Design Checking 2

[stanmorek]

Hello Engineers

First post here. Refusing to sign off as the checker for an unsafe design is one thing but in your opinion does the checker have an obligation to ensure that the design is reasonably buildable and not overly engineered? I realise that this may be subjective and practices will vary from country to country.

I work for the in-house structural group of a large UK public corporation that does it own design work and project management for some projects. A few have run over planned budget by some margin. There's one that's only half built yet the construction contractor's cost claims have already exceeded the original tender price...a debacle down to poor design and management decisions. The project has had numerous design changes resulting in abortive work and contractor's staff being stood down for long periods. The crux was the main steel structural elements were grossly over designed and too heavy to handle/lift and has majorly affected the rest of the design and programme.

I've had no involvement in these projects but these designs have gone through a company system of oversight and reviewed and passed by senior and principal engineers.

 

[hokie66]

A checker does have an obligation in regard to constructability and economical design, as well as to safe and complying design.

There are a lot of reasons for projects to run over budget, and rarely does a checker have input into the budget establishment. Without knowledge of your company structure or the nature of these projects, it sounds like the budget was the problem.

How do you know the main steel elements were "grossly over designed"? UK contractors seem to have a record of encountering problems during erection of steel structures, then trying to pass costs back to the owner. I assume they bid the work, at which time they should have had a plan to erect the heavy members. These days, nothing is too heavy to lift.

 

[stanmorek]

Thank you for replying hokie66 and answering my question. My employer is a government organisation responsible for maintaining rail infrastructure. Safety is never in question but efficient use of taxpayer's money sometimes seems to be forgotten about. Some of our design engineers are known to be very risk averse.

The project in question is a rail bridge protection beam against accidental vehicle collision from the road below or 'bridge bash beam'. Much of our existing bridge stock over public roads have sub-standard headroom clearance for high sided vehicles.

You're correct in saying that budget is a major factor and arguably the job was too big for the contractor. A problem is that the project managers are in sole control of the tender process with little engineer's input. Not enough questions are asked of bidders at this stage. Cost and programme will always win over demonstrating a good technical understanding of the job. However, the original design was a 25 metre single span fabricated beam. Having seen the design drawings there were 80mm thick plates with all sorts of welded stiffeners. It appeared to be more robust than the bridge it was protecting which is carrying trains everyday.

Yes project management and contractor carry blame but I believe the subsequent chain of events may have been somewhat mitigated if questions were raised at the beginning in the sanity check.

 

[MikeHalloran]

The problem is interesting because anything traveling on the road below that could cause structural damage to a rail bridge is necessarily pretty sturdy.

... so a cross-road 'beam' intended to stop such sturdy vehicles before they hit the beam, must be even more sturdy than the bridge.

The money might have been better spent in reinforcing the bridge, or raising it, or lowering the highway.

... but a checker can't influence a decision like that.




Mike Halloran
Pembroke Pines, FL, USA

 

[ukengineer58]

There seems to be a lot of posts lately on here claiming that designers have 'over designed' by clients reps etc. my question is how do you know that they are? Have you checked the calcs and do you have the knowledge to know if its over designed or not? Plus it's easy for a reviewer to claim they could save money from a design when they do not carry the responsibility for it. Comments such as this structure here is lighter or we've got away with that before irk me because you are never comparing like for like and it may be that the old is in theory not adequate. Especially considering old railway bridges, some are not to code requirements now but are simply monitored.
I have encountered this before,someone saying this looks over designed, usually by people who have never designed a structure in their life.
On your point impact loads can be huge and very unpredictable, more unpredictable than the train loading so a little conservatism isn't always a bad thing. Unless you want to replace it all the time. And surely whatever the design was the contractor had site of it at tender and should have priced to construct it.
I suggest the blame lies not with the designers but elsewhere, probably your own organisation.

 

[ukengineer58]

Also consider the cost claimed by train operating companies for delays to service from restricted speeds or cancelled trains over a damaged bridge on a main line can be tens of thouseands per minute and on a secondary line thousands per minute is it cost effective to save some steel for increased risk on protection?

 

[TenPenny]

If the beam is supposed to stop high vehicles before they can hit the bridge, it's going to have to be pretty massive, any high vehicle carrying a large heavy load, travelling at say 100 km/h, is going to need a pretty big crash beam to stop it.

Where I'm heading with that is, what did the plan for the beam intend to accomplish? To prevent any high vehicles from hitting the bridge, or to provide a warning impact so the drivers can stop? If the mandate is to prevent high vehicles from hitting the bridge, the beam is by necessity going to have to be the physical device preventing any over height vehicles from getting by, which means that it is going to have to be designed to STOP the heaviest, fastest possible vehicle on the road that would be overheight. On the other hand, I've seen such devices that are designed to do damage to the high vehicles, but the assumption is that the driver will then stop.

I do find your comment about the engineers being 'risk averse' to be amusing. Isn't the point of your mandate to minimize risk? If not, why bother with this exercise at all? You're being critical of people designing safety systems for doing too much? Perhaps the mandate needs to be reviewed.

