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#16-18 Wire Rope Safety Barrier systems – post footing issues (updated Dec 2016)

Published: | Category: Technical advice note | Audiences: Road controlling authorities, Road traffic engineers & consultants, Roading contractors

Wire Rope Safety Barrier systems – post footing issues (updated December 2016)

Publication details

NOTICE TO INDUSTRY

To whom it may concern

Dear Sir/Madam,

Wire Rope Safety Barrier systems – post footing issues (updated December 2016)

This document extends the guidance given in TAN 16-01 (February 2016) and may be used to replace that document in its entirety with immediate effect. This is a clarification and does not vary our requirements.

The Transport Agency has been made aware of a number of issues associated with the failure under impact of wire rope safety barrier (WRSB) post footings (see photos below). The issues are generic in nature and not specific to any particular system.

Our concern is that the WRSB footing dimensions at some installation locations are inappropriate for the soil conditions. This may result from a combination of:

  • inadequate site investigation during design;
  • inappropriate site geometry;
  • poor soil compaction during earthworks operations; and/or
  • poor installation practices.

Additionally, limited in situ testing of WRSB installations may have masked any installation issues.

If a WRSB system has inadequate foundation support, this is likely to result in poor system performance when impacted. This increases the risk of serious injury to vehicle occupants and foundation remedial works as well as additional maintenance costs.

TAN 16 01 WRSB image 1 TAN 16 01 WRSB image 2

The following text details the Transport Agency’s expectations of the supply chain for the design and installation of WRSB systems. It is wholly consistent with the Transport Agency M23 Specification for road safety barrier systems, AS/NZS 3845.1:2015 Road safety barrier systems, and the Transport Agency’s procurement and quality procedures.

The Transport Agency expects that the Installation Designer (refer AS/NZS3845 Part 1), as the entity that designs the length, location and types of components of a road safety barrier system to be installed, will take due account of the particular ground conditions on the section of the road network being treated.

Design

The design of all permanent (or temporary) road safety hardware system installations, including WRSB installations, shall be compliant with the crash tested design or the road safety hardware system configuration granted acceptance by the Transport Agency, as listed on the Transport Agency M23 webpage (www.nzta.govt.nz/resources/road-safety-barrier-systems(external link)).

The default standard foundation for all WRSB systems is considered to be that used for the NCHRP 350 Test Level 4 crash tests and detailed in the relevant WRSB product manual; this being the minimum performance level for all 4 cable WRSB systems currently accepted for installation on the state highway network. Where applicable, WRSB foundations tested to MASH may also be used.

To assist in foundation design, the System Owner (refer AS/NZS3845 Part 1) must have available the design horizontal force(s) and/or bending moment(s) at a nominated height and at an angle of 90° to the barrier as measured from data recorded during crash testing of the WRSB system.

It is strongly recommended that the Installation Designer (refer AS/NZS3845 Part 1) and any Peer Reviewer have attended and passed the Transport Agency Road Safety Barrier Design Course within the last five years. This course is held annually with the course details and dates available on the Transport Agency website.

Ground conditions

All NCHRP350 and MASH crash testing of accepted barrier systems has been undertaken using standardised soil (to AASHTO M147 Grading A or B) for repeatability and consistency across test facilities.

The design of all permanent and temporary road safety hardware system installations must take account of the particular ground conditions at the proposed installation site and any system constraints.

It is expected that all embankment earthworks supporting road safety barriers, including WRSB, will be generally compliant with NZ Transport Agency specification B/2 and/or M/4. Care must be taken with at grade and cut earthworks to ensure suitable ground is present.

The Installation Designer (refer AS/NZS3845 Part 1) must test and confirm the ground conditions at the installation site as part of the design process. The results of this site investigation must be documented.

Where the ground conditions vary from the standardised soil detailed above, the Installation Designer must amend the footing design and location using one of the following options (in no particular order of preference):

  1. design a revised WRSB footing in accordance with the indicative guidance in Appendix 1;
  2. adopt the Transport Agency standard oversize WRSB footing (refer figure 1 below); or
  3. use an alternative road safety barrier system.

