Dr. Lowery-

I have done both problems for the Roof Pump (i.e. Excel and EES) and they are giving me two different positions for where the pump should be. The final cost difference is only $11 and some change but the position of the pump is greatly different which then changes the length of pipes and number of joints. Is this due to each program’s rounding methods or do you think I might have an equation wrong? Thank you for your assistance. See you Tuesday.

No, you have nothing wrong, and yes, rounding influences the answer. You are correct in assuming that the programs indeed get different answers for the optimum location. Notice that there is only $11 difference in the two answers, and both programs are saying “Close enough. I’m going home.”

If you put EES’s answer into Excel as a starting point, Excel may well give you the same answer , although they use different methods of solution.

Factors such as where the program starts the solution (i.e. the initial x,y guess), how many iterations he should run, how much accuracy you want per iteration, etc. can all be adjusted in the programs, but either answer will normally suffice.

Do we have to perform a hand solution for all iterations of our computer solutions?

No. When I say you have to have a hand solution, I want one calculation for each typical variable for one iteration, just to make sure your equation is giving you what you want. Thus on the roof pump problem I expect you to initially put the pump someplace on the roof, not (0,0) where something might not calculate properly, but at say (10,20), and calculate the length of the pipe from the pump to a typical drop point at (30,10), and make sure that’s what the program is giving you. You don’t have to do all 10 pipes. Then hand calculate the cost of one of the pipes, including the joints, by hand and make sure that’s what the computer gives you. Then make sure the sum of all costs is correct.

That is good enough.

For the footing you should run out the calculations for volumes, weights, costs, sums, etc. for the first iteration and check only that they are correct.

—–Original Message—–

From: Matthew

To: lowery@tamu.edu

Subject: CVEN 322 Roof Problem

Dr. Lowery,

For the roof problem using Excel, it states that a special reinforcement pipe of 3 feet long will be used at the drop points. Does this pipe act purely as reinforcement around the pipe that will already be there to connect to the pump, or will it be a connecting piece of pipe adding length to the other pipe that goes to the pump? Basically, do we use the distance formula like normal and add the cost of the special 3 foot pipe later since it will not add distance if used as reinforcement, or do we need to subtract the 3 feet from the distance formula to find normal length of pipe in the case it does add length to the normal pipes?

I’m not sure which route is intended. Any help would be appreciated.

Best Regards,

Matthew

Matthew:

Danged if I know. Let me call the boss.

L^3

Dr. Lowery:

The Guard Cans are only required on some installations. They are used to structurally protect critical piping at points of entry against impact from equipment used on the roof such as window cleaning scaffolding, etc. They carry no pressure or product and merely guard the existing pipe.

Joe