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Structural Design of Cable and Steel Posts Holding the Cables

Structural Design of Cable and Steel Posts Holding the Cables

Structural Design of Cable and Steel Posts Holding the Cables

(OP)
Hello,
Cables are suspended between two steel posts and anchored as shown in the picture. How these Cable needs to be designed and What are the main components provided in the structural design drawings for construction.
3D View.
Does any one have slightest idea?

RE: Structural Design of Cable and Steel Posts Holding the Cables

The required tension in the cable(s) needs to be determined so that it performs as expected/wanted. When determining the tension different conditions need to be considered; such as ambient temperature, wind loads, ice loads, etc. all of these things will affect the tension in the cable as well as how much it sags.

Standards/codes used for the design of utility lines might be a good place to start. Check out ASCE MOP 74, Guidelines for Electrical Transmission Line Structural Loading. https://ascelibrary.org/doi/book/10.1061/978078441...

RE: Structural Design of Cable and Steel Posts Holding the Cables

You should carry out a non-linear analysis of third order in order to correctly simulate the behaviour of the cable structure.

RE: Structural Design of Cable and Steel Posts Holding the Cables

If you can get ahold of a copy of AASHTO's sign specification, Appendix A contains an incremental analysis method for span wire structures.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Structural Design of Cable and Steel Posts Holding the Cables

The diagrams you present look as if they could have come from some structural analysis software.  If so, perhaps that software already has some capability for geometric nonlinearity?

If not, my website (http://rmniall.com) has a spreadsheet that analyses any single cable, with or without a point load somewhere along its length.  However the spreadsheet requires that the cable's end points be rigidly supported, and your diagrams suggest that the end points might have significant elasticity.

RE: Structural Design of Cable and Steel Posts Holding the Cables

(OP)
I have performed Non-linear analysis, to hold down the cable and specified sag is achieved Target force (5 Kips Pull) is applied as shown

There is a point load also applied during service condition and further sage occurs in the cable.

The Main Post supports Three sub post. Span length of the cable is 400 ft.


Now the reaction forces are too high I have to provide anchor block at the Main post for the balancing Cables.


How the structural drawings should be provided to the client for construction? If some one have a sample drawings related to cable structure it would be very much helpful?



RE: Structural Design of Cable and Steel Posts Holding the Cables

(OP)
@Deniel You are right but I am worried that how should the structural drawings should be provided so that the required tension forces are achieved in the cables otherwise there might be consequences.. and the steel post might collapse?

RE: Structural Design of Cable and Steel Posts Holding the Cables

5K seems awfully light for something spanning 400'...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Structural Design of Cable and Steel Posts Holding the Cables

Assuming a 3/4" span wire (Single steel wire rope; no other cables), 90mph wind load, 600 lb point load w/ 20 sq. ft. projected area, and stiff (nearly rigid) supports, here are the numbers I get from my analysis spreadsheet:

At 5% sag:
Wire tension - 5.7 kips
Maximum sag - 25'

At 3% sag:
Wire Tension - 9.4 kips
Maximum sag - 17'

At 2% sag:
Wire tension - 14.0 kips
Maximum sag - 13.5'

1/2" span wire (other loading same) at 5% sag:
Wire tension - 5.1 kips


Rod Smith, P.E., The artist formerly known as HotRod10

RE: Structural Design of Cable and Steel Posts Holding the Cables

Quote (Wazir_Malang )

I am worried that how should the structural drawings should be provided so that the required tension forces are achieved in the cables otherwise there might be consequences.. and the steel post might collapse?

This is often done with stringing charts. You provide a table that shows how much the cable is sagging (and its tension) based on various conditions (often temperatures). The construction crew would use this chart to identify the condition that matches the "at installation" conditions and install accordingly (either by directly measuring tension or by measuring the sag).


RE: Structural Design of Cable and Steel Posts Holding the Cables

@Wazir Malang,

It appears to me that there is something wrong with your statics. The horizontal reaction at the left is 45.11k, while the horizontal reaction at the right is 3.60k, neglecting the minor reactions at the bottom of the two posts.