The Transport Agency standard oversize footing (refer figure 1 below) is considered an acceptable solution for soils where the undrained shear strength is not less than 25kPa or the internal angle of friction is not less than 25° over the depth of the footing. Where footings are wholly within cohesive materials, the maximum linear shrinkage of the material shall not exceed 8% when tested in accordance with AS1289.3.4.1

16-18 Fig 1 NZ Transport Agency standard oversize WRSB footing

Figure 1:  The NZ Transport Agency standard oversize WRSB footing

WRSB location

Using the WRSB system’s default post footing as detailed in the relevant WRSB product manual, the minimum offset from the WRSB (taken as centreline of system) to a batter hinge point (BHP) behind the barrier is 1 metre with a maximum batter slope of 1V:4H, as shown in figure 2 below.

Where this minimum offset of 1 metre cannot be achieved OR the batter slope is between 1V:2H and 1V:4H, irrespective of the WRSB system, the post footing shall be either:

  1. the Transport Agency standard oversize footing (minimum 400mm diameter, 1200mm deep with additional reinforcement) as shown in figure 1, or
  2. specifically designed with peer review in accordance with the guidance in appendix 1 of this document

NOTE: The offset can be reduced down to a minimum of 500mm from the BHP. If the barrier is to be located closer than 500mm to the BHP, then a more suitable road safety barrier system must be used.

In both cases 1. and 2. above, the use of non-standard footings and their installation locations must be fully documented and reported to the Transport Agency Project Manager.

16-18 Fig 2 WRSB location relative to batter hinge point BHP

Figure 2:  WRSB location relative to batter hinge point (BHP)

Where the batter slope is steeper than 1V:2H and the minimum offset of one metre cannot be achieved, an alternative barrier system or special foundation system will be necessary.

Any other offset/slope combination not specifically mentioned above, such as may be encountered in relation to a retaining structure, must be referred to the National Traffic & Safety Manager for decision.

The foundation design, together with all site investigation results and Peer Review reports, shall be retained as formal project records in accordance with the Transport Agency’s project documentation requirements.

Anchor blocks

Anchor blocks for WRSB systems are a critical element and must be considered accordingly during both design and construction.

The Transport Agency expects that all System Owners shall ensure their WRSB system drawings clearly indicate the required soil parameters to ensure a particular WRSB anchor performs as crash tested. Where the soil parameters fall below the stated minimums, an alternate (generally larger) anchor block will be required.

Testing of installation

All WRSB installations shall be subject to post pull-over testing at the minimum interval frequency outlined below, unless in the opinion of the Installation Designer, the testing frequency can be varied. When varied, the Installation Designer shall document the reasoning. For example, where the WRSB is to be installed in a controlled engineered fill (as confirmed by earthworks compliance testing), the Installation Designer may decide that a reduced frequency of pull-over testing is appropriate over this length.

The System Owner shall have a static pull test procedure available for testing the WRSB post foundations to ensure the system will perform as intended. It is expected that this procedure will be fully detailed in the relevant WRSB product manual.

Unless the Installation Designer determines otherwise, a sample of not less than 3% (minimum testing of one post in every 100m) of each installed length of WRSB shall be tested in accordance with the System Owner’s instructions (generally static pull tests).

The results of all testing shall be retained as formal project records in accordance with the Transport Agency’s project documentation requirements along with the Installation Designer’s determination(s) as regards the need for and frequency of testing. The minimum data to be recorded must include the date of testing, the location of the installation, testing procedure, the results of the load/pull tests, and name and signature of the tester.

Changes to road safety hardware systems

Any changes to the tested/accepted configuration of the road safety hardware system will deem the barrier system non-compliant unless prior acceptance of the change/departure from the tested/accepted configuration has been granted by the National Traffic & Safety Manager.

Such changes may include, but not be limited to, modification of the system components or installation of the system outside the nominal test conditions due to ground conditions or other roadside features.

In the event the ground conditions cannot be verified prior to construction, the NZ Transport Agency oversize footing shown in figure 1 below shall be used and validated with post pull over testing after construction.

Please ensure this notification is circulated to all staff in your organisation who are responsible for the design, specification, procurement and/or peer review of WRSB installations.

Yours sincerely

Fergus Tate

National Traffic and Safety Manager

Highways and Network Operations

Appendix 1: indicative design procedures for wire rope safety barrier footings

Introduction

The design of the foundation system for the line posts in a wire rope safety barrier (WRSB) system is critical to the functionality of the barrier. During an impact from an errant vehicle, the foundations are required to remain fully elastic in the soil while allowing the steel line post element to plastically deform. After the impact, the posts should be easily removed from the foundations and replaced without the need to disturb the foundations.

