15-foot threshold marks a point where pedestrian bridge design considerations change significantly compared to lower boardwalks.
I. Foundation Design (Below Ground)
A. Soil conditions and load-bearing capacity
Similarly to more traditional boardwalks or pedestrian bridge structures, foundation considerations for highly elevated boardwalk structures are dependent on the geotechnical subsurface conditions below ground.
Unless rock is present near the ground surface, shallow footings are generally not the best fit for highly elevated boardwalks as a result of the lateral loading applied at such tall stickup heights (stickup height being defined as the distance from ground line to top of foundation). As a result, deep foundations are generally the best fit for highly elevated boardwalks, especially when silt, sand, and “clayey” soils (not rock) are present.
Types of foundations generally used for highly elevated boardwalks in silty, sandy, or clayey geotechnical conditions include driven piles (composite, steel, concrete or timber), cast-in-place drilled shafts and helical piers. When rock is present, installing driven piles and helical piers is typically not feasible. In the case where rock is present, typical foundation types include drilled-in piles, cast-in-place drilled shafts, or micropile foundations.
B. Types of foundations for high elevations
|
Foundation Type |
Advantages |
Considerations |
Best For |
|
Driven Piles (Steel H-piles, Composite, Timber, Concrete) |
High load capacity, suitable for weak surface soils |
Require heavy equipment for installation, may cause vibration |
Sites with deep layers of weak soil |
|
Cast-in-place drilled shafts |
Can be sized exactly to load requirements, less vibration during installation |
Concrete curing time, potential for contamination of wet concrete |
Sites with varying soil conditions or where minimal disturbance is needed |
|
Helical piers |
Quick installation, minimal site disturbance, good for variable soil types |
Limited to certain soil types, may have lower lateral load capacity |
Sites with limited access or where minimal ground disturbance is crucial |
C. Depth requirements for stability
The depth of the foundation depends on several factors:
- Soil characteristics: Weaker soils generally require deeper foundations.
- Load requirements: Heavier structures need deeper or wider foundations.
- Frost depth: In cold climates, foundations must extend below the frost line.
- Scour potential: For foundations near water, depth must account for potential erosion.
- Lateral loads: Wind and seismic forces may require deeper foundations for stability.
Engineers typically determine the required depth through calculations based on soil data and structural loads. For highly elevated boardwalks, foundations often need to be deeper than for ground-level structures due to increased lateral forces and lateral deflection considerations of the foundation.
In all cases, the foundation design must balance the need for stability with cost-effectiveness and constructability. The right choice depends on a thorough understanding of site conditions and project requirements.
II. Column Design (Above Ground)
A. Material choices (timber, concrete, steel)
|
Material |
Advantages |
Considerations |
Column Bracing Considerations |
Best For |
|
Timber |
Natural appearance, relatively low cost, easy to work with |
Susceptible to rot and insect damage, requires regular maintenance |
Transverse and longitudinal bracing required |
Lower-height boardwalks in less harsh environments |
|
Concrete |
High durability, fire resistance, low maintenance |
Sometimes a higher initial cost, heavier weight, may require larger equipment for installation |
Column typically designed to eliminate need for transverse and longitudinal bracing generally not required |
Long-span, high-elevation boardwalks with heavy load requirements |
|
Steel |
High strength-to-weight ratio, allows for slender columns, corrosion-resistant when properly treated |
Higher material cost, may require specialized fabrication |
Transverse and longitudinal bracing required |
Very tall boardwalks or those with aesthetic requirements for slim profiles |
B. Load distribution and transfer
Columns in highly elevated boardwalks must effectively transfer loads from the deck to the foundation. Key considerations include:
- Axial loads: The vertical weight of the structure and its users
- Lateral loads: Wind forces and potential seismic activity
- Eccentric loads: Uneven weight distribution on the deck
To handle these loads, designers must consider:
- Column cross-section: Larger cross-sections generally provide more stability but increase visual bulk
- Reinforcement: For concrete columns, proper rebar placement is essential
- Bracing: May be necessary to increase lateral stability (discussed more in the next section)
C. Height and size considerations
As boardwalk height increases, column design becomes more demanding:
- Slenderness ratio: Taller columns are more prone to buckling under load
- This may necessitate larger column diameters or additional bracing (generally a controlling factor for timber and steel columns)
- This may necessitate larger column diameters or additional bracing (generally a controlling factor for timber and steel columns)
- Wind loads: Increase significantly with height
- May require stronger materials or larger column sizes to resist lateral forces
- May require stronger materials or larger column sizes to resist lateral forces
- Visual impact: Taller columns are more visible
- Balance between structural needs and aesthetic considerations becomes more important
- Balance between structural needs and aesthetic considerations becomes more important
- Construction challenges: Taller columns require specialized equipment and techniques for installation
- This can influence material choice and column design
D. Connection points with the boardwalk structure
The interface between columns and the boardwalk deck significantly affects structural integrity:
- Beam-to-column connections:
- Must transfer both vertical and horizontal loads
- Options include bolted connections, welded plates (for steel), or cast-in-place joints (for concrete)
- Column caps:
- Distribute deck loads evenly across the column top
- Can be designed to accommodate thermal expansion and contraction of the deck
- Beam bearing connections:
- Transfer loads from the column to the foundation
- Must be designed to resist both shear and moment forces
- Expansion joints:
- May be necessary for very long boardwalks to accommodate thermal movement
- Columns at expansion joints need special design considerations
- Railing attachments:
- While often not primary load-bearing elements, must be securely fastened to withstand lateral forces
- While often not primary load-bearing elements, must be securely fastened to withstand lateral forces
When designing these connections, factors to consider include:
- Load transfer efficiency
- Ease of construction and potential for prefabrication
- Durability and resistance to corrosion or decay
- Aesthetic impact on the overall structure
The column design for highly elevated boardwalks requires careful balance between structural requirements, material properties, constructability, and visual impact.
