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#1
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Tower project - Phase 2 Complete
For what it's worth - may be interesting to those contemplating a
similar project: Phase 1 was finding 'such a deal' on a full sized 7-element Mosley Pro 57-B five band beam. Also researching towers that I wouldn't have to climb and still reach above my trees of 30-45'. e.g. 50' tower. My first choice was a Heights crank-up, but cost moved me to Glen Martin Engineering's M-1350A package deal. (13" wide legs; 50' high w/guys) and their "Hazer" system to raise and lower the beam (with one or two guys temporarily disconnected at the ground). Phase 2 was first digging the 36' x 36" x 45" hole for the base, then securing the tower rod base and pouring the concrete. It took almost two weeks of occasional digging with a handy small "striker" shovel to dig the hole. That shovel holds just what I can lift up to the wheel barrow, and is easy to maneuver in the hole. I used a wood 2 x 4 frame which is also used for the top of the concrete platform. Easy then to keep sides vertical in line with the frame. The soil is a firm clayish-dirt mix. A problem not explained is what to use in the hole to support the metal tower rod base which only goes 28" into the hole. Why don't the manufacturers just make available longer rod supports? I ended up using concrete blocks under each of the 3 tower rods with a couple bricks on top to just equal the requirement for the rods with nuts to extend above the concrete the proper length. One handbook suggested a rebar wire cage in the hole, but I had no way to bend or weld rebar. I did try, driving around one morning to various shops with welding equipment, but none could or would do it. So, I cut various lengths of rebar and wired them at various angles and levels to the base and through the cinder blocks without going into the dirt sides. It made lots of surface area for concrete adherence. The concrete: I'd checked some local concrete companies and had only two that responded with any interest. Actually, the closest to my home quoted me a price of $426 when I stopped by and asked, including a $125 delivery charge. Yikes! And it was close by and my "hole" was accessible so the truck could back right up and dump the load. As it turned out, when I was ready for the delivery I stopped by to prepay, and they lowered their price to $195 for 1.5 cu. yds., 3000+ spec, and no charge for delivery. I'd expected delivery in a few days, but they said they'd be over in about 15 minutes. They were, made the delivery, cleaned up their chutes, and in another 30 minutes I had it all smoothed out and finished! I'll be letting it dry for at least a week. Now I'll be on to Phase 3, driving guy anchors into the ground, assembling the tower pieces, and preparing for the "several guys" walking up the tower into the air with the hinge mount in the base secure. To be continued Marv W5MTV |
#2
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Tower project - Phase 2 Complete
On Mar 7, 4:03*pm, MTV wrote:
For what it's worth - may be interesting to those contemplating a similar project: Phase 1 was finding 'such a deal' on a full sized 7-element Mosley Pro 57-B five band beam. Also researching towers that I wouldn't have to climb and still reach above my trees of 30-45'. e.g. 50' tower. My first choice was a Heights crank-up, but cost moved me to Glen Martin Engineering's M-1350A package deal. (13" wide legs; 50' high w/guys) and their "Hazer" system to raise and lower the beam (with one or two guys temporarily disconnected at the ground). Phase 2 was first digging the 36' x 36" x 45" hole for the base, then securing the tower rod base and pouring the concrete. * * It took almost two weeks of occasional digging with a handy small "striker" shovel to dig the hole. That shovel holds just what I can lift up to the wheel barrow, and is easy to maneuver in the hole. I used a wood 2 x 4 frame which is also used for the top of the concrete platform. Easy then to keep sides vertical in line with the frame. The soil is a firm clayish-dirt mix. A problem not explained is what to use in the hole to support the metal tower rod base which only goes 28" into the hole. Why don't the manufacturers just make available longer rod supports? I ended up using concrete blocks under each of the 3 tower rods with a couple bricks on top to just equal the requirement for the rods with nuts to extend above the concrete the proper length. One handbook suggested a rebar wire cage in the hole, but I had no way to bend or weld rebar. I did try, driving around one morning to various shops with welding equipment, but none could or would do it. So, I cut various lengths of rebar and wired them at various