DIS-TRAN Steel Blog

How To Slip Joint Fit Up on Hermetically Sealed Steel Poles

Posted by Brooke Barone on Jun 24, 2014 10:30:00 AM

When preparing specifications for weathering steel transmission pole projects, utilities will often require that the poles be hermetically sealed.  This means that the inside of the poles are sealed from the outside environment through welding.  The rationale behind this requirement is easy to see.  Weathering steel requires a number of wet/dry cycles in order to form the protective oxide coating that prevents further corrosion.  If water is allowed to rest against a weathering steel surface, this oxide coating cannot form and corrosion can be a result.  By eliminating water from inside the pole, this corrosion mechanism is eliminated.  The most common method of hermetically sealing steel poles is through the use of sealer plates that the top and bottom of a slip jointed section.

Problem:  Difficult slip joint fit up on hermetically sealed poles

Sealer plates are often used on weathering steel slip jointed poles to provide a barrier against air intrusion.  A sealer plate is placed at the very top of the lower section while a second sealer plate is placed above the point of maximum slip on the upper section.   These sealer plates can significantly stiffen the sections against deformation.  Often, steel pole sections will not be perfectly round when formed.  In order for the two sections to mate together properly, the sections need to conform to each other.  When sealer plates are present, the tubes are stiffened against deformation which can make it difficult to achieve the proper slip length.

sealer_plate

Solution:  Eliminate the upper section sealer plate

If the top of a section is sealed against moisture there is no way for moisture to travel up the slip joint.  Furthermore, the air gap between the top and bottom section allows any condensation or other moisture that finds its way inside the tube to drain out the bottom.  Following these recommendations will go a long way towards ensuring proper slip joint fit-up, as well as make the contractor's life easier.

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Back to Basics: Slip Fit Pole Assembly Procedure

Posted by Brooke Barone on Feb 20, 2014 3:34:00 PM

There are a number of ways to slip a pole: some can be very basic, while others require more experience or equipment. It all depends on the type of pole, site conditions, equipment  availability, room to work, etc. Slipping a pole can be done on the ground using jacking devices, while others can be lifted and slipped in the air by sections with a crane or even a helicopter. Whatever the method, it’s important that the required joint seating be obtained, metal to metal, and that worker safety is always a priority. 

With so many factors that can affect how a pole is slipped together, the following explanation will focus on one very basic method: Slipping a pole on the ground using a jacking device.

Assembly:

When assembly of slip fit poles are performed on the ground, the poles are raised to position in one piece. The shafts are typically supplied with some form of marking so that the shafts can be match-marked and properly aligned. These markings should be matched at each splice along the pole before assembly operations.

Assembly nuts or holes should be provided during fabrication; four above and four below each splice.  These attachment points are 180 degrees apart and allow a high strength bolt to be used for attachment of the jacking device.  The nut or hole size shown on the structure erection drawing will dictate the fastener size required.

Depending on section size and availability of equipment, there are several methods that can be used for jacking the sections together. Mechanical jacking devices can be used and include: chain hoists, come-a-longs and turnbuckles, as well as hydraulic jacking devices. 

*We do not recommend ramming the pole sections together with bull dozers or other equipment as damage may occur to the pole sections.

Prior to attaching the jacking devices, the pole sections should be blocked up off the ground and leveled as near as possible.  The pole sections are then telescoped into one another as far as possible taking care to align the match marks.  Jacking devices should then be placed on both sides of the pole to complete the joint assembly.  All attachment points (4 per side) should be used when attaching mechanical or hydraulic jacking devices to avoid damage to the pole.

slip fit pole assembly

The minimum jacking force per side should be at least 10,000 lbs, and may be applied using (2) six ton come-a-longs under the full effort of one man each.  Jacking forces in excess of the 10,000 lbs. minimum may be required to achieve the proper seating specified and shall be used when necessary.  Make sure to check with the appropriate pole fabricator on the maximum values which should not be exceeded. 

While applying the jacking force, the pole sections should be pulled together while “walking” the upper section over the lower by using a back and forth movement with the crane.  This process should continue until the joint is fully seated.  At this point, the sections should show no further movement and there should be adequate metal-to-metal contact on all the flats. Small air gaps sometimes occur on several flats and are acceptable.  Gaps may also be present at the welded corners. If larger gaps exist and there is resistance of the pole sections to further movement, it may be necessary to increase the jacking force toward the maximum limit. It's important to always refer to the manufacturer's designand erection procedures on allowable gaps and slip distances. 

Lubrication may be applied to the lower section to aid in the slipping process.  However, the use of grease, oil, or other petroleum-based lubricants are not recommended, as it results in discoloration, which is difficult to remove.  Liquid soap provides adequate lubrication and is easily removed after assembly.

Once the pole sections have been assembled, the slip distance (the distance between the weld bead marks), should be checked. This measurement should only be taken after the joint has been fully seated.  Failure to fully seat the joint, even if the slip distance measurement is in the acceptable range, could cause unacceptable stress transfer across the joint or further slippage of the joint after the lines have been sagged and clipped in.

Note: The same principles may be used when jacking the pole sections together in the air. 

While the pole is on the ground, the arms, pole steps (if required), and other miscellaneous pieces can be installed; thereby making the pole ready to climb when it is erected.  It may be necessary to leave off climbing devices in the area where they would interfere with the erection lifting straps.

Want to know a basic way to erect the slip fit poles? Click on the image below and find out! 

 

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Tags: slip fit pole assembly procedure, bolts for cantilever arm connections, high strength bolt, slip fit pole assembly and erection procedure, steel structure erection drawing, steel assembly nuts, horizontal weld bead, jacking pole sections together

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