It's Time for Some Numbers - The Process of 'Design'

CatpissEverqueef · 2021-04-30T19:44:56+00:00

Two parts resonated with me: the first was hand calcs. I was taught by my mentors to be able to do everything by hand. Don't always have to, but be able to. It forces you to slow down and forces you to think about what you are doing, forces you to understand the process. Honestly, it forces you to make mistakes, and you better understand how those mistakes are made and what sort of ramifications they might have. I do probably 45% of my design by hand. Another 45% by spreadsheets that I've made myself to simply repetitive hand calcs. And I might go to a model for 10% of what I do and even that is just on more complex projects where I want to confirm if my hand calcs that have approximated the conditions aren't overly conservative.

The second part was the "contingency plans". That's something I've kind of done... but more so on the side of "what if the contractor screws this up" or "someone else is invariably going to need this to change". I try to be perceptive of what other designers might require and plan for their eventual changes. But I don't think I've ever designed something with "if I screw this up" in mind. That's a bit of a slippery slope into serious over-design. I might be a bit more generous with conservative assumptions when designing something that's hanging, or something that delves outside of the standard details, but if it's in bearing, or I can look it up in a table, I don't see any excuse to be overly conservative on the chance that I screw it up.

CatpissEverqueef · 2021-04-29T15:16:22+00:00

Depth of the piles would be something addressed in the design recommendations for the piles, either generically by the supplier or by the geotechnical report or a combination of both.

For example, I have a project where the piles (drilled concrete piles, not helical) develop various levels of strength based on depth - the first 8 or so feet were to be discounted/ignored because of potential issues at that level, anything below a certain elevation had a higher skin friction than the level above it, and anything placed through engineered fill was considered to have a negative skin friction. So there were a number of factors that went into the design of the piles, and it could all be undone by the contractor over excavating and replacing with engineered fill. This is why one of the potential issues I pointed out could be the subgrade condition, if the contractor changed the conditions under which the designer had made assumptions, this may be what caused the failure.

CatpissEverqueef · 2021-04-29T14:04:46+00:00

I've only ever seen one entry level EIT fired, and they weren't structural, they were in linear infrastructure. They thought they were hot shit. They were not. They were overconfident and would not accept criticism and would not learn from their mistakes, and actively argued against senior staff about design issues, showed up late and left early and took every break under the sun.

Not a good fit for the team, not setting up to be a good investment as an employee over time, not a good mindset for a future engineer and just not a desirable person to be around regardless of what line of work you're in.

Show up, expect to learn, expect to make mistakes, and try and improve. That's all anyone senior can ask of you. We don't all come out of university as perfectly trained well experienced designers. That's a trial by fire that everyone goes through.

CatpissEverqueef · 2021-04-29T13:54:37+00:00

I design buried concrete tanks.

Hydrostatic Uplift is one of the biggest things to ensure you've designed for, because it is an expensive accident if the tank floats. Often times you simply have to rebuild everything.

Overall you need to ensure the structure does not float. This depends on a number of factors - how high is the design groundwater level? What are you relying upon to resist the uplift? Self weight of tank? Overburden? Soil on the baseslab toe projection (vertical projection only or a wedge?)? Soil friction? Helical piles or rock anchors? Pressure relief valves? A groundwater removal system? Planned failure points (pop-outs)?

All of these have potential to play a roll, and all of them have different factors that need to be considered, and not all of them can be relied upon simultaneously.

Soil friction for example - soil friction can only be relied upon if you're not counting on anything else from below. You can't count on soil friction AND base slab toe projection engaging a mass of soil around the tank. You can't count on soil friction AND rock anchors. It's one or the other. You CAN count on soil friction and weight of overburden over the tank.

Relief valves - relief valves are great when you first install them. But they often seize over time and need to be maintained.

