I've been tasked with designing construction guy wires for a 23' dia x 122' high steel stack being built on the ground. I started with quite a few simplifying assumptions but want to put a finer point to my pencil. I'm using ASCE 7-10 for the wind loads and ASCE 37-14 for construction factors, I'm researching coefficients of friction between the concrete weights and what they rest on ( steel plate and grade ), using standard wire rope strength and termination factors, I'm assuming the stack is rigid but may revisit this later, I'm not sure if cable preload is a major factor, etc. Comments and suggestions are welcome so I don't overlook anything major. Thanks!
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Temporary guy line design for a stack
- Thread starter IFRs
- Start date
You can't go wrong with using some generous safety factors. Remember during construction there can be unexpected happenings,such as someone driving along with equipment and they don't see the cable. Or the tension adjustment is done wrong by a well meaning worker. I always try to follow the rule: "If it can be done wrong, it will". Keep it simple.
If it is being constructed on the ground, I assume it will be tilted up with a crane(s) in a single piece? If that is the case, what is the purpose of the temporary cables? Can the crane simply hold it in place while the anchors are tightened? Otherwise, perhaps some amount of time will pass between when the crane releases the load and the anchors are tightened? Depending on the erection sequence, you may need to look into the cable loads at various stage(s) of erection.
- Thread starter
- #5
There are four steel cans being brought to the site in pieces to be assembled as four separate roughly 30 foot tall vertical hollow can-like sections, then two of these sections will be picked and placed on the other two to make two stacks of two (61' tall), then one stack of two will be placed on the other to make one stack of four (122' tall). Then the stack of four will be placed on the base stack. My job is to make the single, double and quad stacks stable until they are in turn picked, placed and welded to the assemblies under them. The ground is 2 feet of crusher run, 12" crane mats and 1" road plate. The owner has 20 metric ton weights for us to tie to. We plan to use chain binders to take the slack out of the guys but to not pre-tension them at all. Gross stability is all we are after.
The first stage, with four 30 foot stacks appears to be stable without guys, the two stack assemblies can have one or two sets of guy wires, the four stack can have three sets of guys. The guys will be attached roughly at 30 feet, 60 feet and 90 feet above grade. I'm looking at wind loading primarily although I will check seismic even though the construction duration is about 4 weeks soup to nuts.
The first stage, with four 30 foot stacks appears to be stable without guys, the two stack assemblies can have one or two sets of guy wires, the four stack can have three sets of guys. The guys will be attached roughly at 30 feet, 60 feet and 90 feet above grade. I'm looking at wind loading primarily although I will check seismic even though the construction duration is about 4 weeks soup to nuts.
Lomarandil
Structural
You're on the right track.
Make sure that your stack stability checks can allow for some allotment for uneven grade under the stack and for a significant amount of movement before the guys catch up. Horizontal stiffness of guys with any sag (even with the "slack taken out" at reasonable levels) is a pain in the rear.
Dik is right -- you will develop some additional vertical load (perhaps not evenly distributed) in the stack. Depending on wall thickness, this may be an issue. The stack designer won't have accounted for it.
Some contractors prefer to not leave chain binders with any load in them (unless rated, most aren't). Not too hard to work around, if you think ahead and have a separate (all cable) load path that's engaged once the chain binder has been tightened appropriately.
As motorcity alluded, coordinate with whoever is designing the crane pick to make sure your guys can be connected and disconnected while the crane is rigged up. Especially at 90', make the connection between your guys and the stack as foolproof as possible for whoever is up in that manlift.
Don't forget to reduce your concrete deadman friction factors for wet conditions (usually 1/2 to 2/3 normal). It will get wet.
----
The name is a long story -- just call me Lo.
Make sure that your stack stability checks can allow for some allotment for uneven grade under the stack and for a significant amount of movement before the guys catch up. Horizontal stiffness of guys with any sag (even with the "slack taken out" at reasonable levels) is a pain in the rear.
Dik is right -- you will develop some additional vertical load (perhaps not evenly distributed) in the stack. Depending on wall thickness, this may be an issue. The stack designer won't have accounted for it.
Some contractors prefer to not leave chain binders with any load in them (unless rated, most aren't). Not too hard to work around, if you think ahead and have a separate (all cable) load path that's engaged once the chain binder has been tightened appropriately.
As motorcity alluded, coordinate with whoever is designing the crane pick to make sure your guys can be connected and disconnected while the crane is rigged up. Especially at 90', make the connection between your guys and the stack as foolproof as possible for whoever is up in that manlift.
