Views: 289 Author: NANJING TAIDUN Publish Time: 2026-10-15 Origin: Site
Content Menu
● What Is Fender Reaction Force? The Load Your Structure Must Resist
>> Why Reaction Force Matters for Structural Design
>> How Reaction Force Is Tested and Reported
● What Is Fender Energy Absorption? The Work Your Fender Must Perform
>> The Units Confusion: kNm vs kJ
>> Why Energy Absorption Determines Berthing Safety
● The Fender Performance Curve: Where Reaction Force and Energy Absorption Meet
>> Reading the Curve: Key Features
● The PIANC WG211 Approach: Partial Factors for Both Metrics
● Comparison Table: Reaction Force vs Energy Absorption
● Temperature and Velocity Effects: Why Both Metrics Change in Service
● The Optimization Paradox: Can You Get High Energy and Low Reaction Force?
>> Strategy 1: Longer Deflection Range
>> Strategy 2: Multiple Fender Contact
>> Strategy 3: Optimized Curve Shape
● Practical Selection Checklist for Port Engineers
● Conclusion: Two Metrics, One System
● Frequently Asked Questions (FAQ)
>> 1. What is the difference between reaction force and energy absorption in fender design?
>> 2. Can I select a fender based on energy absorption alone?
>> 3. How does temperature affect reaction force and energy absorption?
>> 4. What does PIANC WG211 say about reaction force and energy absorption?
>> 5. How can I get low reaction force without sacrificing energy absorption?
>> 6. What documentation should I request from fender suppliers?
Introduction: The Two Numbers That Define Every Fender System
Every fender specification comes down to two critical values: reaction force and energy absorption. These two metrics define whether a fender system will protect your berth or become the source of its most expensive failure.
As an OEM manufacturer with over 23 years in this industry, I have reviewed specifications where designers focused exclusively on energy absorption—only to discover during commissioning that the reaction force exceeded the quay's structural capacity. I have also seen the opposite: reaction force carefully controlled, but energy absorption inadequate for the design vessel.
The truth is simple: you cannot optimize one without understanding the other. This guide provides a complete engineering comparison of these two metrics, explains their relationship through the fender performance curve, and offers practical selection criteria for port engineers and OEM procurement specialists.
Reaction force (often designated as R) is the force the fender transmits to the supporting structure at a given deflection. It represents the load the quay, pile, or dolphin must absorb during berthing.
Reaction force is typically specified at the rated deflection—the compression level at which the fender achieves its guaranteed energy absorption. For rubber fenders, this is commonly 52.5% or 60% deflection, depending on the fender type.
The quay structure—whether concrete caisson, steel pile, or timber dolphin—must be designed to resist the reaction force without exceeding allowable stress limits. This has direct cost implications:
- Higher reaction force = larger pile diameters, thicker concrete sections, more reinforcement
- Lower reaction force = smaller structural elements, reduced civil engineering costs
The consequence of underestimating reaction force is severe: structural cracking, pile yielding, and potentially catastrophic failure during extreme berthing events.
Reaction force is measured during compression testing of the fender unit. According to PIANC and ASTM standards, the fender is compressed at a controlled rate, and the force required to achieve each deflection level is recorded. The test produces a force-deflection curve from which both reaction force and energy absorption are derived.
Critical note: Reaction force values are not constants. They vary with:
- Temperature: Cold rubber stiffens, increasing reaction force
- Compression velocity: Higher berthing speeds increase reaction force
- Fender geometry: Taller fenders generally produce lower reaction forces for equivalent energy
Energy absorption (designated as E or EA) is the total work performed by the fender during compression. It represents the kinetic energy of the berthing vessel that the fender can safely absorb without exceeding its design deflection.
Energy absorption is calculated as the area under the force-deflection curve up to the rated deflection. This is why fender performance curves—showing both force and energy—are essential for proper selection.
Energy absorption is expressed in kilonewton-meters (kNm) or kilojoules (kJ). These units are dimensionally equivalent: 1 kNm = 1 kJ. When reviewing specifications, verify the unit convention and ensure consistency across all documents.
If the fender cannot absorb the design berthing energy, the excess energy must go somewhere—into the vessel hull, the fender support structure, or both. The result ranges from cosmetic damage to catastrophic structural failure.