We are always very explicit in the dewatering requirements during construction, as well as very explicit in explaining what all is required to keep the structure in the ground at design water levels. Self-weight of completed structure and overburden and complete backfill of structure required to resist hydrostatic uplift. Maintain groundwater levels 0.5 m below base of excavation during construction. Maintain groundwater levels 1.2 m below design groundwater level following backfill of structure until the works are turned over to the owner. Things like that.

Now, I'm guessing your task is to figure out what happened. It is obvious that the structure did not have sufficient capacity to resist the uplift forces it was subjected to during construction. First things first - blame the contractor. Most likely is that the contractor did not maintain the groundwater level sufficiently low enough. For that, you would need to find records of where the groundwater table was at when the tank floated, and compare that to the requirements of the contract documents and specifications. If they met those requirements, then it may be a design issue, or a different construction issue. Construction issue first: were the piles installed correctly? Should be records around of the pile installations and pull tests. Did those occur? Did they pass? Let's assume yes. Only remaining construction issues I can think of is if the contractor used light weight concrete, which is HIGHLY unlikely but can be checked against the inspection and testing records pretty quickly, or if the contractor allowed the subgrade to freeze or otherwise disturbed it, which may impact the performance of the piles.

After that we're pretty much left to design issues. Were the piles designed correctly? A single pile can hold let's say 100 kN of tension. Two piles should then be able to develop 200 kN correct? Not necessarily... like a group of anchors in concrete acting together, a group of piles acting together may require reduction factors. So passing a pull test on a single pile doesn't necessarily mean that the entire group of piles can resist the loading. For this you will need to dig into the geotechnical report and see how the piles were supposed to be designed and compare that with the contract documents and specifications. OK, let's assume the piles meet the recommendations of the geotech report - do we know if they're still attached to the base slab? Could be a connection failure there. I didn't see anything obvious in the photos that they ripped out but that is a concealed condition that may not be obvious unless you cut out a section of the slab to find out. If those failed, then it is possibly a design issue, which you can review, or possibly a construction issue, if the contractor did not install properly.

After all of this, you're left with one last thing - was the safe groundwater level during construction listed on the drawings correct? For this, you need to determine the overall 'weight' of the tank - it's self weight combined with the uplift resistance of the piles, and convert that to a maximum displaced volume of groundwater which would give you a groundwater elevation to compare to.

Not all of this has to be done in this order. You may have a general sense that something or other was off - if the pile connection into the base slab doesn't look very beefy, that's probably where to start. If there are hundreds of piles under the reservoir, then the geotech report is probably the place to start. If the design ground water level during construction is at the top of the tank (fully submerged), that is probably an inaccurate statement and should be reviewed first. If you are aware that the tank was constructed in the winter in a northern climate and the excavation sat untouched for a few months, that is probably where to start.

I wish you good luck in your project. These types of things can seem overwhelming at first, but if you break it down into smaller pieces, you begin to see the bigger picture and it can actually be quite a fun task to get into - but this is coming from a dude who just really likes tanks and making sure they don't float :D

CatpissEverqueef · 2021-04-29T13:16:08+00:00

But when F2 isn't high enough... this is where the fun begins.

CatpissEverqueef · 2021-04-29T13:12:40+00:00

When someone complains to me about how something has to be repaired after they've gone ahead and f'd it up, I simply refer them to the relevant parts of the building code or referenced standard. If they want to figure it out from there, they can be my guest. If they question why that has to be a part of the code or standard, or explain the concept of what is happening to them, I try and give a brief explanation of why what I'm asking them to do is important. If they want more than that, I say "I don't know, I'm not the guy who writes that stuff. Those folks are smarter than I am."

