New to me tower

[SteveShannon]

This is a Rohn BX48. It’ll stand 48’ above the surrounding ground. Buying it was almost certainly the easiest part. It must be attached to a solid base. The documentation describes a block of seven sack concrete that’s 5’9” square by 5’ deep. 
However, my cabin, which is where I intend to install this, is in the Boulder Batholith. I have boulders throughout my property that are larger than a truck. 
I just need to figure out how to anchor this to one of them. 
 

Edited July 31, 2023 by Sshannon
Discovered this tower is a BX48 rather than the HDBX48.

[BoxCar]

Wood screws?

[SteveShannon]

51 minutes ago, BoxCar said:

Wood screws?

Yeah, but short ones to avoid hurting the boulder.

[WRUU653]

Wedge anchor, maybe stainless for that winter rust.  https://www.hilti.com/c/CLS_FASTENER_7135/CLS_MECHANICAL_ANCHORS_7135/r8863108

 You’re going to need a good hammer drill. I would recommend getting one that uses the SDS Plus bits. You may want cordless for your location but the corded ones are usually a bit cheaper.

https://www.homedepot.com/p/Milwaukee-M18-18V-Lithium-Ion-Cordless-5-8-in-SDS-Plus-Rotary-Hammer-Tool-Only-2612-20/204390533

Your location looks beautiful. Congratulations on the new antenna! I’m sufficiently jealous

[Guest]

1 hour ago, Sshannon said:

Yeah, but short ones to avoid hurting the boulder.

Gorilla glue and ductape 

[SteveShannon]

2 hours ago, WRUU653 said:

Wedge anchor, maybe stainless for that winter rust.  https://www.hilti.com/c/CLS_FASTENER_7135/CLS_MECHANICAL_ANCHORS_7135/r8863108

 You’re going to need a good hammer drill. I would recommend getting one that uses the SDS Plus bits. You may want cordless for your location but the corded ones are usually a bit cheaper.

https://www.homedepot.com/p/Milwaukee-M18-18V-Lithium-Ion-Cordless-5-8-in-SDS-Plus-Rotary-Hammer-Tool-Only-2612-20/204390533

Your location looks beautiful. Congratulations on the new antenna! I’m sufficiently jealous

I’ll attach a picture of the cabin.

I have a Makita SDS Plus hammer drill already. Corded.

I’ve been wistfully looking at an SDS Max lately, cordless.

The anchors were what I was thinking about. I’m not sure if I should drill the boulder and epoxy in a bunch of rebar and then pour a thick slab on top, or if sufficiently long anchors and flanges bolted to the legs of the bottom section would be enough. 
Here’s a picture of the boulder I have in sight. It’s in shadow in the picture but the exposed face is probably about six feet tall.  

 

[Guest]

4 minutes ago, Sshannon said:

 

 

[breathtakingly beautiful]

 

Edited July 29, 2023 by WRXD372
shortened quote

[WRUU653]

Very nice Steve. It reminds me of a place my grandfather used to have. It also had boulders.  I broke my arm as a teen jumping from one to another.  

The epoxy works really good too. I’ve used both before in concrete. I’ve never had an issue with either but I lean towards using anchors fwiw. 

[WRUU653]

P.S. on second thought your slab idea might help with a nice flat surface and you could pour cement around large bolts that have a bend in them like what’s used for street lights. Cut a plywood template to run the bolts through for your mounting pattern to attach the antenna, with rebar epoxied into the rock for the slab to rock bond. 

[SteveShannon]

23 minutes ago, WRUU653 said:

Very nice Steve. It reminds me of a place my grandfather used to have. It also had boulders.  I broke my arm as a teen jumping from one to another.  

The epoxy works really good too. I’ve used both before in concrete. I’ve never had an issue with either but I lean towards using anchors fwiw. 

I never used either so I appreciate hearing your experiences. 

[SteveShannon]

Here’s another picture of the antenna. 

[nokones]

Is it possible to guy wire the tower? Would you have the unobstructed space to guy it? What would the requirements be for the anchor points and will the  soil accommodate the anchor points without any difficulty? More than likely the tower wasn't engineered/designed for being guyed since it is a free standing tower and you may have to do some modification to the tower. Just a thought.

[SteveShannon]

1 minute ago, nokones said:

Is it possible to guy wire the tower? Would you have the unobstructed space to guy it? What would the requirements be for the anchor points and will the  soil accommodate the anchor points without any difficulty? More than likely the tower wasn't engineered/designed for being guyed since it is a free standing tower and you may have to do some modification to the tower. Just a thought.

You’re right, the tower isn’t designed to be guyed. In fact, the primary reason I got this one instead of a Rohn R25 is the free-standing design. I would rather not guy it if possible. I would dig the hole and buy five yards of concrete first. 
But, not everything always works out the way I want. So, if it needs guys I will just have to figure it out. The soil consists of a lot of decomposed granite. Based on the guys used for utility structures and cell towers in the area it seems to work well. 