This cannot be correct since there are no horizontal forces acting on the system. The horizontal reactions at each end must be equal.

Also, the units should be kips, not kip-in.



EDIT: if 3.60k is the reaction of a single sub post, then the horizontal reaction of three sub posts would be 10.8k, which still does not agree with the left hand horizontal reaction of 45.1k.

BA

RE: Structural Design of Cable and Steel Posts Holding the Cables

(OP)
I have Revised my FEM model, Now the revised results are.


What you engineers think about it, are the results ok, or should i update my calculations?

RE: Structural Design of Cable and Steel Posts Holding the Cables

What is the vertical sag relative to a straight line between the supports? Is the 600 lb load still applied? If the 600lb load is applied, what's the projected area for wind?

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Structural Design of Cable and Steel Posts Holding the Cables

Quote (Wazir Malang)

What you engineers think about it, are the results ok, or should i update my calculations?

So you now have 20.23 + 0.07 = 20.3k horizontal reaction at the left end.
And you have 4.55 - 0.58 =3.97k horizontal reaction at the right end. If this is the reaction for one sub post, and you have three sub posts, the horizontal reaction at the right end is actually 3*3.97 = 11.9k. That is a little more than half the left hand horizontal reaction, so your results are not ok.

BA

RE: Structural Design of Cable and Steel Posts Holding the Cables

What is the reason for using three ties on each post? Why not just one at the top?

BA

RE: Structural Design of Cable and Steel Posts Holding the Cables

Also, is 2.5ft of sag even reasonable over 400 ft at only 4-5 kips of tension. That just doesn't compute for me even from just a self weight analysis. Your pre-load on the cable would need to be significant just to keep the cable sag to that value.

RE: Structural Design of Cable and Steel Posts Holding the Cables

Quote (That is a little more than half the left hand horizontal reaction, so your results are not ok.)


It is... it just accellerates to the left...lol

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Structural Design of Cable and Steel Posts Holding the Cables

Quote:

Also, is 2.5ft of sag even reasonable over 400 ft at only 4-5 kips of tension.

Unless the left end is much higher than the right end, the tension is very low. By my numbers, with only the load of the wire itself and the wind load on the wire, the tension is 4.3 kips at a 3% sag, equating to 12 feet over the 400 ft span. Less sag, and the tension increases dramatically.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Structural Design of Cable and Steel Posts Holding the Cables

Based on the image posted, the 2.5ft of sag is relative to the horizontal projection of the low post, which is 13.5ft lower than the high post. This sounds reasonable to me (The sag from the straight line between top of posts is 5.78'). I have attached a PLS-CADD/Ultralite report printout(free program from PLS, available here for anyone interested: https://www.powerlinesystems.com/plscaddultralite) that looks to be in agreement with what the OP is showing (as well as some screen grabs).



RE: Structural Design of Cable and Steel Posts Holding the Cables


NYSDOT has a spanwire program available at:

Link

RE: Structural Design of Cable and Steel Posts Holding the Cables

As long as there aren't any loads on the span, and the wind doesn't blow, the 4 kips tension at 5.8' of sag is fairly close to what I got.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Structural Design of Cable and Steel Posts Holding the Cables

I agree with BA, only the top guy cable is doing any good on the low posts.

For the taller post with guys at 120* intervals around, the layered guys make sense.

----
just call me Lo.

RE: Structural Design of Cable and Steel Posts Holding the Cables

(OP)
Dear, BridgeSmith

Quote:

BridgeSmith (Structural) What is the vertical sag relative to a straight line between the supports? Is the 600 lb load still applied? If the 600lb load is applied, what's the projected area for wind?
projected area is just these elements shown in the picture.(just these cables and the steel posts), These results were just due to self weight.

@dauwerda really appreciated your help. I will check my FEM model with it.

Quote (dauwerda (Structural)
Based on the image posted, the 2.5ft of sag is relative to the horizontal projection of the low post, which is 13.5ft lower than the high post. This sounds https://www.powerlinesystems.com/plscaddultralite) [/quote)


The Multiple cables are to stabilize the Steel Post as buckling is an issue right?

