Concrete plinth with hold down bolt design queries 1

[pizzalover]

Hi all,

I am a grad engineer and hoping to get some help with the design of a concrete pad footing as per AS3600 or a similar code. I attached a preliminary sketch to show the intended design.

need to design
- footing size (check overturning due to wind load and bearing capacity of the footing)
- footing reinforcement detail
- cast in hold down bolt (size/ embedment depth)

I found a literature review about concrete pedestal (link:

https://www.sefindia.org/forum/file...forcement_a_petrolium_project_papers_199.pdf)

but it may not be applicable to this case as there is no pedetsal here.
Also the bolt is cast-in. I used to use HILTI Profis to check the embedment depth of post-drilled anchor with epoxy. but I'm not sure the design process of cast in bolt. Can anyone kindly give some advice or have any relevant examples that i can refer to? Much appreciated.

Thanks.

 

[dik]

I'm not sure of the question. It looks like a common spread footing, the size determined by the bearing capacity of the soil. The thickness appears to be a bit robust and could be 300 or 400 thick, again depending on your bearing conditions. Unless you have uplift, I would think a single layer of reinforcing in the bottom in each direction, with about 75mm conc cover to the rebar if cast on soil.

You can use chemical anchors, but my preference would be to cast in anchor rods. These are anchors threaded at each end with a heavy hex nut applied. I prefer them to 'L' or 'J' type anchors. If you have heavy uplift (by tension or moment) the footing would have to be thicker to accommodate the length of the anchor rod embedment. It may have to be deeper to have the soil resist the uplift.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[phamENG]

pizzalover - I can't help you with specific code references because I don't practice in Australia. I will say, though, that your question(s?) is (are?) very concerning. I realize that as a grad engineer (it's June, so I'm guessing a very recent grad) you're not going to know everything, and most don't know how to find their office for the first few days. But if you've managed to get a job at a halfway decent company, they will also understand that.

These are the sorts of questions you should be taking to a mentor/senior engineer. They should be guiding you through these things. You may run into specific problems that they don't know - some interesting theory or piece of an equation that they've never stopped to consider that one of us is more familiar with. We'd love to help/explain! But "what's the procedure to design a simple footing" is a question for your office. You're doing yourself and the company you work for a disservice by not engaging them in your training. You need to develop those connections with the people you work with, and your superiors need to understand your limitations and the pace at which you are growing. If they don't, you (and they) could end up in some very uncomfortable situations.

 

[dik]

That's funny... I did when I graduated!

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Enable]

Well said phamENG! It also works the other way too. Superiors are not always good engineers and since mentoring is so important at this age it's imperative to ensure your mentor is a good one. When you start querying your superiors about what should be typical items and don't get satisfactory answers or their technical explanations really don't seem to make sense, you might want to revaluate the kind of mentorship you're getting and potentially look elsewhere. Sometimes technical answers wont make sense simply because they're complicated and you are fresh. But how to size a simple footing should not be one of those things (lets not hassle complicated / unknown soil conditions yadda yadda - those may make the design a little more nuanced but the explanation should still be relatively simple).

 

[milkshakelake]

@phamENG There's also merit to trying to design stuff yourself or trying not to look like you don't know basic stuff. Then you come to the seniors with your questions, and they might pull out a spreadsheet they already have for your situation, but at least you showed some basic knowledge.

About OP's question, I'd find a design guide on the subject. There are numerous ones here but I'm not sure about your code. The one I use is the Structural Engineering Reference Manual. You just follow the examples and get the hang of it. For the footing, design it for:
-Flexure (cantilever on both sides from edge of column or whatever your code says)
-One-way shear (standard beam shear applied at a distance d or something away from the column edge; this distance is based on your code, base plate layout, etc)
-Two-way shear (also known as punching shear)
-Bearing capacity (take axial load and divide it by the bearing area on soil)
-Concrete bearing stress (to see if the concrete can take the concentrated load of the base plate; different from two-way shear)
The distances used for shear and flexure away from the column edge depend on your code and how it treats the base plates.

