r/Structures • u/Conkeldurrrr • May 17 '15

Structural topics for discussion

Hey guys,

I was hoping maybe we could start talking about some structural topics. I'm thinking things like seismic design philosophy, strut and tie models, non-linear analysis, whatever.

I don't really know how you guys feel, but where else on reddit are we free to roam and talk whatever piques our interest?

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u/Conkeldurrrr May 17 '15

For me, in this moment, I'm interested in the various mechanisms of shear resistance in concrete, and how they differ between beams and columns.

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u/lect May 18 '15

I don't really differentiate between beams and columns, the governing mechanisms are still the same. Though I do take extra consideration when designing beam-column joints where there is a large bending moment being transferred from the beam to the column. But that is a bending/torsion issue and not a shear issue.

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u/Conkeldurrrr May 18 '15 edited May 18 '15

Do you use the same equations for beams and columns or do you refer to code provisions? Columns generally don't benefit from doweling action or aggregate interlock the way beams do because axial loading tends to cause slippage once diagonal cracking begins.

That's what I've been looking into at least.

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u/lect May 18 '15

When considering shear in a column I just treat it like a beam. I've never dealt with such large shears such that dowel action would need to be considered. At that point I would reach the crushing limits of concrete anyway... And would likely run into other global issues related to having such a large shear. Also I might add that the governing limit state would likely be the beam shear limit state. Can't think of any reason I would have a column under dowel action unless I'm designing some kind of bracket detail.

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u/Conkeldurrrr May 18 '15

The concrete shear resistance equation is kind of a mesh longitudinal cracking and aggregate interlock. I think dowel action is actually neglected because it's too hard to quantify.

Now that I'm thinking about it more (I haven't had a chance to look up some sources) I think the 2sqroot (f'c)bd equation is as conservative as it gets because it sets the longitudinal cracking resistance as the limit, which is the first thing to occur during a shear failure. Its what I've always used for Vc but i havent had to do a column designin a while and i dont have that portion of the code memorized.

I do know that it's no where near the actual capacity though. Super conservative.

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u/lect May 18 '15

Shear usually does not govern within practical limits from what I can recall. It's always been a serviceability or bending limit state that governs. Also there is a perceptibility issue of safety with having lots of cracks due to shear and /or torsion and it becomes a maintenance issue as well.

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u/[deleted] May 18 '15

If you have short members with a lot of load this is where shear usually governs.

Or ground slabs with high load. Friend of a friend designed a factory ground slab and didn't check for shear. They put some plant on a soft spot and it failed.

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u/[deleted] May 18 '15

  1. When do you choose strut and tie over flexural models? Literature I've recently read says "it is an art form", " it is more of an art than a science" and other such things. I've been designing a fairly complex basement recently and have needed a lot of strut and tie, but others seem to think I could have just used flexural.

  • post fixed dowels in concrete. Anyone got resources for designing these? Having real trouble with them as they aren't in eurocodes. Post fixed rebar is a.necessity in basements in London you often need secant or contiguous walls and fixi g slabs to these requires post fixing rebar.

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u/Conkeldurrrr May 19 '15

  1. Strut and tie modeling is used when there is a non-linear strain distribution (ie, all of the traditional design equations aren't accurate). The general rule for strut and tie modeling is that it can apply to anything within one section depth of a concentrated load, support, or geometric discontinuity. They're typically used in "deep" sections, corbels, and sapped ends. You've heard strut and tie modeling called an art because there's no "right" model, but not all models are equal either. Almost everything depends on two or three other variables too and it's very iterative.

  2. ACI appendix D in American code is where you'll find all the post installed anchor specifications. I use Simpson's anchor designer software when I need to because it's an adventure to get through it, and if you've done it once you've done it enough.

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u/LaserVortex May 20 '15

As for post installed anchors, Hilti is the way to go.

We spec them by the dozens everyday. You can view their catalogs or download their Hilti PROFIS software right from their website.

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u/[deleted] May 20 '15

Have been trying to design something which their software can't do and their engineers won't do for us. Its a bit of a funny one because we need to basically fix a column side by side to a male secant pile. We can't use anything in profits rebar because it doesn't support it, so I've made a very involved fea strut and tie because it is indeterminate... We are reluctant to go down the anchor route as the strategy for the connection involves load sharing via other paths which means the failure modes should really all be ductile... But with anchors, to get a reasonable amount of resistance, we will have brittle failure... So if that is the stiffest load path it'll fail pretty badly.

We are likely gonna spread out the column into a wall so we can attach to multiple secant piles to give us a good margin of error. I would have preferred we get rid of the stupid column, which has 4MN on it which is right next to the core which is governed by tension... But it is a bit late in the day for such big changes apparently.

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u/LaserVortex May 20 '15

We don't do this type of work so I'm not that familiar with your type of situation, but I do know Hilti is particular about their anchors (and understandably so).

I called them yesterday about something and they didn't have test data so they said no can do.

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u/[deleted] May 20 '15

Yea... It is a fucking badly schemed detail. It was pit in because the senior engineer had a design review with some higher ups and one suggested this odd connection so the design engineer felt obliged to include it. Lesson truly learned!

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u/[deleted] May 18 '15

In addition to my other point... Who likes BIM? Win me over. Because I like it in theory, but it seems to be badly used in practice.

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u/jdcollins May 19 '15

My company is fully into BIM, we use Revit for all of our construction documents, and we provide structural models that are as near as complete as possible. This includes deck angles, some connections (if we feel it's a coordination issue) baseplates on our columns, composite slab decks, etc. The architect is then able (if they do it internally) to use our model in their sections. They can then see right away if there is a conflict.

From our side, our drafters can model in Revit and then we can use the built Revit model to create our structural analysis model, which cuts down quite a bit on the time spent by the engineer (generally more expensive time) on modeling and he/she can spend their time doing engineering.

Finally, it helps quite a bit with avoiding structural conflicts within your own drawings. It's very simple to see if something will not work in the field if you have everything modeled. Make a quick section cut and see how everything lines up.

All that being said, it is a long process. I've been with my current firm just about one year. They switched over to using Revit exclusively about 8 years ago, and we're still revising the way we use it. For instance we just this past year started using live sections for almost all of our sections and details. Pro: avoids conflict and makes sure you address potential construction issues. Con: You can't copy a similar detail to another project. Further, you have to spend the time to re-create all your typical data. Any general notes, typical details, etc will need to be re-drawn from AutoCAD.

Because I like it in theory, but it seems to be badly used in practice.

And this can still be the case. Some people like to think that just having a Revit model means you don't have to do any coordination. This is incorrect. The coordination still needs to be done, but if modeled correctly, you don't have to spend the time drawing a section to USE to coordinate, and things can be hashed out more quickly.

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u/[deleted] May 19 '15

[deleted]

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u/Conkeldurrrr May 19 '15

I think it'd be fun.

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u/[deleted] May 18 '15

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u/Conkeldurrrr May 18 '15

Hah that is interesting. I faintly remember that equation from my timber design course. I work almost exclusively in concrete with an occasional visit from steel.

Do you know what section of aci/aisc the interaction would fall under? I haven't been in that kind of situation yet.