I originally wrote this text in the social network of our company in July 2014 for my colleagues around the world. After migrating from Movable Type to WordPress system for my weblog and being silent for a long time, I thought it would be a good idea to republish my engineering posts in my own blog too. Hopefully younger engineers may find them useful in their career.
Understanding a project or my tiring Wednesday!
Posted by Sohail Forouzan-sepehr 10-Jul-2014
Sarah Daniels started a discussion yesterday entitled Wine Wednesday Check-In, asking how was your Wednesday (also look at Wine Wednesday Check-In posted by Sarah on the 3rd of July). I was going to answer it at home yesterday evening as I really like Sarah’s posts and suggestions; but, I was so exhausted that I went to bed instead! Today at lunch time I decided to answer, but I felt my answer wouldn’t be very interesting for Sarah; but maybe more useful for younger engineers in several different engineering forums. So, at the end I will include the name of a few forums/groups this post may be useful for. In the meantime, I’ll try to explain things without going too much into technical details; so, hopefully those who’re not structural engineers also can read it without yawning! Yesterday I had a very tiring Wednesday but at the end we achieved something, a safe design approach for our client through a 4.5-hour meeting from 10.00 am to 2.30 pm entitled “nobody will leave the room until an agreement is achieved”! Recently I’ve checked the design calculations of a third party designer for a de-icing salt silo on behalf of our client and rejected the design! Our client asked a third party company to design and build a baseless barn on the top of an embankment and very close to the crest of the slope (due to the space restrictions) to store about 700 tonnes of de-icing salt for the coming winter time. Stability of the slope has been already checked by Jacobs and is OK. Thus, a straight forward simple industrial structure which needs to bear the loads arising from such amount of salt and transfer them to a strong soil layer via piles. What the designer did was just blindly feeding a commercial software with some numbers and getting wrong results without clearly understanding the behaviour of the structure under the loads arising from the stored salt as well as the consequences to the embankment which consists of several weak layers of soil over a strong layer at the bottom. Here’re a list of issues the designer ignored or failed to observe in the project and so underestimated the loads on the piles resulting in extremely under-designed piles.
- Storing 700 tonnes of salt over the weak soil layers will cause settlement which is manageable but only if it’s taken into account! Settlement of the weak layers would drag down the piles and reduces their capacity. In geotechnical engineering it’s called “negative skin friction”. It was totally ignored by the designer.
- Due to the limited dimensions of the barn and the height of its walls, the salt would be stored there with an angle. This is called “angle of repose” and for salt can be around 40°. When materials are stored behind a retaining wall with a slope, the lateral pressure of the retained material will be with an angle (will have both horizontal and vertical components) and thus not only pushes the structure horizontally but also pushes it down. Storing the salt with its angle of repose of 40° means the vertical component of its lateral pressure can be almost as big as its horizontal component. This additional vertical components will apply additional loads on the piles which were totally ignored by the designer.
- The very basic principal in design is that the loads must be in an equilibrium (Statics). Where you apply loads, these loads must be in equilibrium with the rest of the system’s actions and eventually be transferred to the ground (to the strong layer).
- There were other technical issues too; but as I promised in the beginning, I’m trying not go too much to the technical details.
When I mentioned these issues to the designer as the reasons I rejected his design, his answer was that he realises that I’m a traditionalist and totally against the softwares; but, that commercial software has been used by hundreds of engineers for more than 20 years and so cannot be wrong!!! Well, to be honest I’ve been developing engineering analysis and design softwares for more than 15 years! I don’t have any problem with modern technology and the commercial softwares. The problem arises when somebody uses a software blindly without understanding the project requirements, the behaviour of a system we want to design (in our case, a structure) and eventually the behaviour and actions of the surrounding environment.
The above project took more than three months and we spent 4.5 hours in a meeting to explain the problems and agree on a correct approach to resolve the problem. It would take less than a week if one just sketched a simple diagram of the structure, summarised the existing information as well as the engineering assumptions, applying the loads and figuring out how and where basically the loads will be applied, how they affect the structure and how they are being transferred to the ground in a safe and optimum way.
If the above issues were taken into account from the beginning instead of just feeding a software (which was possibly developed for a different specific conditions) with numbers, even a better and more optimum option could be considered which could save 100-160 cubic metre of reinforced concrete as well as at least 800 man-hours.
The meeting was eventually finished at 2.30 pm yesterday with a mutual agreement while we were all exhausted and hungry. Yet I don’t believe the final option by the third party designer is the most optimum solution for our client’s requirements. And Sarah, no wine at the end of the day as a day after was also a working day. BUT, I’m happy that at least the design will be correct and thus SAFE. So, at the end of the working day, our client could go home with peace of mind that no disaster will be likely to happen during the working life of the barn with a proper maintenance. She was pleased with our works and I’m willing to bet if she requires another highways salt barn next time, she will directly come to us for the design.
Hopefully this post can be interesting for the below groups/forums:
- Civil/Structural Designers Forum
- Structural Engineering
- Structural Engineering Forum
- Civil/Structural Engineering Practice Group
PS: I guess I will need to review and edit my post later. So, please do let me know if you find something I need to add or amend.