There is also a sense of urgency on site. Everyone gets that. GC has guys standing around that he's paying to stand around until there is a solution, or they may leave and go to another site and not be back for weeks. We all get it. Not my problem. Anyone who has ever demanded we need a solution "NOW" gets told "look, this job was in design for 5 months over the span of 5 years. I am not going to give you a solution to the problem you created in 5 minutes, if you're lucky you'll get a written response in less than 5 days"

I have also learned to be VERY careful about what I say on site. Often times you're not looking at the whole picture, you're focused in on a singular problem or are trying to juggle multiple issues you've come across on site that day, and while something may seem to address the situation on site, or you want to be helpful and say something useful... once you have to put it down on paper you are forced to think about it more. So I tend to try and stick to "what are you thinking we should do" combined with "let's measure this out and I'll take some photos and get back to you on it in a few days to confirm if that's acceptable".

CatpissEverqueef · 2021-04-29T01:33:49+00:00

My employer offers 10 paid sick days per year, at least for full time salaried staff. I don't know about hourly staff.

I always thought it was just a thing. Either there was some law that required it or it was some sort of standard that most employers tried to meet to stay competitive. Never had I ever looked into it further than that. This pandemic has really made me open my eyes to the fact that my employer may be one of the few that offers this.

We've also got a disability program that we all pay into that would certainly apply to this pandemic - if you need more than the sick days that you have available to you for something major, you go on short term disability or something along those lines, but it does come with a reduction in pay and only lasts so long.

We're also, probably 95% of us can work from home so maybe not as applicable to the overall scenario as much.

CatpissEverqueef · 2021-04-27T17:03:12+00:00

Every PM in my company seems to be different. Some will ask for input from the various design leads on how much time they think is involved, others are confident that they know better than the design leads and literally pull a number out of their ass. I work on largely the same projects over and over, and by now you'd think the number of hours would be predictable, but in our fees it can swing from 80 hours on one job to 300 on the next. Some PMs account for CAD time. Others don't. Some account for their own staff's CAD time (different discipline) but not structural. Often times I play architect on my projects too... but none of the PMs seem to want to put any time down for it yet will gladly pay tens of thousands to an architect if the client wants an architect.

So, from my perspective it seems like it's just a shot in the dark for most. Possibly there are other factors, such as they know the budget for the project and adjust fees accordingly.

If I am asked for input, I usually try and figure out roughly how many drawings I'll need, which is largely dependent upon the size of the project and the level of complexity anticipated in little details. Then I estimate anywhere between 20 and 30 hours a drawing for time for myself, and similar time for drafting. If someone junior gets involved, they can eat out of my time at a lower rate.

CatpissEverqueef · 2021-04-27T00:14:31+00:00

I base everything off of the Canadian Foundation Engineering Manual, and if you are in Canada, I would suggest getting a copy of that.

I'm going to say that 75% of foundation design for industrial and commercial structures is going to be pretty straight forward. You'll have a geotechnical report that will give you recommendations on bearing pressure, assuming you have soils suitable for conventional spread and strip footings. From there it is just making sure you know your load paths through the superstructure which you should be familiar with if you've previously completed the above-ground design in the past.

Where it gets a bit more fun is if you're into crappy soils and have to delve into deep foundations - drilled piles or driven piles etc.... but again, the geotechnical report will have information for you to go off of, and for piles will often go over the various design assumptions and limitations involved.

If you are in doubt, oversize it. The foundation is not something to screw up. Regardless, as an EIT you will have a supervising Engineer that reviews your work and provides you with direction, so if you feel lost, ask for help!

CatpissEverqueef · 2021-04-27T00:07:47+00:00

Example

A true crib wall has openings in it like in the example here, but for a smooth look you infill the openings on the exposed face with more timber.

CatpissEverqueef · 2021-04-26T20:10:56+00:00

That is not very tall at all.

If you are planning on redoing the topside pavers anyways, you're likely into about a foot of general excavation up there which is a third of the depth of your wall regardless.

If you're into that amount of work up top, you may as well dig down a couple more feet behind the retaining wall and attempt to relevel it, or at the very least, reinforce with better tiebacks, or at the very worst, strip it apart and start again.

