NFPA #13, Trapeze Hangers & Unistrut.

[SprinklerDesigner2]

I got a job where I have to use trapeze hangers with an 11'-6" span. Looks like I will be going with Unistrut.

I know the P1000T strut won't work for the long spans but looking at it as an example it has two Section Modulus; one for Axis 1-1 (.202) and the other for Axis 2-2 (.290). Question is which Axis would be appropriate to apply with NFPA #13 Table 9.2.2.6.1(b)?

http://img136.imageshack.us/img136/9406/unistrutcopyoj9.jpg

Indeed, is either one appropriate for use?

Then there is the "Beam Loading - P1000" chart. I guess I am not clear on understanding exactly of what a "Uniform Load" means. With a trapeze hanger the load isn't spread "uniformly" so I would be leary of hanging 480 lbs from a 84" span.

And while I am showing how much I don't know what does the "Uniform Loading Span/180" etc mean?

This is the first time I've ever had to deal with a trapeze hanger > 10'-0".

 

[TravisMack]

SD2:

I am not sure which axis you would use, but, I have a spreadsheet that will calculate your section modulus for any length span and any hanger spacing. If you email me, I will send it to you. We are not supposed to put emails here, but if you go to the AFSA website, look for designer companies, you will find me..


Travis Mack
MFP Design, LLC

 

[SprinklerDesigner2]

Travis,

Done did.

Thanks.

 

[firepe]

SprinklerDesigner2,

Should you be concerned that you are now crossing a line in the sand?

My opinion is that what you are attempting is no longer the "cookie cutter" simple approach detailed in NFPA 13.

NFPA 13, 2007, Section 9.1.1.1 states "unless the requirements of 9.1.1.2 are met, types of hangers shall be in accordance with the requirements of Section 9.1".

NFPA13, 2007, Section 9.1.1.2 states "hangers certified by a professional engineer .....shall be an acceptable alternative to the requirements of 9.1".

If your situation does not match any outlined in NFPA 13, then you are outside the scope of the "cookie cutter" methodology of NFPA 13.

This should be done by an engineer.

This is structural work, and is outside the scope of what NICET intends for a certificate holder to tackle by themselves.

Seems to me you are taking on added liability that you would not be able to defend, and you are violating the NICET Code of Ethics Item No. 2, and NICET Policy No. 32


NICET Code of Ethics
NICET-certified engineering technicians and technologists recognize that the services they render have a significant impact on the quality of life for everyone. As they perform their duties and responsibilities on behalf of the public, employers, and clients, they shall demonstrate personal integrity and competence. Accordingly, certificants shall:

2. Undertake only those assignments for which they are competent by way of their education, training, and experience.


POLICY 32. NICET CERTIFICATION AND THE PRACTICE OF ENGINEERING
The NICET certification programs are intended to award certification to individuals who have demonstrated appropriate and adequate engineering technician or technologist work experience. The NICET certification programs are not designed or intended to award certification to any individual to perform engineering services as defined under any state law or regulation as the "practice of engineering." NICET is opposed to any effort by any individual or group to misrepresent the NICET certification program as a program designed or intended to demonstrate qualifications to practice engineering as defined under state law or regulations.

Just my 2 cents.

 

[TravisMack]

NFPA 13 tells you how to calculate the modulus. Also, I believe it says you are able to extrapolate the data in the table. If NFPA 13 gives you the information on how to do it, how is it outside of the scope of a NICET certificant?

I am actually quite interested in this because I try to stay very much on the side of the NICET and not even consider crossing the practice of engineering line. If I use a product that has a published modulus (which unistrut does) and I use the calculation method in NFPA 13 to determine the modulus, how is that incorrect. I see it as no different than doing hydraulic calcualtions.

Have a great holiday weekend!

Travis Mack
MFP Design, LLC

 

[firepe]

Travis,

Sorry, but I have to point out out that the "tables" you want to hang your hat on are based on a midspan concentrated load from 15 ft of water filled pipe, and a maximum allowable bending stress of 15-ksi.

I would also note that unistrut (generic) is NOT listed in any NFPA "tables".

So while you see NFPA 13 showing examples of how to determine the section moduls for and angle iron trapeze member, what you are overlooking is what are the limitations of the "table".

All I am saying is be careful, think about what you are doing. Is placing your NICET credentials at risk worth it? Is potentially getting in hot water for practicing eengineering without proper licensing worth the risk?

Have you checked to see what bending stress was utilized by the unistrut (generic) manufacturer? I would bet good money it is NOT 15-ksi, but more likely 25, 26 or even 30 or 32-ksi.

Long story shorter, if you convert so that you're looking at all apples, the published unistrut capabilities compared to the published section modulus in the NFPA "table" don't match up, and you need a LARGER section modulus from the strut to support the weight. You might need different fasteners, you might need multiple fasteners, whatever.

The point is, this is not technician work, this is structural, and it's engineering, which is not what NICET wants technicians doing unless supervised by an engineer.

As far as your other comment "I see it as no different than doing hydraulic calcualtions."