 

[dik]

My experience is that it's the structural system chosen that has the greatest impact on cost... over design or underdesign of the system has only a small impact on the project cost...

Not knowing the details, for example, if Mf= 1000 kN-m (factored design moment) and Mr=10,000 kN-m (Resistant Moment) then I would consider this grossly over designed (maybe deflection)... (some mining stuff I've done lately has a SF=12.5, but that's the requirement) but, if Mr= 1500 kN-m... I would consider this a overdesigned, but wouldn't likely question it... as they say, the devil's in the details... and we don't have any.

Dik

 

[stanmorek]

Thanks for all the replies. I originally wanted to ask a general question about a checker’s responsibilities. The example I cited above was to give some context however some posters seem to have been irked by this. It wasn’t my intention to and I apologise if this is the case. Also, it wasn’t my intention to discuss the finer details of the project. However, at the risk of going off topic it has thrown in some interesting discussion for me.

I should explain that my background is a designer and have been designing railway structures for the last 10 years. I’m aware I work in a safety critical industry. At the risk of repeating myself I work for a government owned entity that is owner/client, designer, project manager, operator and maintainer of railway infrastructure as a single organisation. So yes it’s my own organisation at fault.

I actually know the designer personally. He admitted that his beam design is well under-utilised with respect to structural capacity (~35%). As mentioned by someone the problem was the onerous deflection limit of L/300 (80mm for 25m span). Loading is actually specified by the UK National Annex to Eurocode BS EN 1991-1-7 for a single horizontal 825kN free point load. My own back of envelope calculations are:
MEd = PL /4 = 825 x 25 / 4 = 5160kNm approx maximum applied design moment
(load factor = 1.0 for accidental situation),
Grade 355 MPa steel, section depth 750mm -> Second moment of area required for ultimate moment
I (ULS) = My/f = (5160x10E6 x 375 / 355) / 10^4 = 545,000 cm^4 approx.
Second moment of area required to satisfy deflection limit d = PL^3 / 48EI
I (SLS) = PL^3 / 48Ed = (825x10E3 x 25000^3 / 48x210000x80) / 10^4 = 1,600,000cm^4 approx.

One might argue that the beam should be allowed to deflect closer to its elastic limit or even allow some plastic deformation to dissipate impact force perhaps with some replaceable components if it does not pose a maintenance liability or danger to other road users.

I agree it isn’t just about saving material or labour costs on the railway, site time and access is severely limited and dictates the cost. The design solution and thus the construction method is a major factor. Work has been cancelled or put on hold because a long enough service shutdown cannot be obtained or the local authorities are not willing to grant permission for sufficient road closures. A lighter beam may have only required a smaller crane lift and removed the need for disruptive heavy foundation work. The load needs to be transferred safely into the ground as well.

As a designer I’m used to being challenged on the solutions I propose to problems. For example, I’ve just designed a scheme for a protection wall to stop road vehicles from driving off the edge of a retaining wall and crashing onto the railway cutting below. The project managers think my design is too expensive to build and propose an alternative. I’m writing a paper for senior management justifying why I chose my solution over their alternative which I don’t believe will work for this particular site. Even though it’s a pain for me but I believe they’re entitled to a sensible discussion with designers.

Yes risk needs to minimised, but engineers are also working in a practical and economic framework. When I said risk averse I meant avoiding risk at any cost and covering every conceivable angle which I don’t believe is good engineering. From a legal standpoint, the UK the approach to risk is the ALARP principle “As Low As Reasonably Practicable” there is only a finite pot of money and resources. The UK rail industry has established procedures in assessing risk against the costs to prevent deaths and to justify whether the work needs to be done or not. There are potentially hundreds of sites we have to deal with and the government can’t afford to build a Rolls Royce solution for all of them. But say if it was up to the designer to choose design loading, I would possibly be looking at road conditions (minor/major roads/urban areas) and analysis of past bridge strikes with possibly some full scale testing, to make a reasonable site specific assumption on vehicle mass, speed and deformation.

Only a portion of the cost overruns can be attributed to the original design assumption as the remainder is down to poor management decisions in forcing design changes but the subsequent chain of events could have been avoided.

As for thinking out of the box and removing the hazard at source, raising bridge levels is problematic when it comes to changing track alignment, raising embankments, modifying station platform heights and tunnel headwalls. Raising ballast levels could overload adjacent bridges which would then require strengthening. We deal with a lot of old infrastructure some which are 150 years old. The Highway authorities are rarely agreeable with closing or altering their roads. They don’t see it as their problem and money is tight for them too. All our bridges in question have adequate road signage forewarning road users but it doesn’t prevent bridge strikes from occurring. We document these events and some bridges have a long history of hits even though the majority of incidents cause no damage or have only caused superficial damage so far. For many years this risk has been deemed acceptable but in recent years there is a policy change.

 

[MikeHalloran]

Maybe a beam is the wrong solution for bridge strikes.