Designers must consider not only the immediate loads and conditions but also long-term factors like maintenance and potential environmental changes over the structure's lifespan.
III. Bracing Requirements
A. When bracing is needed
Bracing becomes necessary in highly elevated boardwalks under several conditions:
- Height thresholds: Generally, boardwalks over 10 feet tall require lateral bracing.
- Slenderness ratios: When columns exceed certain height-to-width ratios, bracing helps prevent buckling.
- Wind loads: In areas with high wind speeds, bracing provides additional lateral stability and reduces lateral pile deflections.
- Seismic considerations: Regions prone to earthquakes often require more robust bracing systems.
- Soil conditions: Unstable soils may necessitate additional bracing to ensure overall structure stability.
B. Types of bracing systems
- Diagonal bracing:
- X-bracing: Forms an "X" pattern between columns
Fig.1: Steel X-bracing, Fig. 2: Timber X-bracing, Fig. 3: Cable Bracing
-
- K-bracing: Resembles a "K" shape, allowing for openings between braces
- Horizontal bracing:
- Ties columns together at regular intervals
- Often used in conjunction with diagonal bracing
- Moment frames:
- Rigid connections between beams and columns that resist lateral forces
- Can reduce or eliminate the need for visible diagonal bracing
- Knee braces:
- Short diagonal members connecting columns to beams
- Provide additional support while minimizing visual impact
- Portal frames:
- Rigid frame structures that resist lateral loads through the stiffness of their connections
C. Impact on structural integrity
Proper bracing significantly enhances the structural performance of highly elevated boardwalks:
- Lateral stability: Bracing reduces sideways movement, particularly important for tall structures.
- Load distribution: Helps spread forces more evenly throughout the structure.
- Reduced column sizes: With effective bracing, columns can often be more slender.
- Vibration control: Bracing can minimize uncomfortable vibrations from wind or user movement.
- Longevity: By reducing stress on individual components, bracing can extend the structure's lifespan.
D. Aesthetic considerations of bracing
While structurally beneficial, bracing can have a significant visual impact:
- Visual weight: Diagonal bracing, while effective, can make a structure appear heavier or more industrial.
- View obstruction: Some bracing types may partially block views from the boardwalk.
- Material selection: Choosing bracing materials that complement or contrast with the main structure can affect overall aesthetics.
- Integrating function and form: Creative bracing designs can become architectural features rather than purely structural elements.
- Minimalist approaches: Using moment frames or portal frames can maintain a cleaner look at the cost of larger structural members.
It’s a balancing act!
Designers must weigh the structural benefits of bracing against aesthetic goals. Options include:
- Hiding bracing within the structure where possible
- Using transparent materials for bracing in view-sensitive areas
- Incorporating bracing into railing designs
- Alternating braced and unbraced sections to create rhythm in the design
When designing bracing for highly elevated boardwalks, engineers and architects must collaborate closely. The goal is to create a structure that's not only safe and stable but also visually appealing and in harmony with its surroundings.
This often involves iterative design processes, weighing various bracing options against structural needs and aesthetic considerations.
IV. Aesthetics and Visual Impact
A. Balancing function and form
Designing highly elevated boardwalks requires a delicate balance between structural necessities and visual appeal. Designers must consider how the structure appears from multiple viewpoints - both to users on the boardwalk and to observers from afar.
The boardwalk's appearance can change dramatically depending on the angle and distance from which it's viewed, so it's important to consider these perspectives during the design process.
Creating visual harmony in long spans often involves using rhythm and repetition in structural elements. However, to avoid monotony, strategic variations can be introduced.
These might include changes in spacing, materials, or even the introduction of wider platforms at key points. At the same time, maintaining a sense of human scale, especially at deck level, helps users feel comfortable and connected to the structure despite its height.
B. Integrating with surrounding environment
A successful highly elevated boardwalk doesn't just sit in its environment - it becomes a part of it. This integration starts with a contextual design approach, drawing inspiration from local landscape features or architectural styles. Designers should consider how the boardwalk will look throughout the changing seasons, as surrounding vegetation changes or even disappears.
