ASCE 113 Section 3.1.11.3 Interpretation 1

[cajeng]

Hello,

I was wondering if anyone could help me in understanding Section 3.1.11.3 of ASCE 113 concerning deflection calculations. This section suggests using the un-factored extreme wind and un-factored extreme icing loads if the equipment is operating during extreme wind or extreme ice. Is the factor this section is speaking of the conversion factors mentioned in the sections prior (3.1.11.1 & 3.1.11.2) or the load factors? I'm inclined to think these factors are the conversion factors, which reduce the loads applied, rather than the load factors, which increase the loads applied. It seems odd to me that we would consider a reduced load for extreme conditions (i.e. not using load factors). Any help is greatly appreciated!

 

[transmissiontowers]

We are currently revising ASCE 113 and will clarify some things (I Chair the committee). Section 3.1.11.1 has a slight error in the wording for Table 3-14. It should say "Wind deflection velocity conversion factor" (0.78 for the 5 year MRI). The intent was to multiply the extreme wind velocity by 0.78 to get a 5 year MRI wind speed so you can check the deflection of the various parts of your substation structure. Equation 3-1 has this velocity squared so the effective load includes the 0.78 squared and the force on the structure is 0.608 compared to the force for the full extreme wind load.

Of course the ASCE 113 is only a guide and you are free to consider the deflection at what ever wind velocity and ice thickness you want the equipment to operate at. The intent of the guide was to point the end user structural engineer toward some industry accepted limits. It is up to the substation owner to decide if they want the structure to be able to deflect to the limits in Section 4 of the guide at some wind speed where qualified electrical workers will be in the substation trying to open switches. We chose to limit the extreme wind to a 5 year MRI (for 140 mph in my service area, that would knock the wind down to 109 mph) In my case we would design the structure to deflect less than the limits in section 4 in a 109 mph hurricane. I would not want to be inside a substation fence in a 109 mph hurricane with energized 345kV wires swinging around, but some people are into extreme sports

The second case to consider is extreme ice with concurrent wind in Table 3-15. Since the factor for wind is 1.0, it does not matter that we meant to factor the wind velocity, but that was the intent. The ice thickness for a 5 year MRI is 0.50 of the ice thickness listed in the ice map in Figures 3-3a through 3-3f.

We also intended for the wind velocities to be applied in the wind direction that would produce the most deflection on the structure.

Post back if you need more insight into ASCE 113.

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I have been called "A storehouse of worthless information" many times.

 

[cajeng]

Thanks for clearing that up transmissiontowers, that was exactly what I was wondering. I agree it would be much more clear if the table said "Wind deflection velocity conversion factor." I will be sure to let you know if I have any more questions! Thanks again.

 

[transmissiontowers]

I don't check this site often, so post in this thread if you have other questions. I get an eMail notice when someone replies to a thread I commented on.

We are wrestling with the new wind maps in the 113 revision so it will be different along with the deflection wind cases. We were hoping to get the revised document back to ASCE in late 2015 but I am not sure if we will make that date.

And my real name is George, but I have been doing towers since 1973 so I picked that handle.

_____________________________________
I have been called "A storehouse of worthless information" many times.