A Discussion about Due Diligence for Engineers
This article is being prepared to help guide Professional Engineers being asked to certify or "accept with conditions", the results that are generated from specific Pole Line Design software tools. The focus of the document is dedicated to items of an Engineering nature that a P.Eng could be asked to sign-off on potential projects. Topics related to software cost or user productivity are purposely excluded. This article is very specific to items related to Pole Line Design and is similar to other more general discussion articles around the use of software by Professional Engineers, such this one.
Pole Line Design Software can have an important role to play in the approval of projects by a P.Eng. The engineer could decide to evaluate and/or approve a specific project with complete/partial or no support from any software tool or manual calculations. They can equally decide to use multiple methods and/or tools in order to fulfill their engineering obligations. The options are almost limitless and only bounded by personal preference, access to tools and/or organizational requirements.
The requirements for the final outcome are that the engineering due diligence has been applied to a project appropriately, in a manner that the engineer will support and stamp if necessary. In any event, they are putting their reputation on the line.
In order for an engineer to do this, they must understand or otherwise be comfortable with the results of any tools or processes they utilize, what they address and also what they do not address. All three aspects are very important. They are also responsible to verify the properties of all materials used in the structural models (pole, guying, soil…), the properties of the wire attachments placing loads on the structure and have an understanding of how all these properties influence the results of the analysis. Ideally a supplier of Pole Line Design Software should assist in some way to make these determinations easy to accomplish, or proactively provide them for an engineer to review. A user manual detailing the strength calculations and loading parameters of the structure would be helpful. As always, ultimate responsibility will rest with the Professional Engineer for the correct use of tools they use.
The process of designing new pole lines or assessing existing lines for potential new attachments can be complex and the software used for Pole Line Design may assist with one or many parts of this process. This article provides recommendations related to code compliancy and engineering due diligence only.
There are several key areas where codes(such as CSA and NESC) provide specific requirements; most notably:
- Clearances and separations
- Storm and other assumed loads
- Load Factors
- Strength Analysis of structures:
- The Structure itself including internal strength members and guying,
- Things that support the structure (anchors, foundation, soil)
- Strength Factors
The software supplier should provide sufficient information to the approving engineer to make them comfortable with the methodologies used for assessment or that the results are consistently accurate. Ideal responses would provide direct proof and/or easy instructions to follow by an engineer to validate what the supplier provides. The user manual of the software should detail how each of the usages and capacities of the members are calculated.
The software supplier should have a procedure in place to check their software on many common scenarios through a regression of examples such that when new versions are released the results would be verified to not change. If results have changed between versions they should be explained with a change log or remedied before release.
The software supplier should clearly identify what National and other codes that their software supports. For each of these codes or standards, a detailed list of the various clauses should be provided with a statement about compliancy for each one. (Yes, No, Partial):
- Partial responses should be fully explained as to what is covered and what is not.
- Yes and No responses may provide further explanations for clarity.
With all codes it is upon the engineer of record to interpret the requirements of the code and make sure the software meets their interpretation of the code. Code checks supported in the software should be documented in the user manual or supplemental material supplied by the software provider.
There are some concepts that some codes do not spell out explicitly, but are expected to be addressed, through tools or otherwise, by competent engineers. The extent that Pole Line Design Software Tools cover these is important for an engineer to know and for suppliers to report.
- The first one is that all clauses in the code need to be satisfied. This means that the final design solution needs to consider the worst case scenario for each individual clause. In other words, the worst case scenario to address one clause may not be the right choice for another. The supplier should either indicate what scenario would be used (alternatively how it would be chosen) to address each clause, or provide the means for the engineer to choose the scenario themselves.
- The second one is related to weather/storm loads. Both NESC and CSA codes require that the wind direction be chosen to result in the maximum load applied to the structure and its supports (guys, anchors, pole braces, internal members). In other words – wind can come from any direction, so be sure to evaluate the worst condition. For simply supported poles this means just one direction. For more complicated poles, additional wind directions will need to be evaluated to be sure that each component (guy/anchor) experiences its worst condition as well. Since the wind can come from any direction, it is possible that wind directions where the structure is more lightly loaded could cause higher loads in some of the guys and anchors.
- For Deterministic Storm Loadings, it is implied that the required amount of ice loading at the freezing point with no wind is an additional load case that needs to be considered. This is primarily for clearance tests as it may be the maximum sag condition. The applicability of Extreme Wind, Extreme Ice, and Ice with Wind Loading should be considered.
- The base elevation of poles and changes in attachment heights along the pole line can impact clearances, separations, sags and tensions. Therefore, the most accurate results are obtained when the actual geometry of the line is used in the analysis of structures.
- The materials used in wires for span attachments generally have a non-linear Modulus of Elasticity. Therefore the permanent stretch possible in certain wires under creep or under load can impact the design values used for Sags & Tensions.
- Maximum Sag and Maximum Tension Load Conditions for span attachments are rarely the same.
- There is more than one type of Nonlinear Analysis that can be performed on Structures. The more nonlinear considerations utilized, the more accurate will be the final result. Some codes have minimum requirements for non-linear considerations. If a supplier's software performs a Non-linear Analysis, it should clearly identify the extent of the Non-linear considerations; including but not limited to:
- Non-linearity of member reactions due to structure deflection (Geometric Non-linearity)
- Material Non-linearity of Wire attachments (Non-linear Sags & Tensions)
- Material Non-linearity of Guy attachments (Stretching of guy wires under load)
- Non-linearity of loads due to structure deflection (Load Non-linearity).
- Non-linearity/flexibility of attachment supports .