The Technicality | ONGUARD » Onguard Seismic Tank Systems

Technical

Seismic action produces loadings on tanks that are potentially damaging. To maximise asset protection it is critical that the correct design approach is utilised as this will underpin the performance of the entire seismic tank system.

The ONGUARD system uses a capacity design approach, where ductile anchors are used, which concentrates damage from seismic loading in one small replaceable component, and provides protection to the tank walls.

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 and so that all other elements of the tank system have sufficient strength to remain undamaged.

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.

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Design Approach

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.

How ONGUARD 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.

United States

For applications in the USA, ONGUARD’s design meets the requirements of API 650 and ASCE 7-10. We 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.

Retro-fitting

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 risk it poses. They can then specify a customised ONGUARD system to replace the tank’s existing anchorage system. 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.

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OG Pro Anchor Technical Information

OG PRO anchor range:

Anchor	Design Strength		Application (Tank Volume)*
	kN	kips	kiloliters	gallons
OG PRO 23	23	5.1	2 - 40	800 - 10,000
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
OG PRO MAX	295	66.3	100 - 800+	25,000 - 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:

Component	Specification
Pin	AS/NZS 2637.1:2010 - grade300*
Filler	Ultra-high molecular weight polyethylene
Cap	ASTMA276/479 304L Stainless Steel
Casing	ASTMA276/479 304L Stainless Steel
Coupler	AISI 1215 steel – 300mpa minimum yield
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