Understanding a project – Part 2: Analysis

As mentioned in my previous post, “Understanding an engineering project!“, I have recently restarted blogging after a long period of silence and then migrating from Movable Type to WordPress which made my life much easier. In one of the companies I’ve worked for, we had a series of technical discussions on their social network which I categorised them as “Understanding a project!” The idea came to my mind when one of the younger engineers once asked “How do I become the best engineer possible?” in a post and the most important item of my answer was to advise him not to be afraid of making mistakes as far as a proper quality control procedure is in place and also he’s willing to learn from his mistakes and take a corrective action. I even advised him not to delete his mistake in design and keep it in his archive. I just thought to republish them every now and then on my own weblog to hopefully assist younger engineers in their career.

Well in this post, I was discussing how to understand the loading patterns to any structure before analysing them by sharing a sample of free body diagram. I would suggest reading my first post of this series, “Understanding an engineering project!“, before reading this one and if you haven’t done it yet.

I do hope I can manage my time better to keep posting “understanding a project” series regularly.

Understanding a project: structural analysis

Posted by Sohail Forouzan-sepehr 07-Aug-2014

Salt Barn
Salt Barn

In my previous blog post entitled Understanding a project or my tiring Wednesday! I’ve mentioned how a third party designer blindly dived into using a commercial software to analyse a salt barn without understanding how the loads of 700 tonnes of stored salt being transferred to the structure and then to the ground, how the structure would react and behave, etc. Also, I’ve mentioned eventually after an exhaustive 4.5-hour meeting the designer was convinced to correct his conceptual mistakes in the design. A few days ago I received the revised design calculations and realised though the designer has corrected some of his mistakes, yet he’s confused about the rest of the design and made even bigger mistakes. So, instead highlighting his mistakes line by line, I decided to sketch some diagrams to show him how the loads of the stored salt being applied to the structure and then transferred to the ground. These are the type of diagrams I personally use before analysing a structure. After sending my sketches to the designer hoping they’ll be instructive enough and remembering a discussion here about “How do I become the best engineer possible?“, I just thought to open a new area on ********** for Understanding a Project where people can use their special expertise in explaining for younger engineers how to approach a problem to solve. For instance, Dr John Fay can post some hints using his extensive knowledge in maths and CFD and I will hopefully share some posts in manual analysis and conceptual design of structures. I hope our colleagues will find this series of discussions useful.

As the first post of this series (or better to say second after my above-mentioned blog post), I’d like to share the diagrams I sketched for the analysis of the salt barn below. Having said that the below general arrangement for the salt barn is not my preferred option and I don’t consider it as an optimum solution, but well in this particular scheme, I’m just a technical reviewer, not a designer.

So, in the first figure you see a schematic sketch of the barn and in the below image (I’ve only shown the external loads, not the weight of the structure which should also be considered), you’ll find the diagrams showing how we apply the loads on separate free-body diagrams for analysing different parts of the structure. Using such simple diagrams you’ll realise that even you don’t need any computer programme to analyse the structure. Generally, using simple statics and free body diagrams will help you control any possible mistakes much easier than using a software blindly and basically will help you to understand the behaviour of the structure better.

Free Body Diagrams
Free body diagrams and applied loads (click to enlarge)

PS: I haven’t opened any page or group for ‘Understanding a Project’ yet as I’m not entirely sure about the title. Any better suggestions would be more than welcome.

PPS: 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

Understanding an engineering project!

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 lunchtime 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, in 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 wintertime. 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’s 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. These additional vertical components will apply additional loads on the piles which were totally ignored by the designer.
  • The very basic principle in design is that the loads must be in 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 to 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 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 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 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.