Integrated Buildings & Structures

Strut-and-Tie model design procedure has been presented in the ACI 318 Building Code for Structural Concrete and it is intended to design different types of concrete members modeling them as idealized trusses where the model contains struts, ties, and nodes. I wonder if anyone has attempted to use it or has already use this method to analyze and design thin reinforced concrete shells of double curvature where the surface can be idealized as triangular grid trusses of members in tension and compression only. Any feedback on this subject or any references to  testing that may had been done will be of great contribution to understand the suitability of this strut-and-tie method to general purpose structural concrete analysis and designs.

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Pedro Munoz Ph.D., P.E., M.ASCE
Principal
PRM Engineering, LLC
Methuen MA
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A 2005/2006 report from the University of Texas used thin panels to test strut and tie assumptions. Report was FHWA/TX-06/0-4371-2 Design for Shear in Reinforced Concrete Using Strut-and-Tie Models by Brown, Sankovich, Bayrak, Jirsa, Breen, and Wood. I have (intuitive) concerns about applying methods used with 3-D struts to thin sections but the research seems to indicate that proper application of the strut and tie method works in this sections.

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John Turner P.E., M.ASCE
Wiss, Janney, Elstner Associates, Inc.
Irving TX
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I think we would need to see some validation testing, since the struts are three-dimensional. The expected failure mode in compression is tension across a strut - as with inclined shear cracks in beams. In a relatively thin member, there might be insufficient confinement of the compression struts for them to behave as expected using a traditional model. If the section has reinforcement across the narrow aspect, with adequate confinement, it might behave as expected. Otherwise, it seems likely that the strut could fail prematurely. Santiago Pujol at Purdue has been looking at thin wall sections for quite some time and may have some insight into this.

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John Turner P.E., M.ASCE
Wiss, Janney, Elstner Associates, Inc.
Irving TX
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Good points. The geometry of the strut cross-sections is an important factor.

When I built ferrocement boats in the 1970's we made various struts by rolling wire mesh layers into a rough circular cross-section, then inflating to a very loose fit sausage balloons that had been slipped up in the annulus of the mesh section before the struts were set in place between end supports. It was tricky to lay-up staggered layers of mesh as the mortar was manually applied, especially at the ends of the struts and the adjoining supports. Typical length (1.5-2 meters) 80 to 100 mm ID 4-layer mesh struts had 20 to 25 mm wall thicknesses, based on some random drill holes, with the struts flaring a bit and being solid within 100 to 150 mm of the end supports. Our longest struts were about 3 meters, with wall thicknesses of nominal 30 mm for 5-6 layers of mesh.  And, yes, when we did pop an occasional balloon, we just pressed on with extra mortar at that section.  

They were very strong. Never heard of any strut cracking. Of course, these were real struts, not imaginary ACI struts.

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Robert Bullard P.E., M.ASCE
President
Ahimsa Technic Inc
Ponce Inlet FL
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Original Message:
Sent: 05-08-2017 09:30
From: John Turner
Subject: STRUT-AND-TIE ACI 318 Model for Thin Concrete Shells

I think we would need to see some validation testing, since the struts are three-dimensional. The expected failure mode in compression is tension across a strut - as with inclined shear cracks in beams. In a relatively thin member, there might be insufficient confinement of the compression struts for them to behave as expected using a traditional model. If the section has reinforcement across the narrow aspect, with adequate confinement, it might behave as expected. Otherwise, it seems likely that the strut could fail prematurely. Santiago Pujol at Purdue has been looking at thin wall sections for quite some time and may have some insight into this.

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John Turner P.E., M.ASCE
Wiss, Janney, Elstner Associates, Inc.
Irving TX
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_{I need to calculate distance between driven or bored Piles if I need 2 piles and in case if I need 5 Piles ( under one column )}

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Jobran Almaliky S.M.ASCE
Abha
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Original Message:
Sent: 05-10-2017 15:25
From: Robert Bullard
Subject: STRUT-AND-TIE ACI 318 Model for Thin Concrete Shells

Good points. The geometry of the strut cross-sections is an important factor.

When I built ferrocement boats in the 1970's we made various struts by rolling wire mesh layers into a rough circular cross-section, then inflating to a very loose fit sausage balloons that had been slipped up in the annulus of the mesh section before the struts were set in place between end supports. It was tricky to lay-up staggered layers of mesh as the mortar was manually applied, especially at the ends of the struts and the adjoining supports. Typical length (1.5-2 meters) 80 to 100 mm ID 4-layer mesh struts had 20 to 25 mm wall thicknesses, based on some random drill holes, with the struts flaring a bit and being solid within 100 to 150 mm of the end supports. Our longest struts were about 3 meters, with wall thicknesses of nominal 30 mm for 5-6 layers of mesh.  And, yes, when we did pop an occasional balloon, we just pressed on with extra mortar at that section.  

They were very strong. Never heard of any strut cracking. Of course, these were real struts, not imaginary ACI struts.

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Robert Bullard P.E., M.ASCE
President
Ahimsa Technic Inc
Ponce Inlet FL
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