Dancing about a bit
Appreciate there’s been a bit of a deluge of blogging lately but something came up in my office that I thought was interesting. Also apologies for no pretty pictures – I haven’t been able to get hold of anything relevant yet. You may or may not have read in the news recently about a magnitude 7.7 earthquake that hit the Caribbean yesterday. This was quite a large earthquake in a region that has a risk of seismic events due to the North American and Caribbean plate boundary close to Cuba.
Although Sir Robert McAlpine concentrate their construction operations in the UK they also ‘randomly’ have projects in the Cayman Islands for which MDG do some design work. The Cayman Islands are in the Caribbean region south of Cuba close to the epicentre of this recent earthquake. In the office this morning our team had a call to say that a self-supporting tower crane (on pile foundations) on a Cayman site was ‘dancing about a bit’ yesterday. Secondly a sink hole appeared just over a metre away from the foundations of a mobile tower crane (with ballast foundation). At this time we don’t have a lot more additional information other than there were no injuries and the cranes appear to be still standing. The sink hole appeared in an area of limestone and so far no instrument has been able to determine its depth. Whether there are other voids in the limestone is unknown as I don’t think this was picked up in the GI.
It seems that the crane foundations produced by MDG were designed for tropical weather conditions including hurricanes but not for the seismic conditions. The original brief from site did not mention a seismic requirement but I think it would be reasonable for designers to take this into account. I think due to a lack of experience in this area in the UK it wasn’t considered. The problem facing us now as a result is to determine if the cranes can still be used and a possible redesign of the foundations. Furthermore there is a large risk of foundation failure near the sink hole. Whist the crane is still standing, additional loads either from wind or during deconstruction could undermine the weakened foundations. Clearly there is a cost and time impact to the project as a result. Particularly with addressing the sink hole problem which may require significant amounts of concrete to infill.
Does anyone have any experience with seismic and/or sink hole problems? How do you account for large voids in the rock – I imagine desktop study with GPR might be able to pick this up? I can see if we’re deployed anywhere with seismic activity as PQEs this may be something we have to consider and design for. There is guidance in EC8 for seismic design but potentially other more local codes based on experience might be more applicable (in this example US design codes might be more suitable).
TLDR:
- If you end up doing design work on sites outside of the UK don’t forget to at least consider seismic risks.
- How do you mitigate against large voids in rock during design and how do you resolve a sink hole issue?
- Any suggestions on foundation design for cranes against seismic loads?
- Sorry no pictures yet.
- Yes there is a whole load of other things that come out of this like who holds the risk etc but this blog is already long enough.
Holdfast Training Services Trial Blog 
Al, Shadi’s area of expertise is Seismic Design so might be worth a phone call back to the office tomorrow.
From my limited understanding there are currently two main design approaches. (1) Performance based seismic design (PBSD) where you determine the level of performance required following a set intensity or magnitude event and design accordingly. (2) For simplicity let’s call it ‘safety design’ where you use a code to design for a no collapse requirement however the post event performance cannot be guaranteed. Generally the first will take more time as the stakeholders have to agree what level of risk and performance are acceptable and then design to achieve it (iterative process and you have to assess the performance and likelihood of structural failure). The second approach will probably be quicker as you just design to an existing code but won’t guarantee the crane will be usable after the event.
If using Eurocode the IStructE and their French equivalent have produced a guide to EC8 which is on IHS and worth a look. I’m not sure if the National annexes have ground motion data for the Caribbean (I suspect not). Although the ICE guide has some extra maps for the French overseas islands. You may need to research ground acceleration maps for the area of interest.
If you go down the American route you’ll need to look at the FEMA and ATC guidelines (these are building focused so you will have to determine if there are better codes for temp works or their applicability to your situation):
FEMA P-58 Seismic Performance Assessment if Buildings
FEMA P-454 Risk Management Series: Designing for Earthquakes: A Manual for Architects
FEMA P-547 Techniques for Seismic Rehabilitation of Existing Buildings
ATC-20 Detailed/ ASCE/SEI 41 Tier 1, ASCE/SEI Tier 2, FEMA 352, ATC-52-4 or FEMA 306 may also be a useful as assessment tools.
Although the tower crane will be braced to the building it’s essentially a high structure, with high elevated mass, vertical irregularity at the jib and has a very small base footprint. In simple terms all of these increase seismic risk for the structure. Best of luck for the team going forward. I’d be interested to know how it pans out.
The interesting point from Mark Steven’s comment is regards risk management. I don’t know how long the tower crane would have been erected for, but would it have been reasonable to design against earthquake actions if it was erect for 6-12 months versus a 20 yr life span? What was the probability of a 7.7 magnitude earthquake occurring in that region?
The crane would have been put under significant stresses during the earthquake; a check of welds etc near the bottom of the crane and at the boom may be worthwhile.
The sink hole is a much bigger concern and due to the nature of the rock could be more prevalent throughout the site. Concrete backfilling seems excessive. I would be interested in the solution.
Interestingly enough seismic design does not form part of the role performance specification for a PQE. The questionable nature of this has, err, lead to questions but no actual movement in the RPS. I suspect the earth will need o move beneath a RE project before it enters the curriculum. Pity because even parts of the UK should have a seismic consideration within their design. May be one day when someone reading this gets a chance to influence the RPS for PQEs it might appear and legitimise the small amount of dynamic analysis we have introduced this year…