Hello,
This is appears to be occurring because the default design constraints for certain elements are being violated. I will note that I tested this in each of the last three versions of SewerCAD (including yours). In each, the same results were found.
The design solver will try to find the best solution within the contraints applied, but there are a set of design priorities that must be followed as well. For instance, the design solver will not allow for a downstream conduit to be smaller in diameter than an upstream conduit. So even if an optimal design would be to have a smaller conduit in place, if the conduit directly upstream was larger than that design option, the smaller size could not be used. In other words, the design conduit must be the same size or larger than the upstream conduit. There are a number of these design priorities, which can be found below. The designed size of the conduit must be meet these in order to be used. Some priorities are given greater weight than others, which is high some design priorities (like velocity constraint) can be violoated at times.
When using a simple part-full design of 75%, conduits CO-1 through CO-7 do not meet the minimum velocity constraint. In other words, the velocity in the conduit is lower than the minimum velocity design constraint that you have entered. In some of these, like CO-1, a smaller size cannot be chosen because the smallest size is already being used. Other conduits may see other design priorities given a higher priority than the velocity constraint being violated if a smaller size is used, such as design discharge or slope constraints. For instance, if CO-7 were smaller, the velocity constraint would not be violated. However, CO-6 is would then be larger *and* the design discharge constraint would be violoated. Both of these are higher priority design priorities than the velocity constraint, and therefore CO-7 cannot be designed at a smaller diameter.
When you use the table part-full design, where some of the conduits have a lower part-full percentage, some other constraints are also not met. The same design priorities are used in this case, and must be satisfied before a possible conduit size can be used. As an example, for CO-6 violates the velocity constraint. CO-5 is smaller than CO-6, so a smaller size could be used. However, if the smaller size is selected, the design discharge would be above capacity. Since the design discharge constraint is a higher priority, the smaller size conduit cannot be used.
The design of the conduits and manholes are all interconnected, so you will want to make sure that you review not just the conduit receiving the user notification, but also to look at the conduits and nodes around it, keeping in mind that there may be reasons not only why the conduit in question is not allowed a different diameter, but why those around it might not as well. You may also want to review the settings in the Default Design Constraints to assure that the correct data is entered for your system. In addition, I would review the information below, pertaining to the design priorities used in SewerCAD's design solver. Higher priority items are at the top, meaning they are given greater consideration in the design of a system than those with a lower priority.
Please let us know if you have any additional questions on this issue.
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Design Priorities
A Designed Pipe Should Fit within Adjacent Existing Structures
If a pipe connects to an existing structure, the pipe rise should be completely within the existing structure. The only time this may be violated is if there are no available section sizes that would not violate that condition (i.e., the existing structure height is so small that all available pipes have rises too big). In this very unlikely condition, the smallest available section size will be selected, with the invert elevation placed at the bottom of the structure.
A Designed Pipe Should Not Have a Crown Above an Adjacent Designed Structure
Where pipe inverts are fixed, it is possible that the required section size would cause the pipe crown to be higher than the top elevation of an adjacent designed structure. If all available pipe section rises are greater than the depth of the pipe invert, the smallest pipe size will be chosen.
Note: This situation will only be encountered in situations where the structure's top elevation is set equal to the ground elevation - otherwise, the structure will be designed with a higher top elevation.
Pipe Capacity Should Be Greater Than the Discharge
If the pipe is not limited by adjacent structures, the pipe should be sized such that the design capacity is greater than the calculated discharge in the pipe. The design capacity may be based on one or more pipes, flowing full or part-full, depending on user-set design options. If site restrictions or available section limitations result in a situation where no sections meet the required capacity, the largest available size and number of barrels will be chosen.
Downstream Pipes Should Be at Least as Large as Upstream Pipes
Designs typically avoid sizing downstream pipes smaller than upstream pipes, regardless of differing slope and velocity requirements. One of the primary reasons for this is debris that passes through the upstream pipe could become caught in the connecting structure, clogging the sewer.
Pipe Matching Criteria Downstream Should Be Met
Whenever possible, the designed pipe should have its downstream invert set such that the pipe meets the matching criteria, such as matching inverts or crowns. Note that because of higher design priorities, such as the pipe fitting within existing structures, the matching criteria may not always be met.
Minimum Cover Constraint Should Be Met
Pipe inverts should be set such that the upstream and downstream crowns of the pipe are below the ground elevation by at least the amount of the minimum cover. Note that higher design priorities, such as existing structure locations and matching criteria, may prevent the minimum cover constraint from being met.
Pipe Matching Criteria Upstream Should Be Met
The upstream invert of the designed pipe should be set to meet the matching criteria of the upstream structure. Higher design priorities, such as minimum cover constraints, may result in a pipe that does not match upstream as desired.
Maximum Slope Constraint Should Be Met
Wherever possible, the designed pipe should not exceed the desired maximum slope. In some situations, elevation differences across the system may result in a case where a drop structure can be used to offset pipes. This is used instead of a pipe that is too steep, or instead of upstream piping that would require much more excavation. Note that the maximum slope constraint may be violated if higher priority design considerations, such as existing structure location or pipe matching criteria, governs.
Other Constraints and Considerations
There are many degrees of freedom when designing a piping system. Several constraints that are not mentioned above, such as minimum velocity constraints and minimum slope constraints, may also result in adjustments to the designed pipe. Other constraints may be too limiting, such as maximum cover constraint and maximum velocity, resulting in designed pipes that could violate too many other constraints.
This wide range of choices and priorities emphasizes the need for careful review of any automated design by a professional. It is not always possible to meet every desired condition, so it is very much the responsibility of the engineer to make final judgments and decisions regarding the best design for the client.
If you have any other questions or concerns, please let us know.
Regards,
Scott Kampa
Bentley Technical Support