Design Practice for Flange Plates versus Slip Joint Connections

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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.

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. 

The 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.