Quote:

BAretired (Structural) What is the reason for using three ties on each post? Why not just one at the top?]

Should I go with this results, means my design is adequate to be constructed by using cables suspended between steel posts, is it a reliable design?

RE: Structural Design of Cable and Steel Posts Holding the Cables

Quote:

Should I go with this results, means my design is adequate to be constructed by using cables suspended between steel posts, is it a reliable design?

With just the selfweight applied, it may be working (I still can't tell, comparing to my spreadsheet), but we're still a few steps away from the point where we can say the model works under additional dead loads and wind loading. The wind loading, in particular, adds a whole layer of complexity to the analysis, because the forces are horizontal, and the deflection of the cable is no longer vertical, but at an angle between vertical and horizontal, so there's some vector addition to the applied loads.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Structural Design of Cable and Steel Posts Holding the Cables

and add a coating of ice to that, and it gets more interesting.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Structural Design of Cable and Steel Posts Holding the Cables

Quote (Wazir)

Should I go with this results, means my design is adequate to be constructed by using cables suspended between steel posts, is it a reliable design?

Your presentation to date has been very confusing. In your first post, you indicate one high post and three sub posts, each with a cable attached to the high post. In your next post, you show a tangent force of 5k "to hold the cable and specified sag is achieved", a span of 400' and a concentrated load of 600# at about the quarter point nearest the sub-posts. But you did not specify a sag.

Later on, you specify 4k tension, elevation difference of 13.5' and a 2.5' sag measured from the top of the sub-posts.

Your design has not been demonstrated to be reliable, and it does not appear to satisfy statics.

BA

RE: Structural Design of Cable and Steel Posts Holding the Cables

Wazir_Malang:
If you don’t understand what BA is trying to tell you, you probably shouldn’t be doing the engineering on this problem. I don’t care what software you are using, why you don’t know how to model your problem, or understand the output, or why you can’t approximately check the results with some long hand calcs., and simple statics, etc. You really don’t seem qualified to be doing this engineering work. You shouldn’t be asking the questions you are, the way you are, and then expect this to be built with some degree of confidence. You can’t even describe your problem with enough engineering detail and specificity so the real engineers can understand what you are trying to do. And, when this appears to be the case, we don’t have much confidence in your hidden assumptions, modeling or untold requirements. Talk with your boss about this problem, so he/she knows what you know and what you don’t know, so they can guide you, and keep you and your company out of trouble. I can imagine that more than one tie-back is needed at the posts. The lower ones are for lateral stability of the entire system, at the posts, but you haven’t spelled this out either.

RE: Structural Design of Cable and Steel Posts Holding the Cables

I find that in most structural applications of cables, the sag is small enough that catenary theory can be ignored with reasonable accuracy. The actual cable length is close enough to the horizontal length so that the error will be minor.

In keeping with dhengr's post, simple statics are preferred. The diagram below illustrates a simple method which I suggest using. I am considering a single post at A and B differing in top elevation by 13.5'. I am assuming a weight of 1.5 #/' for the cable, based on an area of 0.44 in2 given earlier by the OP. That should be checked as it seems a bit high, since that is the area of a 3/4" dia. solid bar. However, it does not consider the possibility of wind or ice forces as mentioned by others. And it does not consider the concentrated load of 600# as shown on one of the OP sketches, but that would be easily handled in the same manner. If the high post supports more than one cable, the reactions will be different.

I have not attempted to draw the cable profile, but the low point of cable is below point C. I found the low point to be 2.27' below point B, which would be about 6' below point C.





BA

RE: Structural Design of Cable and Steel Posts Holding the Cables

BAretired, your "simple statics" approach may give results that approximate other analysis methods, but in order to satisfy the statics of the distributed weight of the wire, the vertical reactions at the ends of the span would have to be nearly equal, varying only slightly with the amount of the sag, approaching exactly equal as the sag decreases.

At zero sag, the tension also becomes infinitely large.