[Guest]

19 hours ago, Sshannon said:

Here’s another picture of the antenna. 

No base plate or "feet" with mounting points for the "legs" ?

The legs do not differ in design from the other elements,,,

Connecting the elements to each other seems to be done with three bolts per leg and the nested design supplies the needed integrity for the connection. - The nesting design is not repeated for the feet ...

Seems that the bottom of the tower needs some attention as to not create a weak-spot on the base. - - Does the documentation give any details ?!?

[Guest]

21 hours ago, Sshannon said:

I’ll attach a picture of the cabin.

 

You calling this a cabin, reminds my of that one Crocodile Dundee scene - when that 'kid' is threatening him with a knife ...

Do you sometimes pull that line ?

When sone SoCal guy shows you a picture of his 250 sq foot / single room / a-frame "cabin" and you pull out your picture:

"That (his) is not a cabin -- This (yours) is a cabin !!!"

    

 

 

Edited July 30, 2023 by WRXD372
quick fingers ...

[SteveShannon]

38 minutes ago, WRXD372 said:

 

You calling this a cabin, reminds my of that one Crocodile Dundee scene - when that 'kid' is threatening him with a knife ...

Do you sometimes pull that line ?

When sone SoCal guy shows you a picture of his 250 sq foot / single room / a-frame "cabin" and you pull out your picture:

"That (his) is not a cabin -- This (yours) is a cabin !!!"

    

 

 

This is pretty small. It’s 24’x32’ with a loft but it does have a basement. And a big deck. 

[SteveShannon]

48 minutes ago, WRXD372 said:

No base plate or "feet" with mounting points for the "legs" ?

The legs do not differ in design from the other elements,,,

Connecting the elements to each other seems to be done with three bolts per leg and the nested design supplies the needed integrity for the connection. - The nesting design is not repeated for the feet ...

Seems that the bottom of the tower needs some attention as to not create a weak-spot on the base. - - Does the documentation give any details ?!?

Yeah, they sell a base that gets embedded into the concrete and provides three stubs to bolt the bottom section to. 
Only one of the bottom section legs had three bolts. The other two were two bolts each. I don’t know if that’s because there was a different design previously or this wasn’t truly the bottom section but it’s beefy. 

[SteveShannon]

In researching this better I’ve discovered it’s a BX48, not the heavier duty HDBX48. That doesn’t bother me. It means that I am more limited in the antenna I mount. The HDBX48 was rated for an antenna that weighs 120 lbs and has an area of 6 square feet. The HDBX would have handled 360 lbs and 18 square feet. 

[SteveShannon]

16 hours ago, Sshannon said:

Yeah, they sell a base that gets embedded into the concrete and provides three stubs to bolt the bottom section to. 
Only one of the bottom section legs had three bolts. The other two were two bolts each. I don’t know if that’s because there was a different design previously or this wasn’t truly the bottom section but it’s beefy. 

The more I look into this the better I understand (yes, it would have been better to understand all of this before buying my first tower, but that’s just how I’m wired, I guess. )

The way they built this series of towers was with a series of tapered eight foot sections numbered #1 through #8. #1 is the smallest, measuring 7 3/4”. Each section is about 2 inches wider than the one above it.  Depending on which series of sections you bought, you could assemble them as a tower from 32 foot to 64 foot.

That means you could assemble six of the eight foot sections in three different ways to form a 48 foot tower.  The strength of the tower depends on whether you use the #1, #2, or #3 section as the top piece:

Sections 1, 2, 3, 4, 5, and 6 could be assembled as a BX48. That’s the lightest 48 foot tower in the line. It can handle 120 pounds in thrust and a six square foot area antenna.

But, if you use sections 2 through 7, with #2 as the top and the #7 as the bottom, you end up with a slightly larger (in girth) tower that’s 9+inches across at the top, the DBX 48. That’s capable of handling 240 pounds in thrust and 12 square feet.

So, if you use the #3 section as the top and the #8 at the bottom, you end up with a tower that’s greater than 11 inches at the top and capable of handling 360 pounds of thrust (dead weight) and an area of 18 square feet.

Another way to look at this is that I could assemble the sections to form an HDBX32, a DBx40, or a BX48.

And it’s also important to realize that even though each section is eight feet long, because you lose four inches of overlap at each joint, the actual length of a BX48 assembled is about 46 feet.  But however you assemble the pieces, you assemble them to a four foot long bottom section which is embedded into a block of reinforced concrete.  The bottom section, whether it’s a #6 (in my case) or #7 or #8, is assembled to the base piece before the concrete is poured.  It must be kept plumb so you have a vertical tower.

Finally, the reason I replied to this particular previous post: my tower only has two bolt holes for each of the bottoms of the three legs.  That’s all that the #6 section has when attaching to the base.  Only the #7 and #8 sections have three bolt holes at the bottom of each leg where they bolt to the base.  Based on the little bit of rust inside it, that third hole that is present on one of the legs was drilled after the galvanizing process. I will apply some kind of cold galvanizing treatment to it.