Typically validation of the performance of the foundations of WRSB line posts involves conducting dynamic impact testing on either individual posts or complete barrier systems to verify the performance characteristics of the system are achieved. The majority of this work is undertaken in laboratory conditions with standardised soil properties. Given the large number of variables associated with the design of a foundation system, it is cost prohibitive to validate the performance of these foundations under all conditions. As such the design of the foundations is typically completed through the use of theoretical models in conjunction with a limited number of physical tests to validate performance in certain impact conditions.

This appendix provides guidance on indicative design procedures for use when assessing the performance of the foundations of line posts in non-standard soils. The indicative design procedure is derived from experimental evidence obtained from multiple physical testing programmes and considers both static and dynamic loading combinations.

Limitations

This appendix addresses the design procedure for foundations of the line posts in wrsb systems only. It makes no assessment as to the suitability of the barrier system for the intended application. The design of wrsb systems requires an assessment of the mass of the impacting vehicle, its speed and angle of impact.

Design procedure

Static Load Case

The critical load combination for static loading to the foundations are typically applied from radial forces imposed on the posts by the wire rope cables as the system forms a horizontal curve.

Under these conditions, the forces applied to the post can be determined from the vector change in the cable angle over the barrier and the tension force in the cable. When determining the tension force in the cable it is important to consider both the initial static tension applied to each cable and the maximum likely change in tension due to thermal variations. Guidance can be provided by the WRSB System Owner on both of these variables.

The ability of the foundations to withstand this applied loading can be determined using conventional geotechnical analysis, such as the unrestrained post analysis or the use of specialist geotechnical analysis software such as LPILE©.

Dynamic Load Case

During an impact, an errant vehicle will exit the roadway and engage directly with the wire ropes and posts of the WRSB system. The loads that are imparted into the foundations of the line post are limited by the full plastic capacity of the post. Once this capacity is exceeded, the posts will be plastically deformed and be pushed to the ground by the impacting vehicle. Typically, the direction of impact will be longitudinally along the barrier which is typically the weak-axis orientation of the line posts. However, the potential remains for the vehicle to impact the posts in the strong axis direction and as such this direction of loading must be considered as most critical.

The design loading applied to the foundation elements for a specific barrier system can conservatively be estimated as the nominal strong axis plastic capacity of the post when acting as an un propped cantilever. This loading is defined as:

M = Sx.fu

F = M / e

Where:

            M = Applied moment to the top of the foundation (N.mm)

            F = Applied shear load to the foundation (N)

            Sx = Plastic section capacity of the steel line post in strong axis direction (mm3)

            fu = Nominal ultimate tensile capacity of the material (MPa)

            e = Nominal height of applied loading (mm)

It is typically taken that the applied height of loading (e) is equal to the height of the middle cable used in the cable barrier system, on the assumption that the post is being forced to bend backwards due to the pressure applied from the wire rope cables. Alternatively, the height of loading can be assumed to be identical to the underside of the bumper of the impacting vehicle, with a lower bound estimate taken as 350 mm.

The ability of the foundations to withstand this applied loading can be determined using conventional geotechnical analysis, such as the unrestrained post analysis or the use of specialist geotechnical analysis software such as LPILE©. It is important to note that these methodologies have been developed for static loading conditions and may provide more conservative designs than when subjected to dynamic loading, accordingly dynamic soil properties can be used where applicable and sufficient evidence is presented. 

Determination of Soil Properties

Accurate determination of the properties of the soil in the desired line post foundation is important to validate the overall performance of the system, particularly so in the case of mixed soils. It is recommended that a suitably qualified Geotechnical Engineer be engaged to provide an assessment of site conditions and suitable soil properties for use with the anchor designs.

Soil parameters should be assessed for the depth of the anchor with the average of the measured properties used in the design. In the case of mixed soils, the lowest of the respective undrained shear strength or internal angle of friction shall be used.

Where footings are wholly within cohesive materials and the maximum linear shrinkage of the material exceeds 8% when tested in accordance with AS1289.3.4.1, the footing design shall specifically consider the effects of shrinkage on capacity.

In situ soil measurements can be undertaken using test methodologies endorsed by the NZ Geotechnical Society or the Transport Agency’s Principal Geotechnical Engineer (or his authorised representative).

For sites which have been constructed of engineered hard fill as part of a construction contract, results obtained from calibrated Nuclear Densometer testing may be used.

Expert advice should be sought from a suitably qualified Geotechnical Engineer for sites with mixed soil types.

Further information

Contact Julian Chisnall or Stuart Fraser of the Highways and Network Operations’ Safety team.

Kevin Reid

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