How it works

The ONGUARD anchors are welded to the tank around its base and connected to the tank’s foundation. During an earthquake, they prevent the tank from overturning by holding it down, whilst dissipating seismic energy through yielding of the anchors.

Unlike traditional tank anchorages that only work in a one-pull, tension manner, ONGUARD anchors provide controlled yielding in both tension and compression throughout cycling earthquake accelerations. This gives excellent energy dissipation, produces a more stable tank response and provides additional resilience to withstand earthquakes that are larger than the design case.

Each ONGUARD application is carefully and specifically designed by ONGUARD engineers, specifically for the seismic hazard at your location, to make sure that destructive earthquake energy is confined and dissipated within the patented anchors, and that all other elements of the tank system have sufficient strength to remain undamaged while the ONGUARD anchors are working during earthquake shaking. Unlike traditional systems, ONGUARD continues to work throughout the cycling of the earthquake and its aftershocks – not just for a single earthquake pulse.

The energy dissipating element within each ONGUARD anchor acts as a seismic fuse. Once the fuse has done its job it can be inspected and if needed quickly, easily and cheaply replaced using everyday tools and with no specialist skills or experience. Full earthquake readiness is reinstated with minimal fuss.

How it is designed

New Zealand

Tank wall thicknesses and the anchorage system are designed in accordance with New Zealand Society for Earthquake Engineering (NZSEE) (2009) “Seismic design of storage tank” guidelines. In New Zealand, earthquake loads are determined according to the loadings standard NZS1170.5:2004.

The anchorage system is designed assuming a structural ductility factor of either 2 or 3 (from NZS1170.5:2004), depending on the level of protection desired, and is based upon a ductile elastic anchor force distribution from Figure C5.2 of the NZSEE (2009) guidelines.

A capacity design approach is then adopted to design other components such as the tank walls, tank skirt, and epoxied fixings into the slab, with reserve capacity above overstrength actions. Overstrength actions are based upon anchor overstrength assuming the same ductile elastic anchor force distribution referred to above. Epoxied fixings into the foundation slab are designed taking into consideration the likelihood that the foundation slab will be cracked due to flexural actions.

Experience has shown that a holistic design approach is required, which considers the tank, anchors, foundation slab, and catwalks, to ensure efficient seismic performance. ONGUARD can provide tank fabricators with design solutions for anchors and tank wall thicknesses. In addition, ONGUARD can also provide design requirements for the foundation slab and catwalks and can work with your foundation and catwalk designers to achieve the desired outcome.

United States

For applications in the USA, ONGUARD’s engineers' design to meet the requirements of API 650 and ASCE 7-10, and work with our accredited Professional Engineer partners and tank designers on the ground as necessary to make sure that compliance with local building rules is met, and the appropriate hierarchy of strength throughout the system is maintained.

The anchorage system is designed assuming a response modification factor of 5 to 8 (ASCE 7-10) and a ductile elastic anchor force distribution. This method ensures that the over-strength capacity of the anchorage system is no greater than the design load on the tank shell.


Existing tanks with traditional anchorages pose a greater threat to your operations than new tanks fitted with the ONGUARD system. For many older tanks that may not have been designed to meet modern design codes, this threat is greater still.

Using gathered information such as tank geometry, material thicknesses and foundation construction, ONGUARD engineers are able to calculate the earthquake strength of an existing tank system and quantify the level of earthquake risk that it poses. They can then specify a type and number of ONGUARD anchors that, when added around the base of the tank to replace the tank’s existing anchorage system, will activate in an earthquake before any other element of the system fails. The new ONGUARD anchors protect the remainder of the system from damage by introducing much-needed ductility and resilience, delivering improved earthquake resistance and performance, and giving you peace of mind.

As you would expect, by reducing your exposure to earthquake risk and the negative consequences of damage from seismic events, retro-fitting the ONGUARD seismic system to your existing tanks can provide tangible financial benefits in the form of reduced insurance premiums and deductibles.

OG PRO anchor technical information

OG PRO anchor range:

kN kips kiloliters gallons
OG PRO 33 33 7.5 5 - 60 1,500 - 15,000
OG PRO 51 51 11.5 40 - 100 10,000 - 25,000
OG PRO 81 81 18.2 75 - 150 20,000 - 40,000
OG PRO 117 117 26.3 110 - 300 30,000 - 80,000
OG PRO 172 172 38.7 180 - 800 50,000 - 200,000


* The most appropriate anchor will depend on the height-radius ratio of the tank and the available slab thickness

OG PRO anchor components:


 Pin AS/NZS 2637.1:2010 - grade300*
 Filler  Ultra-high molecular weight polyethylene
 Sleeve  ASTM A554 - grade 316
 Baseplate  AS/NZS 3678:2011 - grade 300 or
AS/NZS 3679.1:2010 - grade 300
 Foot  AISI-1040/45 - 400MPa minimum yeild
 Topplate  AS/NZS 3678:2011 - grade 300 or
AS/NZS 3679.1:2010 - grade 300
 Topplate isolator  Nylon6
 Isolator washers  Nylon6
 Epoxy anchors  Grade 8.8 galvanised threaded rod with Hilti HIT-HY200 or HIT-RE500SD


* Prior to fabrication, mill and test certificates for each batch is checked for suitability and quality control


OG PRO anchor testing:

OG PRO anchors have been tested under quasi-static cyclic loading to verify their performance. Prototype testing was completed in accordance FEMA461 (2007) using a displacement-controlled loading protocol.


Prototype test Yield strength in tension (kN) Maximum tension (kN) Cycle displacement
on which 20% strength
degradation occurs in
compression (mm)
Last cycle displacement
completed prior to
fracture (mm)
 OG PRO 33-1  41  62  11.0  15.5
 OG PRO 55-1  62  95  15.5  21.7
 OG PRO 51-2  65  95  15.5  21.7
 OG PRO 81-1  102  152  21.7  21.7
 OG PRO 81-2  99  152  21.7  21.7
 OG PRO 117-1  158  225  15.5  21.7
 OG PRO 117-2  150  222  n/a  7.9
 OG PRO 172-1  212  326  30.3  30.3
 OG PRO 172-2  209  324  30.3  30.3
 OG PRO 172-3  214  324  21.7  21.7