Don't forget to reduce your concrete deadman friction factors for wet conditions (usually 1/2 to 2/3 normal). It will get wet.
----
The name is a long story -- just call me Lo.
- Thread starter
- #7
Is going through the ASCE 7 wind pressure coefficient worth it, or just assume uniform pressure on each section?
I'm looking at the friction between concrete weights and (dry and wet) steel plate and the grade beyond the plate. I may ask that stakes be driven into the ground or steel be welded to the road plate to help resist horizontal loads. The vertical component initially bothered me but then I realized that the stack had to be designed for much more given it will not have guys in its final assembly.
I'm looking at the friction between concrete weights and (dry and wet) steel plate and the grade beyond the plate. I may ask that stakes be driven into the ground or steel be welded to the road plate to help resist horizontal loads. The vertical component initially bothered me but then I realized that the stack had to be designed for much more given it will not have guys in its final assembly.
IFRs,
If you're using ASCE7-10 to find your loads, then you need to find the pressure coefficient. Some key things though: You should be in Chapter 29, specifically equation 29.5-1, which finds the pressure at the centroid of your area (in effect, using a uniform pressure). However, depending on your diameter, it appears that your Cf will be less than 1 (see figure 29.5-1).
A comment on your guy wires: In order to take the slack out of the line, you are preloading the cable. When I worked for a construction company, their guy wire policy was a minimum preload of 500 pounds in the line. Note that when the guy wire is resisting wind, it will be inducing even more compression in the stack beyond the preload. Likely not an issue, but it at least merits a look.
For your concrete dead weight, don't forget to check overturning of the block during the wind event. Many times this would govern my dead weight design over friction.
If you're using ASCE7-10 to find your loads, then you need to find the pressure coefficient. Some key things though: You should be in Chapter 29, specifically equation 29.5-1, which finds the pressure at the centroid of your area (in effect, using a uniform pressure). However, depending on your diameter, it appears that your Cf will be less than 1 (see figure 29.5-1).
A comment on your guy wires: In order to take the slack out of the line, you are preloading the cable. When I worked for a construction company, their guy wire policy was a minimum preload of 500 pounds in the line. Note that when the guy wire is resisting wind, it will be inducing even more compression in the stack beyond the preload. Likely not an issue, but it at least merits a look.
For your concrete dead weight, don't forget to check overturning of the block during the wind event. Many times this would govern my dead weight design over friction.
I'm wondering about distortion being introduced bythe guys especially those tied near the top of the opening. In order to not have unintended warping of the upper open ends, could a temporary brcket be made to fit inside the opening, say a cross shape with ends located near the tie pleces, perhaps with an upside down "U" to fit over the wall. Then any excessive pull on the guy will be resisted by the whole end opening. Makesure the "U" won't pop out by having it quite deep, say at least 8 inches. I assume some guide lugs will be up there also to help properly in the setting of the next section.
I assume that full penetration welds are being used to join the segments....how are the segments prevented from going out-of-round at the open ends before joining the segments together with the added possibility of radial loads induced by the guy wires?....
- Thread starter
- #11
Thank you all for your contributions.
Lomarandil - One thing these guys are not is afraid of heights. They work safely at height all the time, including repairing wind turbines. Me on the other hand...
Sail3 - correct but not my problem, the joint prep, weld procedure and NDE are all specified by the GC. See next reply.
OldestGuy - Temporary cross bracing is unlikely due the issues of installing and most of all, removal after assembly. Distortion of the upper region of the can by the guy cables will be dealt with the way boiler makers always deal with poorly fitting shell plates - leverage and muscle ( and bitching ). They will likely slack the cables while the crane provides stability during the attachment process, the crane will stay until the new cables are attached and all cables are snugged up. Their key plates and bull pins will do the rest. If a storm approaches they will maybe snug each binder a few clicks (the way boiler makers are prone to do) but no one cares if the assembly moves and shakes as long as it does not fall over.
WineLandV - ASCE 7-10 29.5 is precisely where I am, thank you for the validation. The owner has already specified the wind speed and many of the modification factors, and come up with a 13 psf load but (1) I just don't trust them and (2) I just don't trust them. The curve of the AISC velocity pressure coefficients from ground to 122 feet is not linear which adds complication to my desire for simple. My first pass was to use the pressure at the middle of each segment for that entire segment and calculate each guy for that force only. But, that seemed too simple considering the interaction between the unequal length guys and then the flexibility of the stack and I began to be concerned that I was not being conservative. Block rotation and overturning was not on my radar screen - thanks - I'll check that!