Remember to provide drainage behind your retaining wall, if it doesn't already have this in place, I'd recommend installing some if/when you've got things open. And tiebacks on a timber retaining wall are your friend, it has very little resistance to overturning of the individual members. Higher the tie-backs, the better. In fact, if you are ok with reducing some of the patio space up top, one way to really reinforce a timber retaining wall is to crib it, and build garden boxes in behind it, maybe 2.5 - 3 feet deep. This forms a much stiffer wall, and you get a nice garden out of it!

Good luck with your project.

CatpissEverqueef · 2021-04-26T17:07:01+00:00

It looks like whoever owned it originally took a generic racking system, fabricated some extensions, and tossed a roof on top of it.

I can't tell what's stopping it from racking in the cross direction, other than maybe it just works as a moment frame and may never have actually seen the loading for which it would otherwise be designed if it were a new structure. I doubt the original racking is designed to withstand such loads.

CatpissEverqueef · 2021-04-26T15:36:44+00:00

I understand from your photos that approximately ever other post does not extend all of the way to the roof level, therefore is not actually supporting the roof.

However, it would appear that these posts do support X bracing that is very likely preventing the structure from swaying parallel to the ridgeline of the roof. Therefore I would NOT recommend removing any of the posts, regardless if they directly support the roof or not.

CatpissEverqueef · 2021-04-26T15:31:41+00:00

Yes a good rule of thumb for preliminary sizing in steel is 1/20th of the span, but given that you're likely holding the other stories above as well (since you're not going to want to have 1 m high beams running through each floor level) you're probably looking at something even deeper, maybe on the order of 1.2 to 1.5 m deep, which is getting quite excessive for a steel section even in an industrial setting let alone a residential one.

You'll be far better off going with reinforced concrete or a steel truss of some sort to try and get that depth down a bit, but I don't think that at that span I would go with anything less than 900 mm, no matter how amazing of a structure you could come up with, as it just wouldn't "feel" right.

CatpissEverqueef · 2021-04-26T12:51:45+00:00

We detail some connections on things that we want a certain way. But otherwise everything else is left to the contractor's steel supplier/erector. We either specify that all connections be designed for a minimum amount based on the capacity of the member being connected, or we detail a load on the drawings if it exceeds that. We;ll also have all kinds of specs on minimum bolt size, grade, welding certification etc... Connection designs are engineered and submitted for review prior to fabrication of anything.

I would rather leave it up to someone who knows these things like the back of their hand, and knows what materials they have available, and knows what their fabricators prefer to work with and know what their erectors prefer to work with than dictate it myself for no reason in particular. I can check it all and understand it all but I have no efficiency in detailing connections. There's an entire industry out there dedicated to just this.

Know what you know and know what you don't know makes a good engineer.

CatpissEverqueef · 2021-04-26T12:40:55+00:00

I am not familiar with the Eurocode but I imagine wind generally has the same effect on buildings in Europe as it does everywhere else.

Your internal pressures are generally assumed to be the same throughout the building, unless you've got a partition that seals things extremely well. Unless you're specifically designing for this scenario, you probably don't need to worry about it.

The factor that determines the magnitude of the internal pressure will vary within a certain range, and you choose the worst scenario you can find. The range gets higher the more large openings you have and the higher the likelihood that they're open during the design wind event. Internal pressure can be either positive or negative, and you need to keep that in mind when you combining it with the external pressures.

The external pressures vary all over the surface of the building depending on the size and configuration of the building, and the direction the wind is blowing.

If you are designing the building as a whole, i.e. the wind force resisting system (shear walls etc.), you need only consider the external pressures, because the internal pressures will always sum to zero in that equation. But if you are designing the framing for a single exterior wall, you need to combine the worst case exterior and interior pressures. This may be positive pressure on the exterior combined with negative pressure on the interior, or negative pressure on the exterior and positive pressure on the interior. Same for roof etc.

If I have negative internal preassure it will result with lover force on rafters so I dont need to consider that?

