Design Considerations

When designing tie bars the following should be considered :

  1. Tensile resistance of anchor bar
  2. Elongation of tie bar under load
  3. Settlement of the fill and the effect upon the tie bar and choice of connection design.
  4. Corrosion protection of the tie bar

1. Tensile Resistance

Anchors are generally manufactured from round steel bar with forged or threaded ends that allow a variety of connections to be made to the structure, dead man anchorage or to other bars. Connections can provide articulation or adjustment of the anchor in length (see Figure 2 - Typical end connections for tie bars).

Figure 2 - Typical end connections for tie bars

The tensile resistance of a tie bar will not only depend upon the cross sectional area of the bar but also the cross sectional area of the thread or connecting part to the structure.

On a plain steel round bar threads can be produced by cold rolling or machining such that the minimum tensile area will occur in the threaded portion of the bar (see Figure 3 - Thread direct rolled onto bar).

Figure 3 - Thread direct rolled onto bar

Alternatively threads may be manufactured on upset forged ends which involves forging the end of the parent bar to create a larger diameter over the threaded portion, the minimum tensile area is now in the shaft of the anchor (see Figure 4 - Upset forged thread)

Figure 4 - Upset forged thread

In accordance with EN1993-5 the tensile resistance Ft,Rd of an anchors is calculated as the lesser of the tensile resistance of the thread, Ftt,Rd or the tensile resistance of the shaft, Ftg,Rd at any time during the life of the structure:

A reducing factor is applied to the thread to allow for the possibility of bending being introduced to the thread when settlement occurs of the fill and the sometimes less than ideal installation conditions.

The effect of the Kt reduction factor applied on the tensile resistance of the threads means the most economic design of anchors will often use upset forging technology which allows the cross-sectional area of the thread to be greater than the shaft. Typical anchor tie bars can be 30m+ in length so the reduction in diameter for the shaft allows great weight savings over the length of the tie bar.

Generally, and in particular in seismic areas, it is recommended that failure should occur in the shaft first, and not the threads, as this will allow greater and more uniform deformation to occur before failure. However variation in steel strength characteristics may make this difficult to ensure in practice.

Common steel grades of material available for anchors include 355, 460, 500 and 700 grades (which typically have yield values, fy, of 355, 460, 500 & 700 N/mm2 respectively). The choice of steel grade depends on a number of factors, whilst the higher strength steel will always produce the lightest weight anchor this may not be suitable for stiffness requirements, onsite welding or lead-times.

It is recommended that high strength steel grades (fy,nom between 500-800N/mm2 ) are used with caution. Clause 3.7 EN1993-5 is ambiguous in its guidance which states that for steels with fy,nom > 500N/mm2 double corrosion protection measures should be applied as for ground anchors in EN1537, however this is not practical or economic for typical deadman anchors. Clause 7.2.2(3) of EN1993-5 recommends that very high strength steels, fy greater than 800N/mm2, are not used as deadman anchors. Where high strength steels are used it is recommended to ensure these are not susceptible to Stress Corrosion Cracking (SCC).

2. Elongation of tie bar

Elongation of tie bars under the design load should be checked, this should be based on the shaft diameter of the tie bar. The increased stiffness of the threaded area (if larger) can be neglected as tie bars are typically sufficiently long for the effect to be negligible.

Movement under imposed loads may be reduced in many cases by pre-loading the anchor tie bars at the time of installation which removes ‘slack’ from the system and develops the passive resistance of the ground.

3. Settlement

When detailing tie bars it is important to consider the effect of sag of the anchor and forced deflection due to settlement of fill or for any differential settlement where ground conditions change (eg from existing ground to new fill). Bending and shear stresses can be induced at a fixed connection when a tie rod is displaced or when fill settles causing the tie bar to deflect (see Figure 1).

The effect of fill settlement is to cause the tie bar to deflect placing additional stress in the vulnerable threaded portion of the tie bar (see Figure 5).

Figure 5 - Bending stress

 

The Bending at the thread can be minimised by ensuring tie bars have end fixings that allow articulation (see Figure 2). Where large settlements are expected further articulation may be required between the front and anchor walls. Settlement will always occur in a fill (even if mechanically compacted) and it is good practice to minimise the effect upon the tie bar by using articulated joints at appropriate locations and allowing the bar to rotate with settlement.

Figure 6 - Articulated joints

Typical tie bar arrangements with articulated connections can be seen in Appendix 2.

Settlement ducts are sometimes used as a means of reducing the effect of settlement on the tie bar. Tie bars are placed inside large (typically 200-350mm) plastic tubes, the bar lies on the bottom of the tube and as fill settles the tube settles leaving the bar suspended in the tube. The bar will still deflect due to its own self weight and articulation should still be supplied to the wall connection.  

In practice settlement ducts can make installation more difficult as tie bars become difficult to handle and if not sealed at the ends carefully increase the risk of corrosion as water can flow along the tube. In sea water conditions it is also recommended that the tubes are fully drained before ends are sealed and fill placed on top.

Figure 8 – Tie rod settlement duct installation issues

4. Corrosion Protection

Sheet piles are used in many aggressive environments and consequently corrosion protection factors influencing effective life must be considered for the tie bars. It is especially important to consider the corrosion protection of the tie bars at design stage and of particular importance is the connection to the front wall as the anchor is typically subjected to the most aggressive environment at this point.

Several options are available to the designer

4.1. Sacrificial steel

Often the most practical and robust method is to allow for steel loss in the tie bar i.e. the tie bar is increased in diameter to allow for anticipated corrosion. In this situation, consideration should be given to the probable corrosion rates and consequential loss of tie bar section, in the thread, shaft and fittings depending on their position in the structure. The thickness of sacrificial steel to allow should be based on local conditions but guidance is outlined in EN1993-5 tables 4-1 & 4-2 where typical corrosion rates are given for steel in temperate climates.

Further protection can be given by detailing the tie bar connections so that placement is away from the most aggressive regions of corrosion.

Figure 9 - Protection of tie bar connection, shows how the threaded part of the bar has been placed behind the sheet pile and away from the splash zone.

Figure 9 - Protection of tie bar connection

Where it is either not possible or desirable to place the tie bar head connection inside the sheet pile additional protection can be provided by increasing the diameter of the thread to allow for the anticipated corrosion loss. This system is robust as no special site considerations are required during installation.

Figure 10 - Sacrificial steel protection of thread

4.2. Protective coatings

Several options are available, such as painting, galvanising or wrapping. The most commonly used method is to wrap the anchor to give an appropriate level of corrosion protection. Often the anchor shaft is wrapped in factory conditions and shipped to site but connections cannot be wrapped until installed on site.

Figure 11 - Wrapping of connection on site

The vulnerable anchor head should be fully protected and Figure 12 shows a suggested detail. However this is a very difficult detail to achieve on site and should be avoided wherever possible. It is important to ensure that protection to connections and the anchor head are correctly performed during installation, any damaged or unprotected areas must be repaired before backfilling. Any breaks in the wrapping system could lead to aggressive pitting corrosion and premature failure of the anchor.

Figure 12 - Wrapping protection of anchor head

4.3 Cathodic protection

Generally tie bars are connected to the retaining wall above mid tide level, as cathodic protection is not effective above this level it cannot be used to protect tie bars