For the anchor bolt, it gets a bit complicated but just follow the detailed calculations in the Hilti program along with your code. That's probably the best way instead of using a design guide. Last time I used it was years ago but it had an option for cast-in-place. If it doesn't, there's another similar free program by Powers that can do it, though I don't know if it supports your code. I also don't know if that one still exists; last time I used it was a decade ago.

 

[phamENG]

Enable - thanks. And you're spot on about it being a two way street. If you're not getting the instruction you need, you should look elsewhere for employment. By the way, since the weather is getting warm, are you getting out on the water yet? (Not sure how "warm" it is where you are.) I picked up an old Catalina 22 last fall - new rudder arrives in a week or so, and I can't wait to get her off the trailer.

milkshakelake - I agree on the concept, but not on the context. There's still a big difference between figuring out at least part of it yourself and going to somebody else to tell you how to do it before you go to your boss. The first one will a) give you better insight into what documents you have at your disposal, b) experience reading through them and getting a feel for how to use them, and c) give your boss/supervisor insight into how much you know and how much you can figure out on your own. It seems to me that asking somebody outside of your chain to give you a kick start and tell you how to do it robs both you and your mentor/trainer/whatever of some valuable opportunities for growth and assessment.

This forum is great - both for senior engineers to discuss complex/uncommon topics and for junior engineers to come to seek help. But I'm opposed to "how do you design A" sort of questions. In this case, the OP did go look for some documentation and tried to play with Hilti. That's good. BUT...most of the questions were the sort of thing that get covered in an undergrad soils class and undergrad concrete design class. At the very least there should be a: "I've never done this before, but I tried doing it like this example in my textbook but x, y, and z don't seem right." If a junior/grad engineer can't try a problem using their textbooks, I think we're doing the senior engineers at their company a disservice by helping them cover it up.

 

[milkshakelake]

@phamENG I guess I agree to differ. The problem seems simple to me, but to someone starting out, it can be head-scratching to figure out where to start or what to Google. I tried to put in some key words that are Googleable. It could also be embarrassing asking a senior something that seems basic, almost trivial, but having no idea how to begin tackling it.

 

[human909]

Embarrassing shouldn't be a reason for an engineer (or anyone) to avoid asking questions. There is no such thing as a stupid question.

However as pham pointed out this question is pretty trivial and should be recognised as such. There are plenty of design guides and text books on the matter. This is where pizzalover needs to look.

For what it is worth there are plenty of 'pretty trivial' problems in structural engineering that I'm not adept with. I'm specialised more in steel, my concrete and timber need work! But I just don't go asking ENG-TIPS the basics. I do my own research!

(it's June, so I'm guessing a very recent grad)

BTW. Fresh grads don't normally start in June in Australia, we are in the southern hemisphere. Also he was posting last year.

I'd say he just needs to ask colleagues or do his own research.

 

[milkshakelake]

@human909 I get what you and phamENG are saying for sure. I just have a different way of looking at it. I have sympathy for people with basic questions. And I don't know their motivation for not going to a senior engineer, so I assume there's a valid reason.

Story time. I was trying to figure out if my server storage transfer speed was enough for my office. I got into the weeds of SAS vs SATA cables, 6 Gbps vs 12 Gbps RAID cards, etc. After researching and pulling out my hair for hours, I posted in a sysadmin forum and I got like 100 comments saying how stupid my question was, and how I should hire someone to do this for me because I don't know the first thing about IT. Even my brother, a sysadmin, said that my setup can serve 5 people or 3000 people depending on the use case, so that wasn't helpful. One person DM'd me, asked questions about what kind of stuff my office does, and recommended what to buy and how to set it up. I'm eternally grateful to that one person, because the server is working fantastically and I just couldn't solve a trivial thing on my own.

In conclusion, I give people the benefit of the doubt with trivial questions because I've been there myself. I know how hard it can be to just get started sometimes. I assume they've done at least some work and hit an intellectual roadblock, not a laziness one. I also don't expect people to be fantastic, independent, DIY machines of engineering because I see junior and senior engineers get tripped up on basics. Though OP posting before, not replying, and posting again is new information to me, and I wouldn't have responded if I knew that before.

 

[human909]

@human909 I get what you and phamENG are saying for sure. I just have a different way of looking at it. I have sympathy for people with basic questions. And I don't know their motivation for not going to a senior engineer, so I assume there's a valid reason.