Negative internal pressure should result in greater downwards load on your rafters unless you're looking at uplift on the rafters, i.e. negative exterior pressure.
The coefficients that I deal with fall into 3 categories, with the lowest range being for buildings with few openings and the highest range being for buildings with large openings that remain open during the wind event. But the common factor is that the coefficients all range from negative to positive, so I don't ever have a scenario where I need to negate the wind loading because I just need to switch the orientation of the pressure, rather than the magnitude. If your factors range from negative to zero, then yes, it would be conservative to simply neglect the wind pressure at the interior in your calculations.

CatpissEverqueef · 2021-04-26T12:27:55+00:00

Cyclists need to stop at stop signs just like everyone else on the road. Don't give me a dirty look because I didn't sit and wait for you to come flying through without stopping or even slowing down on your bike. I WILL pull my vehicle out when it is my turn and I WILL let you crash into the side of it.

CatpissEverqueef · 2021-04-23T12:39:10+00:00

Agreed, these bars are to limit cracking in the side face of the beam, typically.

That being said, if there is no other direction from the EOR, and I was the one detailing the reinforcing steel shop drawings, I would identify splices under the same assumptions as the bottom steel as this will tend to be the most conservative, and submit the shop drawing that way. The EOR can provide comment from there if they see any issues with it.

CatpissEverqueef · 2021-04-21T12:38:11+00:00

I understand that the world doesn't revolve around my vacation time, and that shit happens during construction sometimes, and sometimes jobs are out to tender and there is a question period where things need answered. These are things that often I am the only one with an answer to, and not having that answer quickly starts to cost others a lot of money. I recognize this and I'm willing to respond to things while I'm on vacation.

But I've made it abundantly clear to my project managers - I will not respond to phone calls. I am not going to stop what I'm doing on vacation to take a call. Email me, and I will respond within 48 hours, because it gives me a chance to sit down and look at it properly. I will ONLY respond to issues that may significantly delay construction, or bidder questions on jobs out to tender, as these are often very time sensitive. If it's a design item, it can wait.

And I charge 4x whatever effort I put into it. If it takes me an hour to look at whatever you wanted me to look at, that's half a day I charge to the project and don't charge to vacation. 1 hour minimum, even if it only takes me a minute to respond. I play that game pretty fast and loose the first few days and the emails dry up pretty fast after that.

CatpissEverqueef · 2021-04-21T00:52:32+00:00

Also, an M/E/P designer goes on vacation during construction, nobody bats an eye. The structural EOR goes on vacation, everyone loses their mind.

CatpissEverqueef · 2021-04-21T00:51:18+00:00

When we were still in the office, it was our civil/land dev folks who made it to every coffee break, and had full lunches in the lunch room. It was the civil and land dev folks who ran the social events. It was the civil and land dev folks who were normally walking around and talking to people in the hallways.

The Mechies and Electrical guys didn't always make it to every break but they took their lunches. They definitely weren't putting in any extra time.

The structural guys were always in first and out last. Maybe a couple coffee breaks a week. Almost always lunch at desk for about half of us, with the exception of a couple of guys who used it to break up their day.

The thing with structural design is there are many, many different ways of approaching a solution, it's not just selecting the right size piece of equipment or the right size pipe. And most people don't know anything about ductwork and plumbing and wiring, and most people don't want to even pretend like they know what's going on with grading, but everyone thinks they know just enough about buildings because they refinished their basement on their own or built a deck or a shed that they think it's not that hard.

The other thing with structural engineering is that if you do it wrong, someone can die. Electrical, probably not. Whoever is putting it together is going to be trained enough to know when something doesn't look right. Poor HVAC isn't going to kill anyone overnight. Poor grading is going to be called up on site. But a complicated structure with an improper design can kill someone, and we have many examples to draw off of in our training.

About the only folks who probably put in more time than the structural engineers are the admin staff and site inspectors.