While I'm also guessing that stack tension on the leeward side may also be a concern, crippling of the windward side does need to be avoided. I'd hate to see a wind come by and the thing look shorter on one side than the other with horizontal wrinkles. Then again, as long as I don't impose greater vertical load than it will see as a free standing stack I should be OK. NOTE: the 122 feet that we are constructing will be picked up and welded to the top of a 118 foot lower section that another contractor is building and is well anchored into what looks to be a substantial foundation.
Lomarandil - One thing these guys are not is afraid of heights. They work safely at height all the time, including repairing wind turbines. Me on the other hand...
Sail3 - correct but not my problem, the joint prep, weld procedure and NDE are all specified by the GC. See next reply.
OldestGuy - Temporary cross bracing is unlikely due the issues of installing and most of all, removal after assembly. Distortion of the upper region of the can by the guy cables will be dealt with the way boiler makers always deal with poorly fitting shell plates - leverage and muscle ( and bitching ). They will likely slack the cables while the crane provides stability during the attachment process, the crane will stay until the new cables are attached and all cables are snugged up. Their key plates and bull pins will do the rest. If a storm approaches they will maybe snug each binder a few clicks (the way boiler makers are prone to do) but no one cares if the assembly moves and shakes as long as it does not fall over.
WineLandV - ASCE 7-10 29.5 is precisely where I am, thank you for the validation. The owner has already specified the wind speed and many of the modification factors, and come up with a 13 psf load but (1) I just don't trust them and (2) I just don't trust them. The curve of the AISC velocity pressure coefficients from ground to 122 feet is not linear which adds complication to my desire for simple. My first pass was to use the pressure at the middle of each segment for that entire segment and calculate each guy for that force only. But, that seemed too simple considering the interaction between the unequal length guys and then the flexibility of the stack and I began to be concerned that I was not being conservative. Block rotation and overturning was not on my radar screen - thanks - I'll check that!
While I'm also guessing that stack tension on the leeward side may also be a concern, crippling of the windward side does need to be avoided. I'd hate to see a wind come by and the thing look shorter on one side than the other with horizontal wrinkles. Then again, as long as I don't impose greater vertical load than it will see as a free standing stack I should be OK. NOTE: the 122 feet that we are constructing will be picked up and welded to the top of a 118 foot lower section that another contractor is building and is well anchored into what looks to be a substantial foundation.
Lomarandil
Structural
I'm not sure that total vertical load is your primary checkpoint, I'd be more concerned with local crippling effects.
I forgot to mention overturning, but wineland is right -- especially with the guy pulling against the deadman weight.
I certainly wouldn't bother with the non-linear wind pressures by height, discrete segments with a constant pressure is fine. But I think you're right to offer some consideration to stack flexibility and interaction between guy levels. (I'm not saying that you have to go to FEA).
----
The name is a long story -- just call me Lo.
I forgot to mention overturning, but wineland is right -- especially with the guy pulling against the deadman weight.
I certainly wouldn't bother with the non-linear wind pressures by height, discrete segments with a constant pressure is fine. But I think you're right to offer some consideration to stack flexibility and interaction between guy levels. (I'm not saying that you have to go to FEA).
----
The name is a long story -- just call me Lo.
IFRs:
Regarding the guys, there is enough slack in them, at first, that “chain binders” won’t do the trick for initial wire rope take-up. You need a come-along, a chain hoist, or even a hauling cable fixed to the end of the guy wires and a skid steer or some such, pulling it through the end hardware. You might be able to save a set of guy wires by changing your approach a bit. You have four 30' high cans, 1 (bottom),2,3 & 4 (top). They will likely be stacked in a specific order, with the lower ones being thicker. So be careful on this account. But, you claim the individual 30' cans are basically stable as they stand alone. Many times the bigger issue is that you stiffen/stabilize the free edge at 30' so it doesn’t buckle into a wavy shaped circle or oval, or some such. So, on your right hand sketch (your 5 Mar 18 23:50 post) guy wires may not be needed, or they can be very basic. Fix a stiffener ring to the mid to upper ht. of can 2, with hanging guy wires of L2 length. Fix a stiffener ring to the mid to upper ht. of can 4, with hanging guy wires of L4 length, considerably longer than L2, for later assembly use, but [use mid length of this wire rope for the first stability anchoring.] These stiffener/guy wire rings are also intended as lifting rings for use later on. Mate can 2 to can 1, do the welding and temporarily fix the guy wires to some hold down blks. Mate can 4 to can 3, do the welding and temporarily fix the guy wires ( [use mid length of this wire rope for first stability anchoring.]) to some hold down blks. These first two main lifts can be done with a fairly small crane or two, only one can weighing about 25 tons each and only about 60' high lift. Now, your middle sketch has only one set of guy wires at about 45-50' high for each set of two cans.