E.g. I have seen canal lock gates protected by very sturdy wire rope, secured to the lock walls by separable connections, and supported (and lifted out of the way when appropriate) by lightweight truss beams articulated like small drawbridges.

In the case of a bridge bash "beam", there's no need to move the cable on any regular basis. You would still want some kind of connector so that you could quickly replace the cable (and of course the supporting trusswork) after a strike.

I.e., the requirement to withstand a bridge strike by X mass at Y velocity without permanent deflection may be unnecessarily conservative. You could just make the bash beam a consumable, and place it far enough from the bridge to avoid a strike of the actual bridge for any reasonably foreseeable mass velocity combination.







Mike Halloran
Pembroke Pines, FL, USA

 

[truckandbus]

This could be more a question of design review versus print checking. My experience has been that a traditional 'checker' function was to make sure there are enough dimensions to build the part, verify tolerance scheme, make sure that "detail" isn't spelled "detial".

Items like manufacturability, mechanical properties, etc. should be covered in design reviewss ought to be defined and reviewed before the design is started and then again before any real investment in time is made in doing the detailing. Makes no sense to have a package ready for release and have someone say 'oh, should be twice as thick and cast instead of welded'

 

[cvg]

does the checker have an obligation to ensure that the design is reasonably buildable and not overly engineered?

good QA/QC practice will include checking or maybe more appropriate to call it "peer review" at every level. drafters check drafters, designers check designers, engineers check engineers and even senior management decisions should be reviewed by someone at the peer level.

however there is no obligation on the peer reviewer to "ensure" anything, only to review and comment.

Many projects such as this could benefit by doing a formal risk assessment. Such an exercise might prove that “As Low As Reasonably Practicable” is not always achievable or necessary.

 

[TomDOT]

Mike,

I think you are onto something with your wire rope idea, though I would also look at redundant/sequential ropes. Wire rope "Cable Barriers" have become very popular in recent years here in Texas as a substitute for guardrail, particularly to prevent "crossover" incidents on high-speed divided highways. There are some pretty dramatic test videos out there showing a 3-cable barrier diverting large trucks.

Installed cost is much lower than traditional guardrail, though they seem to need fairly frequent maintenance.

 

[stanmorek]

Mike, your idea of a wire rope, I keep picturing an arrestor hook snaring a Tomcat on a carrier deck. Most structural engineers I know prefer to use mathematics over physical testing to give certainty of their designs. In my line of work I don't know many structural engineers who are comfortable with structures purposely designed to deform greatly or working on energy absorption problems.

I'm working on an anti-collision bollard problem to stop a head on smash from a truck over the edge of a precipice. I'm proposing a reinforced concrete wall on a piled foundation but management want a light steel bollard design. Unfortunately amongst other things I'm can't be sure without testing how in the event of collision the bollards won't energy transfer to a weak retaining wall underneath. Management are saying to me prove to them it doesn't work.

Having met with suppliers their market is in anti-terrorist protection while the only purpose designed highway barriers I know of are for sideways impact not head-on. None of these suppliers so far can any satisfactory assurance that debris ejected from a wrecked vehicle won't land on the railway.

Maybe they should be giving the problem to a mechanical engineer to solve.

Link
Link

 

[MikeHalloran]

Without running any numbers, I'm thinking that stopping a moving vehicle requires dissipation of a lot of energy.

Carrier arresting wires connect to a multi-block arrangement of sheaves and cable, and a really big hydraulic damper.

I'm thinking that, regardless of whether the interface with the vehicle comprises cables or bollards or walls or anything else, the interface has to be allowed to move, and to deform _something_ in order to stop the vehicle.

Surely USACE has sponsored work on barriers intended to stop high speed high mass hostile vehicles?



Mike Halloran
Pembroke Pines, FL, USA

 

[MacGyverS2000]

Yeah, I thought those concrete barriers were designed to re-steer a vehicle back onto the road, but that only works if they're heading towards the barrier at less than a 45 degree angle (give or take). I didn't think they were designed to withstand the impact of a vehicle barreling headlong into them.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[Debaser]

In my line of work I don't know many structural engineers who are comfortable with structures purposely designed to deform greatly or working on energy absorption problems.

Presumably they stick to using the train then, and never drive on a motorway (or indeed any road with steel barrier road restraints)?

 

[TomDOT]

Steel guardrail with a modern end treatment is designed specifically to dissipate energy by deforming. In the event of a crash, the rail itself is forced to deform through the end treatment block by the crash, dissipating energy as it goes (and if a larger crash, multiple posts are sheared off along the way.) The "wave" in the rail is flattened out, and the rail is coiled up. Example:

http://www.highwayguardrail.com/products/etplus.html

 

[IRstuff]

"who are comfortable with structures purposely designed to deform greatly"

Actually, almost every car on the road is designed to do precisely that in a crash.

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[stanmorek]

Quick update. The powers that be have relented and I've been able to persuade them to go with a concrete wall and not bollards.

A forum search has thrown up a useful thread.

http://www.eng-tips.com/viewthread.cfm?qid=244205