The low point also changes significantly with the amount of sag, from nearly midspan for high sag values, to actually being at the right end of the span for low sag values.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Structural Design of Cable and Steel Posts Holding the Cables

Quote (BridgeSmith, BAretired)

BAretired, your "simple statics" approach may give results that approximate other analysis methods, but in order to satisfy the statics of the distributed weight of the wire, the vertical reactions at the ends of the span would have to be nearly equal, varying only slightly with the amount of the sag, approaching exactly equal as the sag decreases.
When the supports are at an equal elevation, the vertical reactions would have to be nearly equal, but when Point B is 13.5' lower than Point A, they must differ by 2*13.5H/L where, in this case, H is 4000# And L is 400 feet (statics).

At zero sag, the tension also becomes infinitely large.
I agree, and that condition is impossible.

The low point also changes significantly with the amount of sag, from nearly midspan for high sag values, to actually being at the right end of the span for low sag values.
The low point of the cable changes in elevation, but not in the position in the span. So if a tension of 5000# is specified, the sag below point B becomes 1.82' but the low point of cable remains directly below Point C.
EDIT:
Oops, typing before thinking, a bad habit. If H = 5000#, the reaction at Point B becomes 300 - 5000*13.5/400 = 131.25#, so the low point moves to 131.25/1.5 = 87.5' from Point B. And the elevation changes as well.

If H is large enough to remove the vertical reaction at Point B, the low point of the cable is at Point B and The vertical reaction at Point A is the total weight of the cable, i.e. 600#. That would occur when 13.5H/L = W/2. In this case, when H = 8889#.


I have modified my sketch to show the sag of the cable (area shaded green).



BA

RE: Structural Design of Cable and Steel Posts Holding the Cables

Wazir Malang,

Correct me if I'm wrong, but this project is located in Pakistan and cannot have freezing conditions, so icing of the cables is not likely to be a factor.

The same is not true for wind. You must consider lateral forces due to wind in combination with the cable weight and any additional concentrated loads which may be suspended from the cables.

I don't agree with your arrangement of bracing for the vertical posts. If you wish, this could be discussed further.

Just as a matter of curiosity, what is the purpose of this cable assembly?

The view below is of the Grand Tower pipeline suspension bridge crossing the Missouri River. The main cables directly above the pipe have a substantial drape in order to carry the weight of a heavy pipe filled with natural gas. The pipe is suspended by vertical cables attached to the main cables, not easily seen in the photo. Wind forces are resisted by a cable on each aide attached to rigid arms projecting out from the vertical posts.



See details on: http://www.bridgemeister.com/pic.php?pid=1238
BA

RE: Structural Design of Cable and Steel Posts Holding the Cables

Quote:

When the supports are at an equal elevation, the vertical reactions would have to be nearly equal, but when Point B is 13.5' lower than Point A, they must differ by 2*13.5H/L where, in this case, H is 4000# And L is 400 feet (statics).

I had to think about this for awhile, but eventually, I came to the conclusion that you're right about this. The angle of the cable to a chord line between the attachment points is equal at both ends, but in this case, when the chord line is not level, the tension force in the wire (and the reaction) have a different angle to the pole at each end. The conclusions are slightly incorrect in that respect. The tension in the wire is not equal to the horizontal reactions. The tension is the vector sum of the horizontal and vertical reactions at each end. The tension is at it's maximum at the left end of the span, although unless the sag is excessive, the tension changes very little across the span.

For your sketch, the tension at the left end would be sqrt(4352 + 40002) = 4023.6 lbs (4003.4 lbs at the right end). The tension in the cable is 4000 lbs at the lowest point of the sag. Also, the horizontal component of the force in the cable is 4000 lbs at any point along the cable.

The tension changes as the angle changes, so the angle of the wire for each segment along the span can be calculated, and the length of the cable in the segment can be approximated. We use those calculations to analyze spans with flexible supports (poles without guy wires), by calculating the deflection of the poles and iterating until the deflection under additional load (such as wind) matches the 'apparent' change in the cable length, due to the additional deflection (horizontal 'sag').

