Introduction:

SwingStoppers worked well through Wilma’s pounding. It inspired us to research strength testing through Structural Finite Element Analysis (FEA) to verify that our design will withstand stronger storms and larger boats. FARRIS Automated Systems L.L.C. (FAS). http://www.farrisautomation.com setup the model we provided them using worst case parameters. Because our system becomes stronger when the pilings increase in number or are closer together we tested with a piling distance of 13.5 feet. Most lifts use a piling distance of 12.5 feet.

In modeling our two boat sizes we chose a 25 foot Grady-White and a 33 foot Grady-White but for structural analysis we used the flat silhouette of each boat. The actual boats are curved which will reduce the wind force applied to the boat. http://www.gradywhite.com

During the FEA testing cycle we made some modifications to the original hardware to further increase its durability. Below are the FEA results modeling our final design

Formal Report:


structure_clip_image002Job:047-01

Project Name:  FEA Analysis of Structural Members

The following information provides a summary of the FEA (Finite Element Analysis) done on a proposed anti-swing mechanism designed by Paul and Holly Warters.

Farris Automated Systems LLC performed the analysis using Solidworks Cosmos Express – 3D. The structure was tested in a static mode and utilized surface mesh calculations.

Pass / Fail Criteria

The design conditions and method of use allowed us to use deflection as the driving criteria for a Pass/Fail grade. This is due to:

      • The intermittent use of the structure
      • If the brace or angle does bend slightly, it will still prevent swinging.

It was determined that an FEA resulting in a permanent deflection of less than 0.25” would result as a passing grade.

Test assumptions

      • The boat is secured to the I-beam and does not slip.
      • The entire weight of the boat is taken up by the lift cables
      • The boat is lifted higher than the tidal surge
      • The boat is non-aero-dynamic and acts as a wall to the air
      • The wood pilings are secure and will not fail
      • The metal/wood has not been affected by the elements
      • All bolt connections are tight and will not cause the joints to slip
      • Pilings distances are 13.5 feet apart.

“Hurricane Force Winds”

The equation used to translate the wind speed into force upon a boat was:

Force = ½ x Surface area x (density of air) x (wind speed)^2 x (shape factor)

Wind speed was taken from the US metrological website data using the Saffir/Simpson scale. The chart on page 2 shows the wind force on a 25 Ft boat and a 33 Ft boat in both directions. Both boats were Grady White silhouettes.

Safety Factors of evaluation

      • Wind speeds for each category was taken at the highest speed.
      • Wind speeds for 3.5 and 4.5 were averaged between the highs of the two categories.
      • The boat was modeled as wall. Aerodynamics will decrease the actual forces.
      • Boat inertia was not taken into consideration. Inertia will result in less deflection but could not be calculated due to the differing boat styles and random wind gusts.
      • Calculated wind forces were multiplied by 1.2 in order to account for a boat or gust that does not distribute the wind’s side loading force evenly throughout the dock structure. The model used a flat sided boat while an actual boat will have contours that will variably increase or decrease the wind’s side loading force.
      • The Cradle L-Bracket was designed with a 2x safety factor.

RESULTS

The results are summarized under the following tables.

Both Sides of Lift Restrained
3″ I-Beam 4″ I-Beam
25ft Boat, Secured on Both Sides
25ft Boat, Secured on Both Sides
Hurricane
Side
Back
Hurricane
Side
Back
Cat
Wind Speed
Force
Def.
Force
Def.
Cat
Wind Speed
Force
Def.
Force
Def.

1

95
1700
<.25″
1200
<.23″
1
95
1200
<.23″
1200
<.17″
2
110
2300
<.25″
1650
<.23″
2
110
1650
<.23″
1650
<.17″
3
130
3200
.25″
2300
<.23″
3
130
3200
<.23″
2300
<.17″
4.5
143
3600
.28″
2750
<.23″
4.5
143
3600
<.23″
2750
<.17″
5.5
163
4800
3500
.23″
5.5
163
4800
.23″
3500
<.17″
3″ I-Beam 4″ I-Beam
33ft Boat, Secured on Both Sides
33ft Boat, Secured on Both Sides
Hurricane
Side
Back
Hurricane
Side
Back
Cat
Wind Speed
Force
Def.
Force
Def.
Cat
Wind Speed
Force
Def.
Force
Def.

1

95
2500
<.25″
1200
<.23″
1
95
2500
<.25″
1200
<.17″
2
110
3400
.27″
1650
<.23″
2
110
3400
<.25″
1650
<.17″
3
130
4900
2300
<.23″
3
130
4900
<.25″
2300
<.17″
4.5
143
5250
2750
<.23″
4.5
143
5250
.25″
2750
<.17″
5.5
163
7500
3500
.23″
5.5
163
7500
.36″
3500
.17″
 

 

One Side of Lift Restrained
3″ I-Beam 4″ I-Beam
25ft Boat, Secured on One Side
25ft Boat, Secured on One Side
Hurricane
Side
Back
Hurricane
Side
Back
Cat
Wind Speed
Force
Def.
Force
Def.
Cat
Wind Speed
Force
Def.
Force
Def.

1

95
1700
>.30″
1200
>15″
1
95
1200
.24″
1200
10″
2
110
2300
1650
2
110
1650
>.30″
1650
3
130
3200
2300
3
130
3200
2300
4.5
143
3600
2750
4.5
143
3600
2750
5.5
163
4800
3500
5.5
163
4800
3500
3″ I-Beam 4″ I-Beam
33ft Boat, Secured on One Side
33ft Boat, Secured on One Side
Hurricane
Side
Back
Hurricane
Side
Back
Cat
Wind Speed
Force
Def.
Force
Def.
Cat
Wind Speed
Force
Def.
Force
Def.

1

95
2500
>.30″
1200
>15″
1
95
2500
>.30″
1200
>12″
2
110
3400
1650
2
110
3400
1650
3
130
4900
2300
3
130
4900
2300
4.5
143
5250
2750
4.5
143
5250
2750
5.5
163
7500
3500
5.5
163
7500
3500

Passing conditions are shown in Green.

Final Recommendations

  1. The piling brace I-beam should be at least 3” high. For a larger boat with piling spacing over 13.5 feet a 4” I-beam should be used. C-channel has been tested and failed before the target wind speeds of 3.5 hurricane level could be reached.
  2. The Cradle L-Bracket was designed with a 2x safety factor. Since the L-Bracket is the strongest part of the design a number of different shapes will fall well within specification. It is key to keep the bottom plate 4” wide to maintain the 2x safety factor. The important design factors should be:

        • 3/8 inch aluminum plate making a 4” wide base.
        • At least 1 gusset that is a minimum of 1/4” thick and 1.75” long
        • 1/8” – 1/4” fillet welds

  3. Clamp the L bracket using 4 long bolts or threaded rod. The will add additional clamp load and friction to prevent the components from slipping.
SwingStoppers-2

Farris Automated Systems LLC makes every attempt to interpret the application and relevant data to forecast accurate results. Do not base your design decisions solely on the data presented in this report. Use this information in conjunction with experimental data and practical experience. Field testing is mandatory to validate your final design. COSMOSWorks helps you reduce your time-to-market by reducing but not eliminating field tests.