Strut-and-Tie Models 2

[UcfSE]

I studied the strut-and-tie model recently this semester using it for deep beams and corbel/bracket design as well as seismic design of joints in special moment frames after a method in a book by Paulay and Priestly (spelling?). The method seems pretty logical but also takes quite a bit of work to really do a good job and do it right. How many of you out there use strut-and-tie models in practice? Do you find you get better designs, i.e. more economical use of materials over more tradional methods? What are your opinions on the design code itself? Do you have any advice for those of us who would like to become more proficient at this, such as pitfalls or areas that aren't very clear ro codified well? At my company we don't encounter situations much that would require us to use strut-and-tie methods so I have no practical experience or even opinions on their use and how well they perform outside of academia. I appreciate any responses,

 

[UcfSE]

I take it no one uses strut-and-tie models as in ACI 318 Appendix A, unless there is a baser reason for why no one has responded.

 

[haynewp]

Do you not do many pile cap designs? I think it can be unconservative to not do strut and tie models when it is appropriate.

 

[UcfSE]

I haven't had the pleasure of pile cap designs as of yet.

 

[haynewp]

Are you in graduate school and working as an engineer at the same time like I am?

 

[UcfSE]

Yes. I took a year off after I got my BS. Currently I work full time as a structural engineer and work on my MS part time. My boss likes things easy and mediocre so I haven't seen anything like pile caps lol. I was just wondering about opinions about the "new" method versus old, old being previous methods of deep beam design, pile caps, corbels and brackets, beam-column joints in seismic regions, and so forth.

 

[mascottse]

The confinement for the strut and tie approach is closely scrutinized on the DOE project I am currently working on. It is a sticky issue with our oversight reviewers where we transition vertical rebar where the shear walls step down in thickness. The importance of the structure and the magnitude of the demand (high seismic zones/regions) are important to consider in these regions to maintain the ductility of the structure and confinement of the vertical reinforcing. If the confinement cannot be maintained, you may end up with a thinner shear area below the transition to resist the inplane shear nad a reduced effective beam depth to handle the moment introduced at the wall face by the intersecting floor. The practice does have its champions.

 

[UcfSE]

BUMP....


Anyone? Bueller? Bueller?

 

[MikeT14]

I'm in the same boat as you UcfSE!

I'm trying to design a deep beam and the only real option given in the code is Strut-and-Tie. I couldn't find a good example that was easy to impliment in the time I have to work on this, so it's now become a "in my spare time" project to learn STM.

I ended up using the method given by Park and Pauley (ACI reference 10.22) since it was the method most fimiliar to one of the members of my firm. Nobody hear uses STM or even knows were to start.

 

[JedClampett]

I went to an ACI 318-02 Seminar where they spent some time going over strut and tie. It seemed as if it was something they were trying to ram down everyone's throat (for a throat ramming example, see LRFD). Plus when I asked them to do a more applicable example (a corbel) the instructor laughed at me and said, "well, we can't do every example you might run into".
If your bound and determined to do strut and tie, try the PCA "Notes on 318-02..", chapter 17.

 

[UcfSE]

Thanks for the replies. All I was really looking for was some opinions on the method versus previous methods of deep beam and corbel/bracket design, and others. The method does address drawbacks in the older methods of design but it's also harder to use imho. I was curious as to how it has worked out for anyone using strut-and-tie models. Do you get better designs or do you just do more work, that kind of stuff. I do have some texts with examples as well as the PCA notes and class notes, but those don't address how well the method performs "in real life" so to speak.

 

[WillisV]

In my opinion, the method for deep beams provided in the appendix of the ACI code is cumbersome, overly-conservative, and needs a great deal of refining to be a viable method of design. The method by Park and Pauley mentioned previously has been used for years and has yieled no failures I am aware of. For deep beams the ACI method tries to get you to go through each and every node, nodal face, etc. checking stresses blah blah blah. In reality I have found that if you provide enough steel for the tension tie within a reasonable depth at the bottom portion of the beam and bearing is adequate at each end support then in most cases you are fine.

