Background of Strut and Tie Method

[Brandon]

What are the reasons for using a strut and tie model for deep beams? Will traditional design methods for flexure (Whitney stress block) and shear yield unconservative results for deep beams? Or does the strut and tie model just produce more economical designs?

I have never used strut and tie, but have a current project where a structure meets the deep beam criteria, and I'm trying to decide if it is worth the extra time to learn strut and tie to model it. Using tradional flexure design methods, the flexural steel requirements are very low.

 

[csd72]

Once you learn it, it is actually easier than flexural design.

http://www.gostructural.com/pg.asp?id=20

 

[gwynn]

Using tradional flexure design methods, the flexural steel requirements are very low.

Strut and tie models are generally more economical when high shear loads are present. It may not be as big a deal for beams, but adding hairpins to slabs can be very costly.

Strut and tie models are more accurate for deep beams. Which is more economical will depend on the ratio of length to depth I believe.

 

[cooperDBM]

Traditional beam theory (linearly variation of strain with depth, Whitney stress block approximation, etc.) is only applicable when the span is greater than about four times the depth. It's also not applicable where the section changes significantly (a D-region or disturbed region) such as at a dapped end, large hole, or large change in section - particularly depth. So beam theory is not applicable for deep beams. It's not a matter of economics. To analyze deep beams or any other D-Region the strut-and-tie method is a well recognized type of analysis.

It can be tricky to get started with, particularly when trying to analysis an existing beam. It can be easier for new beams where you can place steel to suit your model.

ACI puts out a publication, SP-208 Examples for the Design of Structural Concrete with Strut-and-Tie Models, that's quite useful.

It's definitely worth learning.

 

[bridgebuster]

The article in Structural Engineer gives a good overview, which would be good to read prior to reading SP-208.

 

[Qshake]

Once you have the SP-208 I recommend that you check out the CAST program at University of Illinois. Of course, you can do the method without a dedicated program but it helps.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[civeng80]

Pardon my ignorance but what is the basis of this strut and tie model and are there any traditional texts on mechanics of solids that explain it with examples?

 

[cooperDBM]

Strut-and-tie wouldn't be mentioned in a book on solid mechanics since they generally deal with elastic isotropic materials. Strut-and-tie is specific to reinforced (including prestressed) concrete which is a composite anisotropic material and potentially cracked. It's a plastic design method and is only used for strength checks (ultimate limit state loading). It was developed by Schlaich in Germany in the 80's.

I haven't looked recently but most newer concrete textbooks should cover it. A Google search will bring up a lot of stuff on the Internet (searcher beware).

 

[rtmote]

The axes of the tie and strut model are key to the design. The imaginary strut line from the point of applied is the concrete in compression and the imaginary tie line over the top of the piles are taken care of by steel in tension. Very simple design. Outside of the tension realm the bars are hooked up to anchor in the mass of concrete.

This design means the top face need not be reinforced, common to many concrete codes.

The advantage is if the beam is deep enough then shear links can be avoided.

I am pretty sure this concept pre-dates schlaich.

Robert Mote

http://www.motagg.com

 

[abusementpark]

Can the Strut and Tie Method be applied to typical 3-pile pile cap with concentrated moment?

 

[tails142]

Absolutely, look up corbel/nib design - you could design the element as a sort of three corbel combo

 

[abusementpark]

tails142,

Do you have a specific reference for this? Thanks.

 

[WillisV]

The steel used for typical 3 pile pile-caps in the CRSI manual was derived using strut and tie - see CRSI manual for more details.

As Qshake mentioned the CAST program found here:

http://cee.uiuc.edu/kuchma/strut_and_tie/CAST/

is a good (though outdated and unsupported) free software tool for strut and tie.