Speaking of guy wires, don't neglect to check the tension in the guy wires; it will be significantly greater than the tension in the span wire. There is also the vertical component of the tension in the guy wire that produces axial compression in the pole. The buckling capacity of the discreetly braced pole and the bearing capacity of the foundation will need to be considered.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Structural Design of Cable and Steel Posts Holding the Cables

No argument from me, BridgeSmith.

BA

RE: Structural Design of Cable and Steel Posts Holding the Cables

I believe many parts of northern Pakistan are in fact subject to freezing conditions.

----
just call me Lo.

RE: Structural Design of Cable and Steel Posts Holding the Cables

(OP)
The Actual purpose of this posts are for training as shown in the picture. But only single person will use at time on a single cable. sample picture shows.


No ice loading is possible.
Do you think that wind loading consideration would be required in the analysis?

RE: Structural Design of Cable and Steel Posts Holding the Cables

(OP)
Here are the complete results to clear your Concern Dear @BAretired and @BridgeSmith
You are right that the reaction at the left support should be the same as the forces coming from the Main cables but as you know these pair of cable does not just resist the main Cables but also the balancing cables in which i have shown. The reaction is not just from the main cables suspended between the Main Post and SUB Posts but also from the balancing cables that are used to stabilize the MAIN POST which are two in number rotated at 120degrees as shown. Now I have showed Manual calcs as @dhengr was referring to inspite of helping he was criticizing, which you can see that My FEM seems to be OK. I will look forward to your response.

@BAretired yes it is in PAKISTAN and ICE is not a problem, Where can you confirm if wind would cause any significant effect as the projected area for wind is very small @BridgeSmith.?

Quote (BridgeSmith (Structural)
Quote: When the supports are at an equal elevation,...but when Point B is 13.5' lower than Point A, they must differ by 2*13.5H/L where, in this case, H is 4000# And L is 400 feet (statics)......incorrect in that respect. The tension in the wire is not equal to the horizontal reactions.)

Quote (BAretired (Structural) I find that in most structural applications of cables, the sag is small enough that catenary theory can be ignored with reasonable accuracy. The actual cable length is close enough to the horizontal length so that the error will be minor.
In keeping with dhengr's post, simple statics are preferred. The diagram below illustrates a simple method which I suggest using. I am considering a single post at A and B differing in top elevation by 13.5'. )





"""WazirMalangSE

RE: Structural Design of Cable and Steel Posts Holding the Cables

Quote:

Where can you confirm if wind would cause any significant effect as the projected area for wind is very small @BridgeSmith.?

By my analysis, a 90mph wind on a single 3/4" cable produces a greater load (8300 lbs) than a 300lb point load at midspan (8100 lbs). It appear you have a pair of 3/4" cables going to each tower to the right, for a total of 6 cables to the tower on the left. That's a horizontal load of almost 50 kips at the left tower, and tension on the anchor cable of around 70 kips (assuming a 45 degree angle of the anchor cable). It's also about 50 kips of axial compression on the tower pole and more than 50 kips of bearing capacity required for the pole foundation.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Structural Design of Cable and Steel Posts Holding the Cables

(OP)
@BridgeSmith Ok. I will consider wind loading. as well.

Quote (BridgeSmith (Structural)1 Mar 21 14:58
Quote:
By my analysis, a 90mph wind on a single 3/4" cable produces a greater load (8300 lbs) than a 300lb point load at midspan (8100 lbs). ...kips of axial compression on the tower pole and more than 50 kips of bearing capacity required for the pole foundation.)


@BridgeSmith Do you agree with my simple statics approach as explained above?

RE: Structural Design of Cable and Steel Posts Holding the Cables

(OP)
@BridgeSmith Do you think wind load should be applied based on the projected area of the elements in any direction?
I have applied 30 lb/ft wind loads on the posts but my FEM software does not apply wind load to cables do you think that is necessary as well?



When I apply 1000 lbs of point load then the axial compression is about 51.6 kips for which the left post is designed but anchor block is designed for 40 Kips

RE: Structural Design of Cable and Steel Posts Holding the Cables

That structure is called a Zip Line, or in this case, a triple Zip Line.