 

[DanTex]

Strut & Tie for bridge design in the US is becomming more and more common. It is being pushed by AASHTO. Most bridge engineers don't use it too much "yet", but... I expect it will be used more and more over the next 5 to 10 yrs.

Dan

http://www.dtware.com

 

[thothduke]

As an individual involved with ACI, I can tell you that the research cited at many ACI committee meetings, has indicated the current methods of dealing with shear in deep beams, footings, etc is unconservative. Strut and Tie has been used in other codes/countries for some time. S & T is touted as the more accurate method. It is only a matter of time before Appendix A becomes incorporated into the body of the code. It will not be long.

ACI does have a really great publication SP208, Strut and Tie models which is nothing but practical examples. If you want to see examples and how they are worked, this is the book.

I do not agree that it seems cumbersome at first. BUt if you see practical examples and work them a little bit, they do get easier. It will be a learning curve for all structural engineers.

 

[14159]

I'm kinda like you -- an engineer and a student!

I designed a lot of heavily-loaded and complicated concrete, all buildings, before I came back to school. Our firm (a pretty large and very reputable one) never, ever, ever used STM for anything.

Last semester, I had advanced concrete and we spent a good while on STM and I found it to be just about worthless from my perspective. I think it's really handy and useful for big-time bridge people, but I doubt that many building designers use it.

Structural engineer is a tremendously wide and deep discipline with many other more useful topics to study, in my opinion. Examples would be computer programming, technical writing, publishing a paper, basket weaving, etc.

DBD

 

[dbuzz]

The strut and tie truss analogy is used extensively in Australia for the design of deep beams and column corbels and can also be applied to design of beams in torsion.

There's nothing difficult about it other than, perhaps, that it's a new concept to many of you.

 

[astructurale]

thothduke - Thank you for the reference for design examples. I did my first deep beam design by STM not too long ago. I did find it to be difficult at first. I think the most confusing part about it is that there seems to be several correct ways to design one beam using STM, dependent on how you size and place the struts and ties. Other more experienced engineers I've spoken with on the topic also agree that there is a lot of room for assumptions when doing STM to help simplify things, but for others it just makes it more confusing when you don't know what assumptions to make. I am definately going to pick up the reference book becuase I would like to have a better understanding of how to use the basic principals of STM.

Does ACI 318-05 elaborate any more on the topic than 02 does?

 

[WillisV]

ACI 318-05 only makes minor changes - nothing groundbreaking.

 

[BoydZander]

I'm teaching myself use of ACI 318-02 Apprendix A Strut-and-tie models. I'm more familiar with designning deep bemas per ACI 318-99 methodology which was specific on beam shear strength and the shear strength of added horizontal and vertical steel.

Commentary states that for a B region, to follw traditional shear design procedures from 11.1 through 11.5. However, if the entire beam is such that No B regions exist, the entire element is a D region, how does one handle shear design? Is consideration of shear inherent in the design of struts and ties?

I'll take any responses at boydzander@aol.com if anyone is interested in responding that way.

Thanks

Boyd

 

[jc77]

it's the first time i've used this website, i'm a british structural engineer with 3years experience since graduating. anyway, no-one's interested in that.... but i am interested in this thread (and the whole idea of this engineering forum really - i'm not aware of a similar discussion group in the UK?)

i find the discussion on this topic very interesting. i have no idea what the codes referenced are or of the design procedures you refer to - but i do use the 'strut & tie' method fairly regularly in concrete pile cap design (we/i call it the 'truss analogy' by the way)... before i waffle on, i'd like to make sure we're talking about the same things so will someone please answer the following before i write further on this topic as i might be completely mistaken:

are your (American) designs (of pile caps and deep beams) primarily driven by seismic requirements? Because mine isn't. We did a bit of earthquake stuff at college but generally there is no requirement to cater for such scenarios (with the exception of some more sensitive structures) in Britain. (Yet!)

if the forces you design for are primarily gravity induced, then maybe we can expand a little on this topic and share some trans-atlantic knowledge?!?!?!?!?! :+)