You practice primarily in AZ? I believe that is what you have said previously. AZ has decided that it is ok for sprinkler layout and hydraulic calculations can be done by technicians, but I read the law and I see nowhere where you could even assume that structural decisions such as this are still layout. What about seismic calculations? Should you be able to do that as well? Not if you don't stray from the "cookie cutter" approach in NFPA 13, but you're walking a tight rope here.

While it's true that some states do allow for technicians to do layout and hydraulic calculations (and we'll discuss what layout exactly means in another discussion) but many more do not.

Are we just limiting the discussion to those areas that do allow technicians to do layout? Shall we discuss whether or not this means that you also get to "stretch" the intent of that ruling in those states that allows technicians to do "layout" to also do what is obviously structural engineering work?

In the original question, SprinklerDesigner2 did not know for sure which axis to use or which load to use. That does not sound to me like a good situation for a technician to be in. Again, I feel that you are making engineering judgements, that exceed your authority from NICET.

 

[SprinklerDesigner2]

firepe,

I appreciate very much your concern but on this one I don't think I will be practicing engineering. I know I won't if I stick with angle iron and, unless I can figure out what section modulus is appropriate for uni-strut, angle iron will be the way I'll go.

So I got this building with 30" joists spaced 11'-6" OC. The maximum size I am looking at is 3" sch. 10

It doesn't look like it in this photo but the joists really are 11'-6" OC. A real optical illusion, steel drawings said 11'-6" but it looked so off to me I had to physically measure it myself... it's 11'-6" between joists.

http://img139.imageshack.us/img139/3224/gymdu2.jpg

I am going to run where indicated, 10'-6" off one joist and 1'-0" off the other.

According to A.9.1.1.6 the equivalent length would be 3.65'.

Using 3" sch. 10 pipe Table 9.1.1.6.1(b) indicates the required section modulus for this particular trapeze member to be 0.29.

2 1/2" x 2" x 3/16" has a section modulus of 0.29. Hangers will be 10' OC instead of 15' but I'll just leave that alone.

Where I would be running into trouble, using angle iron, would be if the equivalent length was more then 10'-0" which is not addressed in the standard. In that case I would be required to have the services of a PE.

But rather then angle iron, steel prices are going through the roof lately, I am thinking uni-strut might be cheaper and I would probably use it if I can determine from the product literature what the section modulus is.

Perhaps it's looking right at me somewhere in the catalog but, if it isn't, I'll just go with angle iron.

Some designers might hang the main off the side of the joist but I never liked doing that. I don't think you need to be a PE to realize all that weight on one side of a joist isn't a good idea. Maybe OK for 3" but I've seen guys hang 8" and I personally don't like it.

To a point I know hanging a trapeze from 1'-0" off one side does much the same thing but if I cut the angle 12'-0" long I would have an extra 3" hanging over on the opposite side of each joist. If I had to I could place two rods one each side of the joist to center the weight.

Reason I am concerned with this is I had to do this once with Ford Motor in Detroit so if it cost me only pennies I'll go ahead and prepare for what might happen.

I also hang mains at panel points. Not called for on this project but I feel it is good practice.

So, with angle iron am I playing engineer?

 

[TravisMack]

FireFPE:

I can very much see your point. We primarily use pipe for our trapeze hangers. So, determining a modulus number from the forumla in 13 and choosing a pipe from the NFPA 13 tables seems appropriate. I can see your point using the example of unistrut.

Please don't take this as argumentative, as I don't mean to be. Is it appropriate for a non structural engineer to calculate the modulus number based on NFPA 13 formulas and choose an appropriate pipe or angle iron to meet the requirements? Or, is this something that only a structural engineer is qualified to do?

Thanks for your time.

Travis Mack
MFP Design, LLC

 

[firepe]

Travis,

As for which engineering discipline should perform these calculations, I don't think it would be limited to justa structural engineer, if the engineer was trained or had specific knowledge or experience. That is the latitude that engineers have as a self policing profession in most (maybe all) states. I do not believe that NICET technicians are afforded the same latitude.

All I am saying is that as a technician, if you are trying to do it all yourself, then at some point you will cross the line and be making engineering judgements, interpretations, and performing engineering work, that is outside of the scope of NFPA or NICET, and if you were ever brought to task for it, by a good lawyer, you would NOT be able to defend yourself. I don't care if your state has arecnt or long standing statement or gentlemans agreement about technicians doing layout work or not. Eventually, what you will be attempting is no longer just layout work.

You think you have liability insurance. You think you have errors and omissions insurance, but push come to shove, if a case is made for practicing engineering without a license, your insurance company will bail on you, and you will lose.

Tha's all I am saying. All of you technicians out there that are practicing and offering all of these services, may very well find yourself in a bind some day. Think about it, that's what I'm saying. Some areas are more gray than others.

My feeling is that if you are playing in the gray, then you are no longer a technician, you better be an engineer, or have one telling you what to do.

 

[SprinklerDesigner2]

Everyone need take firepe to heart when it comes to good lawyers. FPE's and PE's need to watch out too.

A company I once worked for was involved in an errors and ommission case and I learned a courtroom can be a brutal place.

We won but I learned everyone loses. Even winning takes to much of your life.