Before I start, let me disclose that I don't know a lot about the details of infrastructure projects or your file format, but I think the general considerations are fairly universal.
There are a number of considerations when providing your digital model to a contractor, and in my work I have decided to offer them my model in some situations, but not in others. Here are the main factors:
Linear Time. By providing a model, you can probably save the project significant linear time by reducing the amount of work the contractor needs to in order to prepare their shop drawings, allowing the work to start and finish sooner. This is probably the most compelling reason to provide the model. It's worth noting that in some industries where file formats and workflows aren't well standardized, sometimes this efficiency can't actually be realized as things have to be re-drawn or re-modeled anyway for the target platform.
Risk. If there is a discrepancy between the model and your drawing, there should be language that instructs the contractor to follow the drawing. However, you have no control over how the contractor uses your model, and they may uncover an error in the model that you never noticed or didn't care about because it is never shown in the drawings. Depending on the contract language, these errors could expose you and your employer to additional liability.
Proprietary information. By giving the contractor your design as a complete model, rather than just output drawings, you make it much easier for them to copy designs you may have developed over multiple projects and re-appropriate them on other jobs. While a well-written contract will make that clearly illegal, it's very hard to translate that into recovering financial losses. In addition, any custom objects (blocks, toolsets, plug-ins, etc.) you have created for the software may be transmitted. Again, this kind of intellectual property is hard to control once it is out in the world.
Costs. Unless someone made a major mistake, the contractor should have bid the job assuming they had to work off of your drawings (the contract documents) and do or redo any ancillary work required to develop their shop drawings and fabricate/build the job. Likewise, your contract was just to produce the drawings, and the model is simply a tool you used. You may have created a model that is good enough for your drawings, but making it good enough to build off of would cost you money. For the added quality expectation, and the added risk, it would not be unreasonable to charge a fee for providing the model. Even if your model is perfect, the contractor may have questions about some part of it, or ask you to convert it to another format, or otherwise have modest requests that mean you need to devote time to the project that you didn't bid on.
By way of example for the discrepancy issues, when making a drawing for a machined part, I might not draw every feature to scale in my model out of laziness. While this is a bad practice, if I override the dimension and the output drawing clearly conveys the correct information, I have met my contract requirement in the design/engineering role. However, if I then provide the model, and the machine shop doesn't check the written dimension, the piece would be fabricated wrong. Each party will feel confident that this is the other party's fault. If I had never provided two redundant sources of information, blame would have been much easier to assign. This is the same reason that traditionally, we don't dimension segments as well as the whole that they add up to. We try to avoid opportunities for information we provide to be internally inconsistent.
It's obviously not directly applicable to your project, but here in the US, the AISC Code of Standard Practice (AISC 303-10) answers contract questions like these for steel projects when they aren't clearly addressed by a specific contract. See section 4.3 for the details, but the general points are:
- The contractor can only use the model with permission of the engineer, and only for the one job (not to extrapolate to future jobs)
- The contractor has to acknowledge that the model is not a contract document (with an exception for projects where the original engineering deliverable was a digital model in lieu of drawings) and is superseded by information on the drawings.
- The contractor is still responsible for verifying all information
- The contractor has to remove information not relevant to their scope of work.
- (In the commentary:) The engineer may choose to charge the contractor a fee for use of the model, without selling the intellectual property rights to the contractor.
In the end, the decision is very job specific. On a fast job with a team that has good existing relationships, it may be worth it to provide the model for free as a good will gesture. On very large projects with lots of locked up risk, it might be wise to only provide what you are contracted to. There may exist a middle ground where providing your model is an option, but only for a price.