I have complete sympathy for people asking ignorant questions. I have less sympathy for people asking extremely basic questions that are at the very CORE of a topic and have a wealth of readily searchable resources online and in hardcopy textbook. I believe this is one of them.

Story time.
I failed probably 33% of the units I studied during my engineering course and came very close to being kicked out of my degree for poor performance. I never worked in a firm where there is another structural engineer to mentor me. So I ABSOLUTELY understand how difficult it can be and how lost you can feel even on some of the most basic topics. However the quality of the posts in this forums would quickly deteriorate if ever 2nd student or junior engineer posted basic questions here. They should join student forums or do their own research.

My above comments may call into question my competency, though I'll let my posts speak for themselves regarding my knowledge or lack there of.

 

[Retrograde]

Do you have access to a copy of "Design of Portal Frame Buildings" by Woolcock et al? It discusses hold down bolt design in some detail.

It is also covered in the "Design of Structural Connections" by Hogan & Thomas.

Both are published by the Australian Steel Institute.

 

[pizzalover]

Thank you all for your valuable input and concerns. Yes, this is indeed a basic question, and unfortunately, I have discovered a gap between what I learned in uni and what I encounter at work. I learned the fundamentals of concrete/ steel/ footing design. However, when the situation deviates slightly, I get a bit lost. It may be a basic question, but lacking confidence can make it challenging. And knowing that this detail is simple, I was hoping there would be some examples available in order to fully understand the design. When I ask my colleagues for guidance, they often just suggest to use a similar detail from another project.
Also they advised that i can design it as per section 15 plain concrete pedestal and footings AS3600.
Appreciate the helpful assistance regarding relevant handbooks and materials, and I will definitely look into them.

 

[phamENG]

When I ask my colleagues for guidance, they often just suggest to use a similar detail from another project.

When you say colleagues, are you talking about peers only, or do you include supervisors in that mix? If your supervisors are unwilling to help you understand the basics, you should really start looking around for someplace more suitable for a grad engineer. At this stage, you should be soaking up everything like a sponge, and it only lasts a few years. If they aren't going to facilitate that, you need somebody/someplace that will before it's too late.

Most problems you're going to face will fall into one of two groups: complex things that are made up of several small, easy steps; and difficult things that can't be broken down. 99.9% of the issues I deal with fall into the first category. The trick is dismantling the complexity to see the small and easy steps that you already know how to do. Remember, engineering problems aren't things that you hunt for an equation to throw at, plug in, and solve. They are physical things that you have to understand first, and then apply math to in order to get the answer.

For footing overturning: I could throw out a stock diagram and some standard equations, but that won't really help you in the long run. Think of that footing as any old blob in your statics book. You have loads applied to it, it has a self weight, and you have a reaction from the soil. The reaction is a little tricky, but if you check your soils/foundation design textbooks you should find some information to help you with the starting assumptions to get that dialed in. You're going to adjust the bearing pressure until your basic statics equations are balanced.

Reinforcing and anchor bolts are harder for me to help with as there are more code nuances involved there, and I don't know the first thing about Australian codes (or the Australian higher educational system - thanks, human909!)

Confidence will come with time and experience. But you need guidance from a mentor in the industry. I struggled with it as well (and still do - sleepless nights are not uncommon on really tough projects). My first boss had what I considered to be a misplaced over confidence in my abilities. So I would make up dumb questions or purposely put some glaring mistake in something to a) scare him into teaching me something and b) make sure he was actually reviewing my work. I'm not saying you should do the same thing - engineering wasn't my first career and I had some experience dealing with similar people, so I knew how to....produce the desired outcomes without negative repercussions for myself. Be careful if you go that route as it could backfire horribly.

Good luck!

 

[milkshakelake]

@human909 I get where you're coming from about the quality of the forum being flooded with low effort posts.

About your story time, I barely passed engineering college myself. I partied, sold illicit substances, and hit on girls instead of studying. Even went to jail for one of the aforementioned things. Not to have a race with you to the bottom, but it's funny how real life changes people. Working a real engineering job sobered me up. I also didn't have a mentor. I guess that's why I'm very sympathetic to people asking trivial questions, and I fully see your side of it too.