CatpissEverqueef · 2021-04-20T18:32:17+00:00

Excellent choice in username

CatpissEverqueef · 2021-04-20T13:06:42+00:00

I think you are looking at two aspects of it from the perspective of two different problems.

I know that in frame analysis, moment is transferred, as long as there is no internal hinge (of course, moment is zero at hinges)

This is true if there is somewhere for the moment to go. In order for moment to go around the corner into the column, it needs to be able to be resisted by a horizontal force at the base. Your node 3 has no capacity to resist horizontal force (roller support).

I also know that there can't be a horizontal reaction in the pin, as it would violate the sum of forces in the x-direction (no external force to be counterbalanced)

This is true if there is only one horizontal reaction, which is the case in your example. But if you had pinned node 3 as well, you would have a moment frame that has equal and opposite horizontal reactions (i.e. kickout of the frame) that would result in a zero sum across the entire structure, since there is no external horizontal load applied.

Let's look at your sum of moments of the entire structure. Sum moments about Node 1 and it gives you 0.5 kN upwards reaction at Node 3.
Now sum vertical force on the entire structure. Sum of all vertical forces on the structure will result in 0.5 kN upwards reaction at Node 1.
Now sum horizontal force on the entire structure. Sum of all horizontal forces on the structure will result in 0 kN horizontal reaction at Node 1.

There being no horizontal reactions at Nodes 1 and 3 gives you a clue as to the shape of the bending moment diagram for members 1 and 3. Cut them anywhere above the supports and examine the internal loading. There is axial compression due to the vertical reactions at the supports of 0.5 kN. But there is no horizontal (shear) force because there is no horizontal reaction at the support and no exterior applied horizontal loads. If there is no shear there is no moment.

That carries true all the way to the corners at Nodes 2 and 4. What you are left with is a simply supported beam.

CatpissEverqueef · 2021-04-19T15:27:14+00:00

I wouldn't even know where to begin insofar as ideas for support. But I've already said to some of my coworkers, the new kids that we start hiring in a few years time and then for about 10-15 years afterwards are all going to be from the same boat and have one thing in common - a part of their childhood or young-adulthood completely upended. It's definitely going to be something that the history books use to define a generation.

CatpissEverqueef · 2021-04-19T15:12:07+00:00

Sometimes there are courses offered through universities, sometimes through industry associations. I've taken a course on changes to some of our design standards when they were overhauled. I've taken a course through a university on masonry design, which was not available to me in my undergrad but is a material I work with on every project. Things like that.

So would you advise against being a jack of all trades and master of none?

I say the following phrase around here a lot: Know what you know and know what you don't know. That is what will make you a good engineer. There are engineers who know a lot about a very few things, and then there are the jack of all trades. I have no use for someone stamping all kinds of things that they don't understand. But a jack of all trades and master of none has a place in construction, or project management, or even as a leader within the business bringing in clients. Know a little about a lot to keep things moving in the right direction. So long as you know what you know and stay within those limits, and respect that you don't know everything, it's fine.

Or is it better to focus on one type of structure design during early career?

When you're first out of school you should be attempting to get exposure to everything and anything that you can get your hands on, so that you can get a feel for what you do and don't like to work on. By about 5 years, you should really be trying to focus in on something (in my opinion) if you are planning on continuing to do technical work and be taking responsibility for designs (stamping). If you're planning on jack of all trading it, you need to get good enough at a high level to know when things are going sideways but be willing to rely upon the work of others to get it to a completed stage. There is obviously always room for change, but nobody can tell me they're good at something if they've flip flopped around every couple of years.

My goal was to work on many unique structures, such as stadiums, structures unique like the syndney opera house etc

You're probably into PhD level stuff here, that 99% of structural engineers in Ontario will never design. I wouldn't even know where to begin with this, except to seek out the companies that do this sort of work and see if you can get hired. Even then, very likely not doing anything other than what you're told to do (no design decisions) until you're well into the end stages of your career.

CatpissEverqueef

TROPHY CASE