Light Post Base Design 2

[PEinVA]

Hi,
I'm currently working on a project which is a 12 story hotel renovation. There isn't too much going on structurally. However, they are adding some light posts to the outdoor pool deck, which is an elevated 7" slab. This was designed in the late 1960's. The slab is designed as a two way beam supported slab.

My boss was saying that these poles usually require large connections due to larger loads at taller heights. So I started to calculate the wind load applied to the to ornamental part of the light fixture, and also the eccentric weight due to the shape of the light fixture. See attached cut sheet from the manufacturer.

How would I calculate the wind load? I used ASCE 7 Ch. 6 and used design for a round chimney to determine the wind speed to apply to the area at the top of the light.

This particular light is 14' tall. I used a 10 ft^2 area and only 11 psf for wind. My moment from this is only 1540 #-ft. roughly. That doesn't seem like of real concern to me. Am I missing something?

Thanks for the feedback.



RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[JedClampett]

Take a look a the light pole catalogues. GE has a good one. They use a factor called an Effective Projected Area (EPA). This is what they design the pole for. This limits the amount of fixtures that can be installed on any pole. Each fixture is assigned a EPA and you add them up. Once you put the pole up, you have no control over what is installed on it (speakers, cameras, more lights), so you need to design for the pole's limits.
The catalogue will also have a base detail with anchor bolts (number and sizes) for every pole. You can either design for the capacity of the anchor bolts (conservative) or apply the EPA at some conservative height (not all the way at the top), use the wind load from ASCE 7, and design for that.
Whatever you do, don't depend on the light pole manufacturer to design anything. All you'll do is delay the day when you have to do it yourself.

 

[PEinVA]

JedClampett,

I don't think your last statement can be emphasized enough. I ran around chasing my tail, even calling Canada, for them to tell me, oh we don't design anchor bolts, it is not what we do. I wanted to reach through the phone and smack him, because why the heck do you size the bolts and arrange them then? Why can't you just weld to my plate whatever size I want to make it.

Anywho, sorry about the rant, I realized that result. Also, the type that I'm using doesn't publish EPA, so I just looked at some examples, measured the area (my guess), and added a factor of 4 on it.

The loads just seemed smaller than what my boss was hinting at.

Is it odd that 3/4" epoxy bolts would work for this? Or is that something you've seen before?


Jed - star for you, I just wish I posted here before I ran around claiming SOMEONE HAS TO KNOW WHAT THE **** IS GOING ON!

Thanks again.



RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[JedClampett]

I wouldn't be surprised if 3/4 inch epoxy anchors were sufficient for a 14 ft. pole. Compared to some of the other loads that we design for structurally, wind on a narrow pole really isn't that much.
The other thing I've learned, time and time again, is that the pole manufacturer will have nothing to do with isolated pole foundations. Our typical detail (an electrical detail) calls for the pole manufacturer to specify a depth and diameter for the foundation. The next time they do that will be the first time. And I just can't get the detail changed.

 

[civilperson]

Is the pole height measured from grade or from the pool deck? If the pool deck is on the 12th story, (unclear from original post), the height is near 130 feet. The wind pressure for 90 mph is 17 psf before the factors are applied. If the bolt circle diameter has four bolts, then the maximum tension on one bolt is the moment at the base divided by the diameter.

 

[lrhg]

You might want to check the minimum base material thickness for epoxy anchors. Hilti HY 150 or RE 500 will require more than 7" base material thickness for a 3/4" diam anchor.

 

[PEinVA]

civilperson,
I agree. However, when I calculate 90 mph to a wind pressure I get closer to 15 psf. I'm at a height of 35 or so feet above grade.

If I use qz = 0.00256*Kz*Kzt*Kd*V2*I

I =1
V = 90 MPH
Kz = 0.73 (Table 6-3 35' height)
Kd = 0.95 (Table 6-4: Round chimney, tanks, etc)
Kzt = 1.0

qz = 14.38 psf.


Also, if the bolt circle diameter has 4 bolts can't I use 2 bolts in tension with the distance between assuming they are in a 2 x 2 grid arrangement? This obviously puts less tension on the bolts, but I don't see why we can't do that. What do you think?

Thanks for the feedback guys!



RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[civilperson]

The wind can blow on a diagonal axis leaving one bolt in tension, (one bolt in compression and two bolts at the NA).

 

[PEinVA]

I agree.

However, the bulk of the moment at the base is from the area of the fixture. If the wind blows on an angle to the face, how do I reduce that area to account for this?

The wind on the face of the pole, (6" tapers to 3", average 4.5") is only ~15psf * (.6 ft^2)/ft height. This is equal to ~9 #/ft for wind on the pole itself.


What do you think.


Thanks for the feedback and clarifying that for me.

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[PEinVA]

lrhg,
Thanks for pointing that out. I missed your post previously, but you are correct. I'll have to review this to determine what to do.
The reason I'm trying to use epoxy bolts is because there are 14 of these lights on the elevated deck. If I have to through bolt it is going to add a lot of cost, and this job is already as tight as can be.

Thanks again.


RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[lrhg]

RC

You also might want to look at the rated temperature range of the epoxy. I am not sure where you are, but 105 - 120F high end temp for epoxy could be a problem many places in an exterior application.

LRHG

 

[PEinVA]

lrhg,
Well, I agree, I would worry about the epoxy at that temperature. But in order for the epoxy to reach that temperature the surrounding slab (which is exposed to controlled air below) would have to reach those temperatures. There will be a setting bed and light paver on top of the slab. I doubt the slab will reach temperatures higher than those experienced below the slab in the controlled environment.

Thanks for the concern though.

Any ideas about the question posed to civilperson - with regard to wind hitting on an angle. What do you guys think?

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[PEinVA]

lrhg,
Also,
Hilti, shows the depth of the slab should be controlled by 2" greater than the depth of the embedment. So while they give a hard 8-3/4" minimum thickness that is based on hef = hnom, which in that case is 6-3/4". I'm able to get away with a 4-3/4" embedment so I have an extra 2-1/4" > 2" so I'm still ok.


Thanks again.


RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[civilperson]

I doubt that the area of the fixture is reduced by any appreciable percentage due to wind at 45 degrees to the axis of the fixture. Draw it in plan and see the addition of the sides to account for total wind area. The diagonal dimension might be greater.

 

[vims]

I used to climb in light posts when I was a kid. Lots of stupid kids around a pool. Take that into account?

 

[fasboater]

I don’t have much experience with the design of light poles but I read that depending on the style of light poles you can get a lot of oscillation. It was also surprising to me that the manufacturer does not consider this in their design values. The only reason I mention this is because in the article they found failures with the attachment of base plate to the pole in as little a 3 months as I recall. I just thought it may be wise to keep this in mind.

 

[PEinVA]

fasboater,
Any idea of the name of the article or publication?

Thanks.

RC

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[fasboater]

I have attached a link on the subject. It is not the same one that I found, regretfully I didnt save it.

JB

 

[fasboater]

RCraine,

I was going throught the May 2008 Structure magazine and found on page 51 a refernce to the March 2008 artical I was telling you about. I know its probably a bit late but I finally found it..

JB