angles and levels to the base and through the cinder blocks without going into the dirt sides. It made lots of surface area for concrete adherence. The concrete: I'd checked some local concrete companies and had only two that responded with any interest. Actually, the closest to my home quoted me a price of $426 when I stopped by and asked, including a $125 delivery charge. Yikes! And it was close by and my "hole" was accessible so the truck could back right up and dump the load. As it turned out, when I was ready for the delivery I stopped by to prepay, and they lowered their price to $195 for 1.5 cu. yds., 3000+ spec, and no charge for delivery. I'd expected delivery in a few days, but they said they'd be over in about 15 minutes. They were, made the delivery, cleaned up their chutes, and in another 30 minutes I had it all smoothed out and finished! I'll be letting it dry for at least a week. Now I'll be on to Phase 3, driving guy anchors into the ground, assembling the tower pieces, and preparing for the "several guys" walking up the tower into the air with the hinge mount in the base secure. To be continued Marv W5MTV $426 YIKES. I think it runs arounf 90 dollars a yard here with a 2 yard minimum order . There is no delivery fee unless you live way out in the boonies. I think they have a little truck that deliveres the concrete dry then they use your water to mix it for small orders, less wasteful but you still have to pay the 2 yard minimum. Jimmie |
#3
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Tower project - Phase 2 Complete
On Sat, 07 Mar 2009 15:03:33 -0600, MTV wrote:
Phase 2 was first digging the 36' x 36" x 45" hole for the base, then securing the tower rod base and pouring the concrete. A 36 foot deep hole ought to hold up a freestanding 100' tower -- or more! :-) |
#4
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Tower project - Phase 2 Complete
Allodoxaphobia wrote:
On Sat, 07 Mar 2009 15:03:33 -0600, MTV wrote: Phase 2 was first digging the 36' x 36" x 45" hole for the base, then securing the tower rod base and pouring the concrete. A 36 foot deep hole ought to hold up a freestanding 100' tower -- or more! :-) It's actually 52" deep; the tower mfgr said min. of 45"; ARRL handbook says 6' but only 30" sq, depending on the type of soil. Marv |
#5
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Tower project - Phase 2 Complete
MTV wrote:
Allodoxaphobia wrote: On Sat, 07 Mar 2009 15:03:33 -0600, MTV wrote: Phase 2 was first digging the 36' x 36" x 45" hole for the base, then securing the tower rod base and pouring the concrete. A 36 foot deep hole ought to hold up a freestanding 100' tower -- or more! :-) It's actually 52" deep; the tower mfgr said min. of 45"; ARRL handbook says 6' but only 30" sq, depending on the type of soil. I don't know that I'd use the ARRL handbook as a source for structural/civil engineering data. The figures in the "assembling a station" chapter in my 1990 handbook are just an example for a TriEx LM-470 or for a Wilson ST-77B, not a general recommendation for all towers. In fact, the last sentence in the section is: "Once you have that information, contact the engineering department of your tower manufacturer or a civil engineer" Following the mfr recommendations is probably the best bet. (and that's if they're the *current* recommendations.. engineering and construction standards are always evolving, what was acceptable in 1960 might not be acceptable today) That said, there's a lot of possible tradeoffs in tower bases, even for the same kind of tower in the same location. skinny and deep vs wide and shallow is one. If you're not taking the manufacturer's recommendation, then you probably need to get someone with some engineering expertise to tell you whether what you want to do is reasonable. |
#6
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Tower project - Phase 2 Complete
On Mar 9, 12:53*pm, Jim Lux wrote:
MTV wrote: Allodoxaphobia wrote: On Sat, 07 Mar 2009 15:03:33 -0600, MTV wrote: Phase 2 was first digging the 36' x 36" x 45" hole for the base, then securing the tower rod base and pouring the concrete. A 36 foot deep hole ought to hold up a freestanding 100' tower -- or more! *:-) It's actually 52" deep; the tower mfgr said min. of 45"; ARRL handbook says 6' but only 30" sq, depending on the type of soil. I don't know that I'd use the ARRL handbook as a source for structural/civil engineering data. *The figures in the "assembling a station" chapter in my 1990 handbook are just an example for a TriEx LM-470 or for a Wilson ST-77B, not a general recommendation for all towers. In fact, the last sentence in the section is: "Once you have that information, contact the engineering department of your tower manufacturer or a civil