 

[tweiss3]

I'd call Rohn and talk to them about your specific site constraints. With all the rock, they might be able to help you with a matt foundation that would work for your loading scenario. It would still be reinforced, but would be wider than the standard foundation, and much shallower (maybe 2'), and could be placed directly on the bedrock, however deep that is (assuming its barely under the topsoil). I would buy a new base for imbedding in the concrete unless you want to use the bottom section as your base. 

[WRXB215]

1 hour ago, Sshannon said:

it would have been better to understand all of this before buying my first tower, but that’s just how I’m wired, I guess

And you are a retired P.E. Shame! Shame! 

[Guest]

1 hour ago, Sshannon said:

The more I look into this the better I understand (yes, it would have been better to understand all of this before buying my first tower, but that’s just how I’m wired, I guess. )

The way they built this series of towers was with a series of tapered eight foot sections numbered #1 through #8. #1 is the smallest, measuring 7 3/4”. Each section is about 2 inches wider than the one above it.  Depending on which series of sections you bought, you could assemble them as a tower from 32 foot to 64 foot.

That means you could assemble six of the eight foot sections in three different ways to form a 48 foot tower.  The strength of the tower depends on whether you use the #1, #2, or #3 section as the top piece:

Sections 1, 2, 3, 4, 5, and 6 could be assembled as a BX48. That’s the lightest 48 foot tower in the line. It can handle 120 pounds in thrust and a six square foot area antenna.

But, if you use sections 2 through 7, with #2 as the top and the #7 as the bottom, you end up with a slightly larger (in girth) tower that’s 9+inches across at the top, the DBX 48. That’s capable of handling 240 pounds in thrust and 12 square feet.

So, if you use the #3 section as the top and the #8 at the bottom, you end up with a tower that’s greater than 11 inches at the top and capable of handling 360 pounds of thrust (dead weight) and an area of 18 square feet.

Another way to look at this is that I could assemble the sections to form an HDBX32, a DBx40, or a BX48.

And it’s also important to realize that even though each section is eight feet long, because you lose four inches of overlap at each joint, the actual length of a BX48 assembled is about 46 feet.  But however you assemble the pieces, you assemble them to a four foot long bottom section which is embedded into a block of reinforced concrete.  The bottom section, whether it’s a #6 (in my case) or #7 or #8, is assembled to the base piece before the concrete is poured.  It must be kept plumb so you have a vertical tower.

Finally, the reason I replied to this particular previous post: my tower only has two bolt holes for each of the bottoms of the three legs.  That’s all that the #6 section has when attaching to the base.  Only the #7 and #8 sections have three bolt holes at the bottom of each leg where they bolt to the base.  Based on the little bit of rust inside it, that third hole that is present on one of the legs was drilled after the galvanizing process. I will apply some kind of cold galvanizing treatment to it.

 

 

What a clever design !!!

That company really understands modular design and "streamlining" 

Thank you for the details !!!

 

 

[SteveShannon]

2 hours ago, tweiss3 said:

I'd call Rohn and talk to them about your specific site constraints. With all the rock, they might be able to help you with a matt foundation that would work for your loading scenario. It would still be reinforced, but would be wider than the standard foundation, and much shallower (maybe 2'), and could be placed directly on the bedrock, however deep that is (assuming its barely under the topsoil). I would buy a new base for imbedding in the concrete unless you want to use the bottom section as your base. 

Those are good ideas.  I agree about the base. My original plan when I bought this was to buy the base, dig the necessary hole, and pour the concrete.  It was only after looking at all the boulders that are much larger than the block of concrete that I started pondering attaching directly to one.  I like the mat foundation as well.  That’s sort of what I was wondering if I could do.

Rohn no longer makes this particular tower but another company took over manufacturing it, including all of the various pieces. They sell the bases and they also sell the tower sections, so if I wanted to purchase a new #7 section and #8 section to build a 64 foot tower I could.  I think I can call them and ask questions, too.  I might try that.

[SteveShannon]

1 hour ago, WRXB215 said:

And you are a retired P.E. Shame! Shame! 

Yeah, but once I looked at it I knew I was going to buy it.  The galvanizing and the tower is all in good shape and it was $400.  I probably would never buy a new tower of the same model because they sell for several thousand dollars.

[PA141]

Steve,

I know people that swear by epoxied in anchor bolts (as in installing a different tower on an existing concrete base).  Depending on the quality of the anchor rock, that might be viable.  You might want to sign up to the TowerTalk email reflector, http://lists.contesting.com/mailman/listinfo/Towertalk and pose your question there.  I know that towers anchored in rock have been discussed before.

Not sure what kind of antenna you are planning, but some of those BX towers also had limitations on boom length (something like 10 feet) due to torque limitations on the tower.