The selection criterion is straightforward:
Energy Absorption Capacity ≥ Design Berthing Energy
But as we will see, the relationship between these two metrics is not linear, and optimizing one often compromises the other.
The force-deflection curve (or reaction-deflection curve) is the single most important document in fender selection. It displays how reaction force varies with deflection and enables calculation of energy absorption at any compression level .
A typical rubber fender curve exhibits:
1. Initial linear region: Force rises steeply with deflection
2. Peak reaction force: The maximum force during compression
3. Plateau or decline: Force may stabilize or decrease
4. Energy area: The area under the curve represents absorbed energy
Here is the fundamental challenge in fender design: for a given fender geometry and rubber compound, increasing energy absorption generally requires higher reaction force.
This creates a design tension:
- Ports with weak structures need low reaction force
- Berths serving large vessels need high energy absorption
- Achieving both simultaneously requires careful fender selection or innovative designs
The hollow cylindrical fender design, for example, increases the performance efficiency E/RW (energy absorbed per unit reaction force and rubber weight) by approximately 15% compared to solid designs . This efficiency gain comes from optimizing the force-deflection curve shape.
PIANC WG211 (2024) represents a fundamental shift in how reaction force and energy absorption are treated in design. The guideline introduces partial resistance factors that account for uncertainties in both metrics .
Where:
- Ef,d = Design energy absorption
- Ef,c = Characteristic energy absorption
- γf = Fender performance partial factor
- γmult = Multiple fender contact partial factor
Where:
- Rf,d = Design reaction force
- Rbase,max = Reaction at maximum allowable compression
- Cv, Ct, Cang = Velocity, temperature, and angular correction factors
The key insight: PIANC WG211 requires separate verification for energy and reaction force, using different factors and different correction chains. A fender that passes the energy check may fail the reaction force check—and vice versa.
Reaction force and energy absorption are not fixed values—they change with operating conditions. Understanding these effects is critical for accurate specification.
Rubber is a temperature-sensitive material. At low temperatures, rubber stiffens:
| Temperature | Reaction Force Factor | Energy Absorption Factor |
|---|---|---|
| 4°C | 1.12 (+12%) | 0.96 (-4%) |
| 34°C | 0.97 (-3%) | 1.11 (+11%) |
Implication: A fender specified for 20°C conditions may produce 12% higher reaction force at 4°C. If the structure has no margin, this could cause damage.
Berthing velocity affects both metrics, though the magnitude varies by fender type. For the example V-type fender:
- At strain rate 12.5%/s, velocity factor for reaction force = 1.22
- Velocity factor for energy absorption = 1.21
Implication: The nearly identical velocity factors for R and E mean the shape of the force-deflection curve remains relatively consistent across velocities—but both values scale up at higher berthing speeds.
The short answer: not from a single fender unit. But there are strategies.
Fenders that compress further before reaching maximum force can absorb more energy without necessarily increasing peak reaction force. The hollow cylindrical fender mentioned earlier achieves this through controlled buckling .
When multiple fenders share the berthing impact, each fender operates at lower deflection—reducing reaction force per fender while total energy absorption remains adequate. PIANC WG211 introduces the γmult factor specifically for this scenario .
The ideal fender curve rises steeply to a moderate peak, then maintains a plateau. This "square" curve maximizes area (energy) without excessive peak force. Patent literature describes designs that approximate this ideal curve through multi-stage buckling .
1. Define design berthing energy using PIANC WG211 methodology
2. Determine structural reaction force capacity from civil engineering assessment
3. Obtain fender performance curves showing both R and E at all deflections
4. Apply correction factors for temperature and velocity
5. Verify both criteria: Ef,d ≥ design energy AND Rf,d ≤ structural capacity
6. Document assumptions and require third-party verification
7. Consider multiple fender contact if applicable
Reaction force and energy absorption are not competing metrics—they are complementary requirements that must both be satisfied for a fender system to perform as designed.
A fender that absorbs adequate energy but exceeds structural reaction force capacity is a liability. A fender that protects the structure but fails to absorb berthing energy is equally inadequate.