engineer" Following the mfr recommendations is probably the best bet. (and that's if they're the *current* recommendations.. engineering and construction standards are always evolving, what was acceptable in 1960 might not be acceptable today) That said, there's a lot of possible tradeoffs in tower bases, even for the same kind of tower in the same location. skinny and deep vs wide and shallow is one. *If you're not taking the manufacturer's recommendation, then you probably need to get someone with some engineering expertise to tell you whether what you want to do is reasonable. The design of the base depends a lot on the local soil. An antenna installed in the red clay of North Carolina may be a lot smaller than one on a Florida sand hill. Local guidance should always be obtained. If you wing it on your on you may be held responsible for damage to your neighbor's home, insurance may not pay for damages. I was advised to get a permit when I contacted the city engineers office about installing an antenna. If you get a permit and install the antenna properly and it falls on your neighbor's house their insurance pays, you are not responsible. On the otherhand if you didnt do it by the numbers you can be held responsible and your insurance may not help you. Jimmie |
#7
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Tower project - Phase 2 Complete
JIMMIE wrote:
On Mar 9, 12:53 pm, Jim Lux wrote: MTV wrote: Allodoxaphobia wrote: On Sat, 07 Mar 2009 15:03:33 -0600, MTV wrote: Phase 2 was first digging the 36' x 36" x 45" hole for the base, then securing the tower rod base and pouring the concrete. A 36 foot deep hole ought to hold up a freestanding 100' tower -- or more! :-) It's actually 52" deep; the tower mfgr said min. of 45"; ARRL handbook says 6' but only 30" sq, depending on the type of soil. I don't know that I'd use the ARRL handbook as a source for structural/civil engineering data. The figures in the "assembling a station" chapter in my 1990 handbook are just an example for a TriEx LM-470 or for a Wilson ST-77B, not a general recommendation for all towers. In fact, the last sentence in the section is: "Once you have that information, contact the engineering department of your tower manufacturer or a civil engineer" Following the mfr recommendations is probably the best bet. (and that's if they're the *current* recommendations.. engineering and construction standards are always evolving, what was acceptable in 1960 might not be acceptable today) That said, there's a lot of possible tradeoffs in tower bases, even for the same kind of tower in the same location. skinny and deep vs wide and shallow is one. If you're not taking the manufacturer's recommendation, then you probably need to get someone with some engineering expertise to tell you whether what you want to do is reasonable. The design of the base depends a lot on the local soil. An antenna installed in the red clay of North Carolina may be a lot smaller than one on a Florida sand hill. Local guidance should always be obtained. If you wing it on your on you may be held responsible for damage to your neighbor's home, insurance may not pay for damages. I was advised to get a permit when I contacted the city engineers office about installing an antenna. If you get a permit and install the antenna properly and it falls on your neighbor's house their insurance pays, you are not responsible. On the otherhand if you didnt do it by the numbers you can be held responsible and your insurance may not help you. Jimmie Regardless of the soil that your tower is sited upon, the total mass of the concrete must exceed the weight of the tower, the antenna's fixed upon and the estimated wind force that the tower will see. This is so that the center of gravity of the tower/antenna system is below ground level. Thirty yeas ago my local radio club installed a new tower. The tower was 180 feet tall with a 4 bay antenna. We had a hole 10feet on a side dug and filled with concrete. When the tower did fall it bent in two about 80 feet up and folded over. The base formed the bottom of the replacement tower and is still in service. Dave WD9BDZ |
#8
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Tower project - Phase 2 Complete
Jim Lux wrote:
MTV wrote: Allodoxaphobia wrote: On Sat, 07 Mar 2009 15:03:33 -0600, MTV wrote: Phase 2 was first digging the 36' x 36" x 45" hole for the base, then securing the tower rod base and pouring the concrete. A 36 foot deep hole ought to hold up a freestanding 100' tower -- or more! :-) It's actually 52" deep; the tower mfgr said min. of 45"; ARRL handbook says 6' but only 30" sq, depending on the type of soil. I don't know that I'd use the ARRL handbook as a source for structural/civil engineering data. The figures in the "assembling a station" chapter in my 1990 handbook are just an example for a TriEx LM-470 or for a Wilson ST-77B, not a general recommendation for all towers. In fact, the last sentence in the section is: "Once you have that information, contact the engineering department of your tower manufacturer or a civil engineer" Following the mfr recommendations is probably the best bet. (and that's if they're the *current* recommendations.. engineering and construction standards are always evolving, what was acceptable in 1960 might not be acceptable today) That said, there's a lot of possible tradeoffs in tower bases, even for the same kind of tower in the same location. skinny and deep vs wide and shallow is one. If you're not taking the manufacturer's recommendation, then you probably need to get someone with some engineering expertise to tell you whether what you want to do is reasonable. My thoughts, too, to use the mfg recommendation (45" deep) so I made it 52" deep. I can't see it not doing the job. Tower is HD aluminum and I'm staying within all specs. Rated for 12 sq ft @ 87 mph; my wind load is about 10 sq ft. Highest winds we had w/Hurr. Ike were between 85-90 mph. The tower, however, will be protected by trees up to about 40', I could lower the antenna before a storm, and I have a house brace I'll probably use at 10'. The "Handbook" I referred to is a 2009 ARRL Antenna Book. Today I was trying to spot the 3 guy wire supports. Have a problem with one, so I'm thinking of using 4. Instead of all being 120 degrees, one would be 90 degrees; other three 30 degrees apart. Problem is that one of the 120 deg's would be right into my RV pad - not possible. An alternative to four would be to run one onto the end of the house roof, which I suspect would not be as sturdy as it should be? Marv |
#9
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Tower project - Phase 2 Complete
D
Regardless of the soil that your tower is sited upon, the total mass of the concrete must exceed the weight of the tower, the antenna's fixed upon and the estimated wind force that the tower will see. This is so that the center of gravity of the tower/antenna system is below ground level. That's not how the design works.. Consider an extreme example.. you have a big tripod of a tower. The CG is well above ground level, yet it still won't tip over. Or, you have 4 towers that stick out horizontally resting on the ground, and a 5th one sticking up (like clown feet sticking out). Again, the CG is above ground, but it won't tip. For a guyed tower, the base is there to keep the bottom of the tower from sinking into the soil, or from moving sideways. A wide flat pad or a deep pillar both do this, although the wide flat pad might be better from the "floatation" standpoint, as long as the bending forces in the concrete aren't too high. Consider the case where the base of the tower sits on rock, with a pier pin or similar to resist the shear loads. For an unguyed tower, it's a bit different. The base has to prevent rotational motion. There's a bending moment applied to the concrete, which in turn pushes on the soil on the sides and bottom of the foundation (and, obviously, the concrete itself has to take the loads). Thirty yeas ago my local radio club installed a new tower. The tower was 180 feet tall with a 4 bay antenna. We had a hole 10feet on a side dug and filled with concrete. When the tower did fall it bent in two about 80 feet up and folded over. The base formed the bottom of the replacement tower and is still in service. Folding in the middle somewhere is the most common failure mode for unguyed towers (after just falling intact), for the same reason that pencil points break the way they do, and tall chimneys don't fall intact. When it starts to fall, the whole thing is basically rotating, so the top end winds up with very high acceleration, and the bending forces get very high. It's a rotational inertia thing. For guyed towers, it depends where the guys are, and whether the tower is failing by bending loads or by buckling under compression loads. Dave WD9BDZ |
#10
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Tower project - Phase 2 Complete
MTV wrote:
Jim Lux wrote: MTV wrote: Allodoxaphobia wrote: On Sat, 07 Mar 2009 15:03:33 -0600, MTV wrote: Phase 2 was first digging the 36' x 36" x 45" hole for the base, then securing the tower rod base and pouring the concrete. A 36 foot deep hole ought to hold up a freestanding 100' tower -- or more! :-) It's actually 52" deep; the tower mfgr said min. of 45"; ARRL handbook says 6' but only 30" sq, depending on the type of soil. I don't know that I'd use the ARRL handbook as a source for structural/civil engineering data. The figures in the "assembling a station" chapter in my 1990 handbook are just an example for a TriEx LM-470 or for a Wilson ST-77B, not a general recommendation for all towers. In fact, the last sentence in the section is: "Once you have that information, contact the engineering department of your tower manufacturer or a civil engineer" Following the mfr recommendations is probably the best bet. (and that's if they're the *current* recommendations.. engineering and construction standards are always evolving, what was acceptable in 1960 might not be acceptable today) That said, there's a lot of possible tradeoffs in tower bases, even for the same kind of tower in the same location. skinny and deep vs wide and shallow is one. If you're not taking the manufacturer's recommendation, then you probably need to get someone with some engineering expertise to tell you whether what you want to do is reasonable. My thoughts, too, to use the mfg recommendation (45" deep) so I made it 52" deep. I can't see it not doing the job. Tower is HD aluminum and I'm staying within all specs. Rated for 12 sq ft @ 87 mph; my wind load is about 10 sq ft. Highest winds we had w/Hurr. Ike were between 85-90 mph. The tower, however, will be protected by trees up to about 40', I could lower the antenna before a storm, and I have a house brace I'll probably use at 10'. The "Handbook" I referred to is a 2009 ARRL Antenna Book. hmm, rating of 87 mi/hr makes it sound like it was rated under the old EIA-222-C Zone A(30 pounds/sf) which was average 87, peak 113. That's roughly equivalent to what the more modern standard versions call a "basic wind speed" of 70 mi/hr. Given the more modern ways of doing the calculation, the square foot rating is probably half of your 12 sf, and that's at 70 mi/hr. (drag on tubes is different than flat plates, etc.) (and ick.. why can't Glen Martin give their ratings in the more modern TIA-222E, F or G system) If you're in Harris county (where your call resolves to), you're in a 90 mi/hr zone, so the wind loads would be 65% higher, or, more depressingly, your tower's probably only rated to 3.6 square feet. Not to say that it will inevitably collapse, but.... You want to be careful about combining reinforcing methods, because the loads may distribute in ways not intended. A tower is designed to flex under load.. the movement is what distributes the load over the entire structure. Adding a constraint somewhere can concentrate the load causing it to fail. A classic example in beginning structures classes is adding a gusset to a welded T joint that makes it weaker, because it concentrates the loads at the end of the gusset. Another example is the combination of 100# test piano wire and 100# test rope in parallel. The wire is much stiffer than the rope, so as the combination is loaded, the wire stretches very little, all the load goes on the wire, so it fails at not much more than 100#. definitely, you want to consult the mfrs instructions. Today I was trying to spot the 3 guy wire supports. Have a problem with one, so I'm thinking of using 4. Instead of all being 120 degrees, one would be 90 degrees; other three 30 degrees apart. Problem is that one of the 120 deg's would be right into my RV pad - not possible. An alternative to four would be to run one onto the end of the house roof, which I suspect would not be as sturdy as it should be? most residential roof construction is mostly designed to use gravity to keep the roof on and to support down forces. They don't resist upforce well (unless you're in an area where the code encourages straps.. hurricane areas, for instance), nor do they resist side loads. The house itself can usually take a fairly good side load (after all, there's all that surface area exposed to the wind), but the fascia boards on the roof can't, so you have a problem of transferring the loads from the antenna/guy to the structure of the house. Different areas of the country have different construction practices to accommodate the loads that are important (e.g. my house in southern California is designed to resist shaking loads in shear from earthquakes, but I doubt the roof could take many feet of wet snow) Watch out with 4 guys rather than 3.. The limiting thing on a guyed tower is the downforce on the tower under load. If you have 4 guys, you have more static load on the tower than with 3 (assuming you've tensioned them the same), so you're that much closer to the failure point. Glen Martin doesn't have a whole lot of useful engineering data on their web site, so it's hard to even do a back of the envelope calculation of the buckling loads on the vertical tubes. And you gotta love the picture of a bunch of guys standing underneath the tower they're tilting up without any sort of safety precautions. Marv |
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