Strut and Tie Help

[dlawrence529]

I'm trying to develop a strut and tie model for a beam connecting an overhead sign structure to a pair of drilled shafts. I'm having trouble developing the truss for the analysis. I've been following ACI publication SP-208 and SP-273 (Examples of Strut and Tie Models), AASHTO LRFD7, and Design of Reinforced Concrete by McCormac and Brown. All these publications show examples using the general truss configuration shown in the attached file. Is it correct that this truss is considered unstable? If so, how can it be used for design?

All the examples show symmetric applied loads, so the forces can be easily calculated. My problem does not have symmetric applied loads, so I am having trouble resolving the truss member forces. I have a few questions:

-Has it been so long since I have analyzed a truss that I'm completely out in left field?
-Does anyone have a recommendation for a truss configuration for my problem?
-Would it be appropriate to consider two separate analyses for my problem and use the more conservative rebar? This would use two symmetric loading cases(F1=F2=-50kips and F1=F2=120kips)
-Are the applied loads so close to the interior face of the drilled shafts that I don't need to consider a strut and tie model at all?

 

[dik]

Take a peek at the CAST program mentioned... only looked at it for an hour, but, it seems to be an extremely useful program and can easily take care of your model.

Dik

 

[dlawrence529]

Thanks Dik. I was looking through the pdf with examples that you posted. It looks like on page 59, I am still required to modify my truss with "stabilizer" elements to avoid an instability error message. Do you have any experience with this?

 

[TehMightyEngineer]

You need a node on the edge of the concrete shape with a horizontal support and a dummy strut connecting that edge node into your truss. Took me a bit to figure out the arrangement to make this work in CAST. Still learning it myself.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[dik]

TME is on the mark... not an expert with the program... light use only and for 1/2 hour to 1 hour... Appears to be extremely useful.

Dik

 

[dlawrence529]

Thanks TME. I'm still struggling with this. I modified my model to include edge nodes with supports, but I still can't get the model to run. Right now I'm at this:

Is this what you mean? If not, could you provide a quick sketch?

 

[TehMightyEngineer]

I had to add in another diagonal to your truss to make it all add up correctly. Don't like this arrangement as it's not how the rebar would be laid out but I gotta get back to my own work so I'll have to mess around with this later to get the truss rearranged.

Let me know what you come up with as we're all apparently still learning this program and I'm no expert on strut and tie to begin with. I was going to attach my cast file but the darn thing got corrupted. Apparently don't save it in the Program X86 folder as it wont actually save it without setting CAST to have admin privileges.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[dlawrence529]

Thanks TME. I found some examples in the C\Program Files\Cast\Examples folder. I copied the truss from the deep beam example. I'm still working out the strut/tie types and node regions.

 

[Guest262653]

I haven't used CAST in a long time but I'd use a different model, orienting the ties according to the reinforcement position. Please see attached (sorry for the lousy sketch...). If the centre diagonal strut angle is too shallow (e.g. less than 35º), I'd break it up into a beam model, placing an additional central tie.

For the analysis of STM models I tendo to use Fachwerk, which also has a version for 3d models (although it doesn't check the nodes).
The link is the following:

http://fachwerk.sourceforge.net/index_en.html

 

[TehMightyEngineer]

Yeah, that's the arrangement of the truss that I was thinking of avscorreia. Arrange the vertical tie as you would the actual rebar.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[sticksandtriangles]

Any good link to download the CAST program? Some of the websites I found look sketchy to say the least.

Thanks,
S&T

 

[TehMightyEngineer]

Look in the other topic.

http://www.eng-tips.com/viewthread.cfm?qid=427228

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[dlawrence529]

If you use the Wayback Machine, you can download directly from Dan Kuchma's old website

https://web.archive.org/web/20090903064452/http://dankuchma.com:80/

 

[dlawrence529]

TME can you explain what you mean by "Arrange the vertical tie as you would the actual rebar"?

 

[TehMightyEngineer]

See the sketch in avscorreia's post. They have the tension tie vertical under the tension load. You're not likely to want to have sloped tension rebar for that tie. With vertical stirrups you can use typical constructions details.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[Lim Justin]

I have created some types of strut tie mdoel.