A Bridge Maintenance Case Study

By Julio Banks

Abstract

A Bridge Maintenance Frame required a FEA (Finite Element Analysis) for a client with the objective of reducing the lateral deflection of the frame from 7” to 3/4”.  I produced the solution within 10 hours using a combination of Mathcad, IronCAD, FEMAP and NEi Nastran FEA software.

This study was required to understand the structural adequacy of a mobile frame and catwalk designed to be used for bridge maintenance and repair by a crew of maximum two occupants.  Its design needed to meet Occupational Safety and Health Administration (OSHA) requirements. The FEA showed that the lateral deflection was reduced from 7” to 3/4” as desired.

Description of the Problem

A bridge maintenance frame was arbitrarily modified by the fabricator. The base of the frame was doubled in width. This is a typical example where no thought was given to the response of the platform such as lateral deflection due to the increase in width of the platform. The original fabricator maintained the single 2x2x0.124 HSS made from steel. During the modification study, the shop proposed the use of double the original 2x2x0.124 HSS without any offset. I promptly demonstrated that a 6” minimum offset would be required to take advantage of the parallel-axis theorem use of the square-influence of the offset dimension multiplying the cross sectional are of the HSS (See Figure 1)

Figure 1

Proposed Solution

Manual calculations using Mathcad indicated that the solution was to offset the vertical members from the bottom of the platform to the top and around the C-shape of the lateral portions of the frame at minimum, six inches. The final geometry is shown in Figure 1.

Method of Solution

The solution was made most effective by the use of the combined application of Mathcad (for manual calculations), IronCAD (for solids modeling), FEMAP (for FEA Pre- and Post-processing) and NEi Nastran (for FEA solution) for the structural deflections and stresses. I have used Mathcad, IronCAD, FEMAP and NEi Nastran for over twelve continuous years and find such combination of analytical tools to be the most cost-effective method of solution especially for solo-consultants.

Mathcad Manual Analyses

The first step in a structural analysis is to determine, at a global level, the geometry and material to be used for a given set of loads. Mathcad is the best cost-effective means of achieving cost-effective and prompt solutions. Once the most promising design candidates are selected, then the generation of detail geometry proceeds to be performed using IronCAD solids modeling.

IronCAD Solid Modeling

IronCAD, which I believe is the best solid modeling software, easily allows me to create a full 3D solid model of the frame. Additionally, IronCAD is especially useful for analyst engineers since it allows the “slicing” of 3D surfaces and solids to accommodate the FE (Finite Elements) entities (nodes and elements). With its short learning curve and ease of use, I can shorten the project time by spending more time on the analysis versus the design aspects.

NEi Nastran Finite Element Analyses (FEA)

In order to effectively ensure the requirements mentioned in the abstract above, I used NEi Nastran FEA software to run a structural analysis.  The mesh consisted of plate and beam elements for the walking surfaces and tubular elements for the supporting structure. A steady-state gravity load was applied, in accordance with OSHA requirements, to represent the weight of the occupants. Lastly, fixed constraints were placed on the supporting structure to represent real world conditions. The FEA model shown in Figure 1 was created using IronCAD and Figure 2 shows the FEA solution.

The model was analyzed with the objective of reducing the lateral deflection of the frame from 7” to 3/4”.  By using the NEi Nastran FEA software, I was able to adjust localized structural properties (such as moment of inertia by offsetting a new HSS member) to produce reactions at the supports, as well as beam and plate stresses and deflections. With these tools in hand, both the customer’s and OSHA’s specifications were met before any prototypes had to be manufactured.

Figure 2

Having used the software for over 14 years, I believe that NEi Nastran is truly reliable software allowing me to make a commitment to my consulting client and deliver without any delays.

Conclusion

This case can serve as a cautionary case due to the ease of use of current FEA software at affordable prices. It is recommended that, at a minimum, companies should have their design reviewed by either in-house consultant or independent consultants to ensure that a FEA user has not made critical errors during the FEA modeling and interpretation of results. This warning is especially of greatest importance if the design is mass produced such as consumer products with potential harm to the users in the event of a structural failure.

About Julio Banks

Julio C. Banks, MSME, PE & CGC is the owner of J. C. Banks Construction, LLC established in 1993. Julio has had direct experience in the analysis and design of thermo fluids (incompressible & compressible) flow as well as structural engineering consulting.  He has worked in diverse industries such as Fossil and Nuclear (Stone & Webster), Military-grade Weapons (General Electric), Turbojet  (Pratt & Whitney) and Turboshat (Pratt & Whitney and Siemens-Westinghouse), Airframes (Northrop Grumman, and Piper Aircraft), as well as commercial and residential construction including General Contracting. Numerical Analysis and Solution Methods is his passion and he considers the subject of computational algorithms a hobby.

J. C. Banks Construction, LLC
P. O. Box 880187
Port Saint Lucie, FL 34988-0187
USA
Phone: 1.772.204.8888
Fax: 1.772.204.8899
Email: BanksJ (at) asme (dot) org

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