I attempted to measure the maximum angle from the photograph (see below). The thick red line is my estimate of the slope of an unloaded cable at the left support. It isn't very accurate, but it looks like about 18o for a cable without a person suspended from it. The maximum angle we calculated earlier in the post, using a horizontal component of 4000# and a vertical component of 435# was tan-1(435/4000) = 6.2o. I have no experience with zip line design, but I wonder if the angle proposed is a bit shallow. It may be prudent to find the recommended slope from zip line manufacturers.

Insofar as wind design, the structure should be capable of resisting expected maximum wind loads when unloaded, but it seems unlikely it would be loaded with personnel during a 90 mph wind.




Note: the man on the left seems to be deflecting his cable considerably more than the guy on the right. This suggests to me that the cables on the installation are not equally tightened.

BA

RE: Structural Design of Cable and Steel Posts Holding the Cables

Quote:

I have applied 30 lb/ft wind loads on the posts but my FEM software does not apply wind load to cables do you think that is necessary as well?

Most definitely the wind load must be applied to the cables. We typically ignore the wind on the poles as being negligible (Typically less than 2% of the total forces on the pole).




Rod Smith, P.E., The artist formerly known as HotRod10

RE: Structural Design of Cable and Steel Posts Holding the Cables

Quote:

Insofar as wind design, the structure should be capable of resisting expected maximum wind loads when unloaded, but it seems unlikely it would be loaded with personnel during a 90 mph wind.

Agreed. Calculating the loads separately, I got about the same forces/tension with a 90mph wind on one cable as with a 300lb point load on a cable, which about doubled the tension from the selfweight alone.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Structural Design of Cable and Steel Posts Holding the Cables

(OP)
BridgeSmith , BAretired

I have applied wind load on each node of the cables as shown does it seem ok to you.?

RE: Structural Design of Cable and Steel Posts Holding the Cables

I don't know the ASCE, so I can't say if your factors are correct for that. I can say that the loading is about half what I get from the AASHTO Sign spec. for a 100mph wind. The primary difference, I think, is in the gust factor (G?), where we use a 1.3 factor applied to the wind velocity, which then gets squared in calculating the wind pressure. AASHTO design would use a 90mph wind, though, except in hurricane-prone areas, which comes out to be about 60 lbs at each node. I'm not saying you're wrong, just telling you what I got for comparison of the loading on the cables and poles. I get your wind loading at a design wind speed of 68 mph (88mph gust). The cable tension is about 5500 lbs under self weight + that wind load.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Structural Design of Cable and Steel Posts Holding the Cables

plus ice and added wind?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Structural Design of Cable and Steel Posts Holding the Cables

Quote:

plus ice and added wind?

We don't consider ice, since the max wind load is always much larger, and we assume that the ice is not going to stay on a cable in high winds.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Structural Design of Cable and Steel Posts Holding the Cables

Quote (BAretired)

The man on the left seems to be deflecting his cable considerably more than the guy on the right

The photo certainly seems to show this, but it could be because he has only just "launched" and so his tether tension is higher because it has to decelerate him vertically.

RE: Structural Design of Cable and Steel Posts Holding the Cables

Quote (We don't consider ice, since the max wind load is always much larger, and we assume that the ice is not going to stay on a cable in high winds.)


Including the ice build up... area can be increased 3x or more...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Structural Design of Cable and Steel Posts Holding the Cables

Quote:

Including the ice build up... area can be increased 3x or more...

We don't get that kind of ice buildup here in Wyoming, but even if we did, a 30mph wind would shake it off of the cables (and traffic signals, which is what our span wires support). For us, the max wind produces a load on the order of 5 times the dead load. Years ago when I started designing span wire signal structures, I looked at the loading combinations for what we do, and determined that the DL + Ice + 0.5Wind combination was always less critical than DL + 1.0Wind for our span wires and what we put on them.

I think the OP said ice wasn't a consideration for the location, anyway, so for this case it's a moot point.

Rod Smith, P.E., The artist formerly known as HotRod10

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