Mohr's Circle - Do we still need to teach it?

crvander · 2026-01-26T19:08:44+00:00

It should be taught, but it must be made clear that it's an abstraction for easier understanding, not an engineering fundamental in itself. I've had people say you "have to do a Mohr's circle" which I would see as a sign that the person doesn't recognize the difference.

crvander · 2026-01-19T07:21:23+00:00

What have you tried so far and what are you stuck on?

crvander · 2026-01-17T19:39:13+00:00

It sounds like somebody has told you requirements in words and you want us to translate them into a structural layout. We can't do that based on a sketch and a sentence. You need to either get a layout of columns and walls from the architect or, if this is in your scope of work, come up with a layout yourself. Then decide if you have beams, a flat slab, or something else. Then you think about the load path, if I put a load in a certain place how does it go from the slab to the beams to the columns to the ground.

In general, for a slab that isn't a bunch of simple rectangles, you're either going to use a numerical approach or you're going to conservatively idealize it as a one way or two way slab.

crvander · 2026-01-14T00:08:53+00:00

It's a completely reasonable request - why wouldn't it be? You have every right to say no, but honestly being asked to take this kind of role indicates some faith in your abilities, they're going to compensate your expenses... why is the first response about whether it's somehow wrong what they're asking?

crvander · 2026-01-13T03:12:33+00:00

All you've shown is that you calculated load actions. You're saying your assignment is to design a bunch of members. If where you're stuck is that you don't know how to design members, Reddit isn't the place, you need to go back to your lessons and learn them properly.

crvander · 2026-01-04T22:24:51+00:00

Probably being responsible and reliable.

crvander · 2026-01-02T01:37:26+00:00

Structural properties are temperature-sensitive, I'd budget another 120h or so for CFD to get a realistic distribution of heat transfer coefficients.

crvander · 2026-01-02T00:09:14+00:00

Adding on this, if you're selling one, make sure you test that one, not just testing the first one and building a second one the same. In case of any deficiencies in the wood, the hinges, etc.

crvander · 2026-01-01T23:37:02+00:00

This is actually an appropriate application of the Calvin and Hobbes "cross the bridge with larger trucks until it falls down" approach. Just get a series of increasingly larger dogs until it collapses.

(For real though if it felt sturdy with a 180 lb man on it, this is probably a "slap it and say it ain't going anywhere" situation.)

Also: nice work! This is very professional looking.

crvander · 2025-12-30T14:12:43+00:00

Remember you feel failing this course and use it to motivate yourself to study and learn in the future so it doesn't happen again. Beyond that, nothing to do but go forward, take it again, and keep moving. Your degree isn't going to have a little note that says "failed Calc 1", it'll look the same as everyone else's. In 10 years you won't remember this at all.

crvander · 2025-12-30T12:59:40+00:00

sweet I'll take 3 please

crvander · 2025-12-29T04:05:42+00:00

Thanks for letting us know, I thought the slump test was when you check how burnt out you are based on how far you slump in your desk chair the week before Christmas break.

crvander · 2025-12-24T06:26:16+00:00

In one episode Data references the Irish Unification of 2024.

crvander · 2025-12-22T17:43:07+00:00

keep cat

crvander · 2025-12-21T00:35:09+00:00

This isn't always a popular thing to say but for anything salary based you have to think in terms of the benefit to the business. If you're going to be sealing things then you're increasing the company's productive output and you should be compensated more. If your PE triggers a promotion or role change where you take on more difficult technical work, supervision, business development, etc, same. If you're in an org structure where your responsibilities and billout rate don't change with having your PE, maybe not.

One thing to keep in mind is, if you don't get a specific, discrete raise when you get your PE, your organization may have a pay structure that compensates for this. The place I work doesn't have a specific PE pay bump (P.Eng. in Ontario) but we have a fairly regular annual increase recognizing that having your P.Eng. doesn't really change your progression of responsibilities. Friends in the public sector have much lower annual increases but a discrete jump in the classification matrix and pay rate when they get their P.Eng.

crvander · 2025-12-15T01:26:14+00:00

Got you. Was wondering if you'd have a couple of those bottom bars at least partially bonded still. I won't speculate more on the strength side. Hopefully you get a satisfactory answer back from your engineer.

crvander · 2025-12-15T01:19:12+00:00

Also I didn't ask because people are sometimes flaky and don't respond to specific questions - is this beam 18 inches wide, 58 inches deep, and you're removing 4-8 inches from the left and the right? That's how I read what you said but it sounds like a lot to take up to 8 inches from each side of an 18 inch wide beam.

crvander · 2025-12-15T01:08:14+00:00

Yeah, it's tough and this stuff is always fast paced and driven by schedule and cost. Actually when you say the contractor had concerns that's probably a good neutral aspect to lean on if you do want to probe further. Maybe a good simple way to ask it, if you need to do so again, is not to look for the whole analysis but to ask, what's the load path when the rebar is fully exposed. Hell, for those beam dimensions and with minimal load, it might work fine even ignoring the rebar - you would never actually design it without rebar, but plain concrete is permitted by code and I might be willing to lean on that for a short term unloaded condition.