The boardwalk's relationship to existing views is crucial. Rather than obstruct important vistas, the structure can be designed to frame and enhance them. This might involve aligning the boardwalk to capture specific views or using structural elements to create "windows" to the landscape.
Additionally, the boardwalk's route should be carefully planned to minimize impact on sensitive ecological areas, preserving the natural beauty it's meant to showcase.
Lighting design plays a significant role in environmental integration, especially for boardwalks used at night. Subtle, well-planned lighting can enhance safety and usability without contributing to light pollution or disrupting local wildlife. It also offers an opportunity to create a different aesthetic experience of the boardwalk after dark.
For more information about lighting, check out our article “Ways to Add Lighting into PermaTrak Concrete Boardwalks.”
C. Material selection for visual appeal
The materials chosen for a highly elevated boardwalk significantly influence its visual impact. Texture and finish can reduce glare, add visual interest, and affect how the structure weathers over time.
Color selection is equally important - the right palette can help the boardwalk blend with its surroundings or stand out as an intentional contrast, depending on the design goals.
A mixed-material approach can create visual interest and define different zones or functions within the boardwalk. For instance, a change in decking material might signal a transition from a walking path to a viewing platform.
Using local and sustainable materials can help the structure feel more connected to its regional context and can be an attractive feature for environmentally conscious communities.
D. Minimizing visual bulk at height
One of the biggest challenges in designing highly elevated boardwalks is managing their visual weight, especially at great heights. Using high-strength materials allows for more slender structural members, reducing the overall bulk of the structure.
Some designers opt for tapered column designs that become thinner as they rise, creating a sense of lightness despite the boardwalk's height.
Incorporating transparent elements, such as glass panels in railings or sections of open-grid decking, can significantly reduce the perceived weight of the structure. These features allow light to pass through the boardwalk, reducing shadowing and helping it feel less imposing.
For very long spans, visual breaks become important. These might take the form of wider viewing platforms or subtle changes in material or color along the boardwalk's length.
Designers should also consider the spaces between structural elements - these negative spaces can be used to frame views or create interesting shadow patterns that change throughout the day.
By carefully considering these aesthetic aspects, designers can create highly elevated boardwalks that are not just functional, but also visually captivating. The goal is to craft a structure that enhances both the user experience and the surrounding landscape, becoming a beloved feature of the area rather than an imposition on it.
Achieving this often requires close collaboration between engineers, architects, and landscape designers, bringing together technical expertise with a strong sense of place and visual harmony.
V. Project Examples
(Click image to expand)
| Project Name | Foundation, Column & Bracing (if applicable) | Soil Conditions |
| Antiquity Greenway, North Carolina | Timber piles with bracing | Silty-Sand (no rock) |
| Mingo Creek, North Carolina | CIP Drilled-in piles with timber columns & timber bracing | Crystalline Rock |
| Razorback Trail, Arkansas | Micropile foundations with CIP cap at grade & CIP Column (no bracing) | Limestone Rock |
| Wolf Lake, Indiana | Steel H-Piles (no bracing) | Silty-Sand (no rock) |
| PSEG Observation Platform, New Jersey | Helical Piers braced with battered piles | Clayey-Sand (no rock) |
| Thompson Oaks, Minnesota | Helical Piers with bracing | Silty-Sand (no rock) |
| Harbor Walk, Florida | Precast piles (no bracing) | Silty-Sand (no rock) |
| Hickory Aviation, North Carolina | CIP Footings with CIP Columns (no bracing) | Clayey-Sand & Partially Weathered Rock |
| HUB RTP, North Carolina | CIP Footings with CIP Columns (no bracing) | Partially Weathered Rock |
| Cypress Creek Greenway, Texas | FRP Piles (no bracing) | Silty-Sand (no rock) |
| Josey Lakes Park, Texas | CIP Drilled Shafts with CIP Columns (no bracing) | Clayey-Sand (no rock) |
| Lake Highlands Trail, Texas | CIP Drilled Shafts with CIP Columns (no bracing) | Hard Shale (Rock) |
| Hickory Riverfront, North Carolina | CIP Footings with CIP Columns (no bracing) | Partially Weathered Rock |
VI. Your Next Steps for Designing a Highly Elevated Boardwalk
When designing highly elevated boardwalks, focus on key factors like foundation design based on soil conditions, appropriate column materials and load distribution, necessary bracing for stability, and balancing structural needs with aesthetics and environmental integration.
Thorough planning is vital due to the unique challenges of elevated structures.
Assemble a diverse team of experts, conduct comprehensive site surveys early, and consider long-term maintenance needs. Engage with stakeholders and regulators, and use 3D modeling to visualize impact.
With careful planning, your elevated boardwalk can be both functional and visually striking, enhancing access to natural areas for years to come.
