Version 1: Anchor plate at the structure end for e.g. four strands (1), wedge-seating plate with barrel (2), deflection plate (3), support seat for four strands (4), quick coupling (5), four TENSA SM 240 kN stressing jacks (6) and anti-twist protection with integrated ruler (7). As an option one can use a wedge-seating cylinder between (2) and (3).
Stepped anchors are a special form of compression anchors. Both types are used as ground anchors in various fields, for example when it comes to stressing walls or tunnel slabs against the soil. They are differing in that the individual strands of a stepped anchor do not have the same, but different lengths. The load transmission range of the individual strands is staggered in succession in the soil, which ensures a more uniform load transmission into the subsoil. Stepped anchors are used if the soil is not sufficiently load-bearing for a standard compression anchor.
In practice, stepped anchors are often stressed using single-wire stressing jacks, thus ensuring the same stressing force in the individual strands. The disadvantages of alternative methods using multi-strand stressing jacks are: different stresses, the need to thread the strands at the same time and the time-consuming adaptation of the jack to different anchor types.
Paul Maschinenfabrik, experts for prestressed concrete technology, have already successfully made experiences with stepped stressing jacks in the past, have optimized the method and are now offering two versions of stepped stressing jacks. Both versions are using a special support seat with stressing jacks that are individually applied and hydraulically connected. This method offers the following advantages:
- Interchangeability: Identical single stressing jacks can be combined in a modular design with support seats for 4 to 7 strands (or more). In case of anchors with different strand arrangements (hole patterns) or with a varying number of strands one must only change the interior parts in the support seat. Use of relatively simple and lightweight stressing jacks which can easily be replaced for maintenance work, for instance.
- Easy mounting: Application of the support seat onto the anchor, threading of the individual stressing jacks onto the strand.
- Fixing: The individual stressing jacks are connected with the support seat by means of quick couplings, thus preventing the individual jacks from lifting during the test cycle.
- Reduction of the wedge-seating loss: A wedge-seating cylinder (wedge-seating force 120 kN, stroke 20 mm, length 150 mm) for hydraulic wedge-seating can be used as an option. This is already successfully used in other applications.
- Checking of the elongation: Optical check of the elongation on each stressing jack via an integrated ruler. An electronic elongation measuring system can be used as an option.
- Maximum stressing force: 240 kN, ideal for 0.6“ strand. Deflection of 6° saving strand.
Version 1 is made up of a special support seat and is used with stressing jacks type TENSA SM 240 kN which have proven for many years. This version has already been used in the past, has now been optimized and offers the following advantages:
- Use of widely-used and very robust standard stressing jacks type TENSA SM 240 kN
- Extremely economic solution due to the possibility of using the TENSA SM jacks in other fields
- Possibility to convert existing TENSA SM jacks
- Stroke: optionally 100/200/300 or 500 mm
- Automatic installation of the stressing grips as soon as the final position at the front has been reached
- Max. operating pressure: 520 bar
- Strand projection which is customary in the market: min. 650 mm (with 4 strands)
Version 2 uses stressing jacks specifically optimized for this purpose. These jacks are fitted with an exterior stressing grip. It is thus possible for the stressing grip to move closer to the anchor plate at the structure end. The necessary strand projection is thus reduced by 30%. The advantages are:
- Minimum strand projection of 400 mm (with 4 strands) due to front stressing grips
- Stressing jacks more lightweight by 30% than comparable TENSA SM jacks
- Stroke: 300 mm
- Max. operating pressure: 700 bar