Location of "0" Elevation Matter in Hydr. Calc?

8/25/2021

I am reviewing a parking garage that is 5-stories in height fully equipped with an NFPA 13 sprinkler system.

The calculations provided for floors 4 and 5 indicate an elevation of 0 at the 3rd floor riser, with the elevations going in the negative values back to the hydrant.

Floor 3 calculations indicate an elevation of 0 at the Floor 2 riser. Floor 2 and 1 calculations show an elevation of 0 at the hydrant where the flow test was conducted.

Does the location of the "0" elevation affect the calculations demand, if the total change in elevation is still accounted for?

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9 Comments

Alex

8/25/2021 07:12:56 am

Hi Anonymous,

From my experience with the software, no, the negative elevation should not matter as long as your source elevation is properly adjusted as well. For instance, if you incoming water service and or fire hydrant are located on a floor with a ‘negative’ elevation, makes sure your elevation for the service matches. That way when the calc is looking for the elevation from the source to your floor, the difference between the two will be the same.

Note: If you are calc’ing via Revit, cut a section and confirm your floor elevations. The architect or who created the local model may have the floor elevations set up incorrectly.

Hope this helps,
Alex

Dan Wilder

8/25/2021 08:17:28 am

This is common when using XREF's with AutoSPRINK (I can't speak to the other design programs). The designer draws each floor at an elevation as if you were standing on the floor without regard to if the floor is +16' or +2160' above the arbitrary (to us) 0 elevation of the project.

My Casino drawing days had our BIM models elevated based on mean sea level (floors were +2100 to +2700 elevations) which made for a fun coordination effort when the BIM/VDC coordinator started the meeting asking if the pipes at 2194-5½ could be raised to 2195-8½....lots of taped cheat sheets to the sides of monitors.

As long as the vertical elevation is accounted for between the source, piping network, and flowing sprinklers, the elevation loss will be correct

Alex

8/25/2021 08:24:26 am

AutoSPRINK works awesome with CAD when everything is finally modeled currently.... until then, URG! I can picture that monitor and sticky notes now.

Glenn S Berger

8/25/2021 08:31:44 am

"0" being at the midpoint is typically down when this is the main discharge point of the elevator/building.

If I was reviewing the plans for approval, I would probably reject the set if there is a question concerning sprinkler system elevation versus the known supply system data source. I will do this this to be cognizant of any missing pipe segments and fittings required in the system.

If I could be convinced that all information was properly documented in a BIM environment that could alter my thinking.

Steven H.

8/25/2021 08:32:44 am

Alex is correct - the software will only look at changes in elevation, so you could potentially add or subtract any arbitrary amount from all of your elevations and still come up with the same answer.

In addition, the software should only care about the elevations at nodes for water sources (water supply and pump inlet/outlet) and discharge nodes (sprinklers, hoses, etc.). The elevations at your other nodes should, in theory, not have any impact on the results of your calculations (except for BOR to get an accurate bottom of riser result). This is because gravity is a conservative force, meaning any potential energy you lose by increasing elevation, is regained fully when you come back down.

You can try this out by adjusting some of your nodes in the calculation (not a source or discharge node) and putting any arbitrary value and checking to make sure your results stay the same. Of course it is good practice to put in actual elevations of your nodes--particularly to not confuse your plans reviewer--but it is convenient to be able to ignore node elevations when you are just doing a quick proof calc or something similar.

Alex

8/25/2021 09:19:26 am

You just need to be careful with gridded systems where the entire piping network is being used during the calculations.

Casey Milhorn

8/25/2021 08:38:05 am

Alex and Dan pretty much nailed it. I like to think there are three types of elevations in our calcs. The ones that have a critical outcome on the calc. The ones that have some bearing on the calc, but typically don't affect it too much. The last ones don't affect the calc at all. The critical elevations are the nodes for your water supply (source point) and then the driving (hydraulically most demanding) sprinkler. The second type of nodes are ones that do affect the calc, but not in a major way. Nodes that are in your actual calc area can have some minor affects on the calculation, but usually don't affect the calc in a major way. The elevation of the non-driving (but flowing) heads will affect how much delta flow you will see at those heads for example which can increase or decrease the total demand of the system. The last type of node are ones that don't matter at all when it comes to elevation, which are the rest of the nodes. For example, if your test node is at 0 feet and then your underground pipe coming into the building is -8', but then your driving head is at +10', you are still only going to net a 4.33 psi loss due to elevation. The fact that the underground pipe is -8' has no affect on the calc, other than the total footage of pipe. I always tell my new designers that you could have a supply main that goes up to the moon and back down to your building and it would not affect the calc as far as elevation goes. As a plan reviewer, the first thing I would look at is driving head elevation and source point elevation. That will have the biggest affect on the calc and can be a major screw up. 2nd is to check that pipe sizing on drawings matches calcs, and 3rd is to check and see if sprinkler head k factors match from drawings to calcs. If you don't look at anything else, at least check those three things. That is typically where the easiest mistakes are made AND the ones that have the most impact.

Jesse

8/25/2021 09:20:57 am

Its all relative to the supply node.

Assuming all 5 levels are above grade, the supply node would be below grade obviously. If the designer had benchmark elevation set wrong when the design was laid in, it could have shown 3rd level at 0. Its not a calculation problem if all the hydraulic nodes above and below are +/- adequately elevated relative to the 0 elevation.

If they aren't, you might have some problems.

John G

8/25/2021 09:57:31 am

I m not getting something.
If you start at the top, with ZERO, then go negative— your calculations will/ should show a gain in psi as you are loosing elevation. ( and not a loss due to elevation gain ).
So you are working with a fixed , negative, starting pressure. No accuracy.

Location of "0" Elevation Matter in Hydr. Calc?

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