Strut tie models for RC

[WayneStrucEng]

Hi i recently overheard talk of STM's being used in RC design i have a breif idea of what they are but am finding it difficult to understand there benifits and situtations of appication, i was just wondering can anyone shine a bit of light on the them for me???

 

[youngstructural]

Essentially you are looking at reinforced concrete elements and considering them to be trusses...

This is very old engineering know-how, having been first proposed and used for beam elements around 1900. At that time, and until quite recently (around mid to late 70s) it was called "truss mechanism". Park and Paulay's text "Reinforced Concrete Structures" has a good run-down on the actual truss mechanism.

The modern usage, and renaming as STM, comes with the argument that truss mechanisms are formed in shear-governed areas and that these areas can be safely and reliably modelled using STM. This is where the code's discussion of "D" zones (aka discontinuous, disturbed, or non-beam areas) and "B" zones (aka beam, Bernoulli) comes in. Essentially you will find that the theory worked in both, but is much more valuable in D zones. I mean really, who wants to look at a slender beam as a truss analysis when (wl^2)/8 will typically do?

So, you should really read through your code's application of the theory, as well as the commentary in detail, but a good start is to just think that it's truss modelling applied to deep beams (or any beams!) and that all the special issues revolve around making sure that you have stable, sufficiently strong, panel points (which are normally referred to as nodal zones in STM) and stable, sufficiently strong struts (compression legs of your truss) and ties (tension legs of your truss).

There are numerous very good walk-throughs online. I perticuarly like

http://best.umd.edu/publications/stm.pdf

Hope that helps,
Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...