DIS-TRAN Steel Blog

3 Transmission Structures Broken Down

Posted by Wendy Gintz on Jul 7, 2015 9:45:00 AM

When designing transmission structures...

it’s not as simple as saying “ok, I want self-supporting tangent mono poles with delta configuration for my entire 80 mile stretch.”  While that idea might be more economical than having h-frame or 3-pole structures, it’s just not practical.

Ultimately, the wire configurations determine which type of structure will be used, and typically there will be a mix of these structures in order to follow the right-of-way through small or tight turns. Right-of-ways go alongside or through interstates, highways, fields, woods and even water, so an engineer must keep all these situations in mind when designing.

Three common transmission structures:

  •  Tangent
    • Used when transmission route is straight
    • Generally, no longitudinal loads on the structure
  • Angle
    • Used when transmission route changes direction
    • Used from anywhere less than a 5 degree angle to a 90 degree angle
  • Dead-End
    • As name applies, dead-ends are designed to take the full component of every wire's tension
    • Does not necessarily mean end of transmission line 










Whether the structures are tangent, angle or dead-end, wire phases can run in multiple configurations.  Horizontal Configurations provide the lowest profile. Vertical Configurations require the minimum width right-of-way. And Delta Configurations is an attempt to use the value of both horizontal and vertical configurations to maintain phase clearances.

Transmission structures can be classified as either self-supporting or guyed.

Self-Supporting Structures do not use guys: meaning they are not tied to the ground or any other structure in a way that offers additional support. They are better for restrictions to right-of-ways and tend to have loads small enough to not warrant guys.

Guying of structures is used to support the structure and allow for a more economical design in both the steel structure and foundation. Guying reduces bending and deflection. However, the downside is that it requires more right-of-way. 

*Here are some main contributing factors to keep in mind when deciding on whether or not to guy a structure:

  • Structural loading
  • Right-of-way requirements
  • Aesthetic design criteria

**Here are some other contributing factors you may want to consider:

  • Line voltage
  • Electric air gap clearance requirements
  • Ground clearance requirequirements
  • Insulation requirements
  • Number of circuits to be supported
  • Electric and magnetic field limits

There is so much to learn about Transmission Structures.  What questions do you have?  We would love to hear from you so please leave a comment below.

Check out our newest resource for Anchored Transmission Structures.  Click below.
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Tags: transmission structures, guyed structures, tangent, dead-end h-frame structures, configurations

Design Practice for Flange Plates versus Slip Joint Connections

Posted by Brooke Barone on Oct 11, 2012 10:38:00 AM

Having a good relationship with your vendor will ensure a smooth and confident outcome.

Also, by having seasoned engineers and people who have been in the industry for some time and have experienced things that textbooks cannot prepare one for, more knowledge and expertise will be contributed towards the final product.Dead end tran resized 600

Here are some design practices for using flange plates versus slip joint connections for H-frames, single pole guyed and single pole dead-end structures. There is the concern of the uplift/compression on H-frame poles and the excessive amount of vertical load that can be created for guyed structures, especially guyed dead-end structures, as well as how to apply this for self-supporting dead-end structures.

So, what’s the design practice in this regard?

Most customers leave it up to the fabricator to decide when and where to use slip verses flange joints.  This is typically decided in the quote stage, as this can make or break a job.  If you leave it up to the fabricators, just make sure you know or trust the fabricators that are bidding the project.  There are always new players in the game, so you just need to make sure they’re experienced enough with these type structures to know what they are watching for with these connections. 

In other words, if four out of the five bidders quote structures with flange plates and the low bidder quotes slip joints, then understand why they quoted it that way. 

slip joint resized 600The following are a few suggestions when it comes to these type connections. These are not necessarily found in a design guide book or a required industry standard, just a rule of thumb.

  • Dead-End H-frames/ A-frames: Always use flange plates if the joint is located below the beam connection.  (There have been a few times where the conductor beam was located at 45 ft elevation and the structure was 65 ft tall, so in this case, a slip joint was put above the beam connection.)
  • Transmission H-frames w/ X-Bracing:  Depends on loadings and location of joint.  Most of the time, structures are designed with flange joints because the overlap of the slip joint can vary.  Due to the slip tolerances, it could cause a headache in the field to slip both columns the exact amount for everything to line up.  If slip joints are used, the designer still needs to watch the axial loads.  If they get too high, then they will need to switch over to flange plates. 
  • Guyed Structures:  Always use flange plates if the joint is located below the guy attachment points.  The axial loads tend to get high on these type structures.  Of course, there is always an exception to the rule.  There could be some cases where the axial load remains low for a guyed pole.  In these cases, the loads tend to be small, have some uplift or may have a minimal number of guy wires.  If the customer is requesting slip joints on their guyed poles we watch the axial loads closely. 
  • Switch Poles/Riser Poles/Specialty Poles:  Always use flange plates. (Depending on the arrangement, slip joints on low kV Riser poles can be used.)
  • Single Pole Dead-Ends/Tangents:  Always start out with slip-fit joints.  It’s not often flange plates are used for these type structures, unless the customer requires them.  Normally, the axial load does not get excessive for these type structures.   However, never locate a slip between phases.  If the top shaft gets too long due to phase spacing, switch to flange plates in certain spots.  Sometimes a flange plate is used if there is not enough room to fit a slip on the pole due to lots of brackets/equipment/etc.

In the end, always try to use slip-joints where you can because it’s typically more economical.  As you may have noticed, there is an exception to just about every rule above.  This just means there should always be some flexibility in the project so the design engineer has the ability to make judgment calls as needed.  If the customer works closely with the fabricator on each project then it will run smooth.

 Get expert advice for your project

Tags: flange plates, slip joint connections, dead-end h-frame, transmission h-frame, guyed structures, switch pole, riser pole, specialty pole, tangent

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