At NANJING TAIDUN MARINE EQUIPMENT ENGINEERING CO., LTD, we manufacture fender systems with documented performance curves, verified through compression testing per PIANC and ASTM standards. Our 23 years of experience serving ports in 80+ countries means we understand the real-world implications of these metrics—and we provide the data our OEM partners need to verify both.
[CTA]
Need fender performance data for your next specification?
Contact NANJING TAIDUN for:
- Performance curves with reaction force and energy absorption data
- Third-party certified compression test reports (CCS, BV, LR, TUV, SGS, INTERTEK, GL)
- OEM/ODM services for rubber fenders, mooring bollards, steel panels, and UHMW-PE pads
Email: service@taidunmarine.com
Website: www.taidunmarine.com
1. PIANC MarCom WG 211. "PIANC Fender Guidelines 2024." [https://www.kscoe.or.kr/!/download/?path=/media/51/board/2025/01/31/7._PIANC_MarCom_WG_211.pdf]
2. CDIT. "Fender Design Test Guideline." [http://www.cdit.or.jp/en/FenderDesignTestGuideline.pdf]
3. Roubos, A.A., Iversen, R., et al. "Backgrounds partial energy factors for fender design." 35th PIANC World Congress, 2024. [https://repository.tudelft.nl/file/File_db548108-c04f-411d-996e-8cdde51e4be6]
4. ASTM International. "A Forcing Function for Testing Marine Fenders with Comparison to ASTM F2192-05(17) and PIANC Guidelines." [https://dl.astm.org/jte/article-abstract/50/3/1265/22157/]
5. Italian Ministry of Infrastructure. "Fender Design Documentation – PIANC WG211 Parameters." [https://va.mite.gov.it/File/Documento/1242403]
6. US Army Corps of Engineers. "UFC 4-152-01: Design: Piers and Wharves." [https://stg.wbdg.org/FFC/DOD/UFC/ufc_4_152_01_2017_c1.pdf]
7. US Patent 4355792. "Hollow marine fender." [https://patents.google.com/patent/US4355792]
8. US Patent 20010007236A1. "Fender." [https://patents.google.com/patent/US20010007236A1]
9. PIANC Australia. "Navigating PIANC Fender Guidelines 2024 (WG211)." [https://pianc.org.au/wp-content/uploads/2025/12/Presentation-Slide-Master-PIANC-Fender-Workshop-30-July-2025-Brisbane.pdf]
10. DPWH. "Standard Specifications – Fender Materials Testing." [https://dpwh.gov.ph/DPWH/sites/default/files/issuances/DO_007_s2018.pdf]
Reaction force is the force the fender transmits to the supporting structure—it determines structural design requirements. Energy absorption is the work the fender performs in stopping the vessel—it determines berthing safety. Both are measured simultaneously during compression testing and are displayed on the same force-deflection curve. You cannot properly specify a fender without evaluating both.
No. A fender that absorbs adequate energy but produces excessive reaction force will damage or destroy your quay structure. Conversely, a fender with acceptable reaction force but insufficient energy absorption will fail to stop the vessel safely. PIANC WG211 requires separate verification for both metrics.
At low temperatures, rubber stiffens: reaction force increases by approximately 12% at 4°C, while energy absorption decreases by approximately 4%. At high temperatures, the opposite occurs. Both effects must be accounted for using the temperature correction factors published in the fender manufacturer's catalog .
PIANC WG211 (2024) introduces separate partial resistance factors for energy (γf) and reaction force (γR), requiring independent verification. The guideline also addresses multiple fender contact through the γmult factor, recognizing that when multiple fenders share the berthing impact, reaction forces distribute differently .
Several strategies exist: (1) select fenders with longer deflection ranges that absorb energy over a larger compression distance; (2) use multiple fenders sharing the impact to reduce per-unit reaction force; (3) specify fenders with optimized force-deflection curves that maximize area without excessive peak force. The hollow cylindrical design, for example, achieves approximately 15% higher efficiency (E/RW) than solid designs .
Request: (1) Force-deflection performance curves showing both R and E at all deflections; (2) compression test reports per PIANC or ASTM standards; (3) temperature and velocity correction factor documentation; (4) material certificates for rubber and steel; (5) third-party inspection reports (BV, SGS, ABS, etc.); (6) installation and maintenance manuals.