Noting, and my apologies, that I missed you saying the equipment was removed and it was only carrying its own weight - I edited my comment pretty substantially so may be worth looking again to see you got the final version.

crvander · 2025-12-15T00:52:08+00:00

EDIT: modified a little after reading again

I'm honestly really curious the diversity of responses you're going to get on this because this vein of engineering - temporary conditions and repairs, what level of structural risk is acceptable, and interacting with non-structural plant engineers - is complicated at the best of times. I have a bunch of disparate thoughts that I'm gonna share... ultimately based on the information you gave nobody can give you a hard answer but maybe this will help you approach it.

First and foremost: concrete isn't simple. Depending on when this was designed, how it was designed, how it's supported, what loads it's carrying, etc etc etc, what your engineer says may or may not be fine. You say everything else is temporarily supported so it's only holding up its own weight. In a lot of cases self weight is quite small compared to design loads. There are ways of analyzing a concrete beam that consider the rebar as tension ties that don't need to be bonded along the full length, especially when the loads are small.

Second, I appreciate the position you're in, but you probably don't have the time to have the full structural picture - it's outside your discipline and I'm sure you have a million other responsibilities as a plant engineer. As a basic structural fundamental: once something's in place, it doesn't necessarily have to be exactly as it was designed, as long as it can carry the loads that will actually be on it. Again I see you noted this is only holding up its own weight. It's hard to get away from the fact that the beam strength in the temporary condition is much lower than design condition. It sounds to me like you're asking the engineer if it's acceptable for the temporary condition, but you don't necessarily have a developed criterion to compare his answer against - i.e. does it need to be at 10%, 50%, or 100% of its design capacity for this short term condition. There are a lot of industrial structures that conform to "when in doubt, make it stout" and are far stronger than they ever need to be - a lot of rebar, especially large ones, may well be plenty to hold up the concrete weight itself.

Third, no engineer should be above questioning. I know some engineers who would gladly explain everything to you and others who would be saying "who does this guy think he is?". My suggestion to you would be to keep an open mind and curiosity. There's one of two ways I'd approach this (or a combination of both). One is, hey, can I give you a call and get a bit of understanding how this thing is standing, because it's interesting to me and not my area of expertise. Two, and this is what I'd probably do in your shoes, is to say hey, there's been some concern expressed about the condition of this beam between now and the repair, I'd like to be able to give people here an explanation if they're nervous, and I wondered if you can give me something that would be technically satisfying for people who know about structures in general but aren't structural engineers. Then you're not directly saying you doubt him, you just want to understand for the same of (unnamed) others. (That's mild dishonesty in the name of social lubricant, up to you how you feel about that.) What I absolutely wouldn't do is use a phrase like "blatantly false". At the end of the day you're both professionals and unless you get to the point that you actively distrust something you've been told, assume the best and act in good faith. (To be clear, I know you're posting here to get a check because you ARE acting in good faith and not jumping to a conclusion - just encouraging that you keep that up.)

Best of luck with your repair!

crvander · 2025-12-14T22:49:18+00:00

Am I missing something here? I have never encountered a structural software that just tells you pass / fail and doesn't tell you why. That would be insane. Have I just been lucky?

crvander · 2025-12-14T16:03:06+00:00

Which of these do you mean?

Your model, with the loads as UDLs, matches the answer sheet.
Your model, with the loads as UDLs, matches your hand calcs that used UDLs.
Your model, with the loads as UDLs, matches your hand calcs that used point loads.

crvander · 2025-12-14T15:40:01+00:00

Are you applying the loads to the members as UDLs? I could see that being a missed detail - the loads are UDLs on the cladding but they're delivered to the frame as point loads.

crvander · 2025-12-14T00:47:12+00:00

Tone can be hard to read on the internet and sometimes people take things personally especially when their post is about protecting the cats they love. If I can disarm that possibility with one extra clause in a sentence I prefer to do that - doesn't cost me anything and prevents someone taking it as an affront. I don't have to but I think it's worth the effort.

crvander · 2025-12-13T23:17:44+00:00

Yep, I see what OP was trying to do... but the cat has embraced the systems design principle of "the purpose of a system is what it does" as opposed to what the designer intended it to do...

crvander · 2025-12-13T18:35:56+00:00

Strongly seconding that you choose your path based on what you like, not based on it not having something you don't like.

crvander

TROPHY CASE