One thing that I’m a little unclear on is..., you claim two cans high need guying for stability while you are working on them on the ground, and then that three and four cans high do too, while this stack is being built on the ground. But, when it is 240' up in the air, atop the 118' lower part of the stack, stability is o.k., or not an issue, or what? How does this work, the lateral loads are greater at 240' than at 60' or 122'? Is the final 240' high stack guyed or free standing? I understand that you are responsible only for building the top four cans of the stack, total height about 122'. But, I am asking, by working together can the two contractors make things easier or better for each other, by the way they are doing things, with not much added cost to themselves.
Now, with a larger crane or two, lift cans 2&1 onto the 118' high base stack do the welding and start the final guying process if this exists. And finally, lift cans 4&3 onto the top of can 2 and do the welding and start the final guying process if this exists. Alternatively, as it seems you are proposing, mate cans 4&3 to cans 2&1 on the ground, and then a much heavy and longer stack top section is lifted onto the 118' base stack section. This kinda boils down to crane size and cap’y. and availability. But, 100-120 tons at 240' is a lot different than 50-60 tons at 180' and 240'.
Regarding the guys, there is enough slack in them, at first, that “chain binders” won’t do the trick for initial wire rope take-up. You need a come-along, a chain hoist, or even a hauling cable fixed to the end of the guy wires and a skid steer or some such, pulling it through the end hardware. You might be able to save a set of guy wires by changing your approach a bit. You have four 30' high cans, 1 (bottom),2,3 & 4 (top). They will likely be stacked in a specific order, with the lower ones being thicker. So be careful on this account. But, you claim the individual 30' cans are basically stable as they stand alone. Many times the bigger issue is that you stiffen/stabilize the free edge at 30' so it doesn’t buckle into a wavy shaped circle or oval, or some such. So, on your right hand sketch (your 5 Mar 18 23:50 post) guy wires may not be needed, or they can be very basic. Fix a stiffener ring to the mid to upper ht. of can 2, with hanging guy wires of L2 length. Fix a stiffener ring to the mid to upper ht. of can 4, with hanging guy wires of L4 length, considerably longer than L2, for later assembly use, but [use mid length of this wire rope for the first stability anchoring.] These stiffener/guy wire rings are also intended as lifting rings for use later on. Mate can 2 to can 1, do the welding and temporarily fix the guy wires to some hold down blks. Mate can 4 to can 3, do the welding and temporarily fix the guy wires ( [use mid length of this wire rope for first stability anchoring.]) to some hold down blks. These first two main lifts can be done with a fairly small crane or two, only one can weighing about 25 tons each and only about 60' high lift. Now, your middle sketch has only one set of guy wires at about 45-50' high for each set of two cans.
One thing that I’m a little unclear on is..., you claim two cans high need guying for stability while you are working on them on the ground, and then that three and four cans high do too, while this stack is being built on the ground. But, when it is 240' up in the air, atop the 118' lower part of the stack, stability is o.k., or not an issue, or what? How does this work, the lateral loads are greater at 240' than at 60' or 122'? Is the final 240' high stack guyed or free standing? I understand that you are responsible only for building the top four cans of the stack, total height about 122'. But, I am asking, by working together can the two contractors make things easier or better for each other, by the way they are doing things, with not much added cost to themselves.
Now, with a larger crane or two, lift cans 2&1 onto the 118' high base stack do the welding and start the final guying process if this exists. And finally, lift cans 4&3 onto the top of can 2 and do the welding and start the final guying process if this exists. Alternatively, as it seems you are proposing, mate cans 4&3 to cans 2&1 on the ground, and then a much heavy and longer stack top section is lifted onto the 118' base stack section. This kinda boils down to crane size and cap’y. and availability. But, 100-120 tons at 240' is a lot different than 50-60 tons at 180' and 240'.
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