FACTA UNIVERSITATIS Series: Architecture and Civil Engineering Vol. 6, No 2, 2008, pp. 193 - 197 DOI:10.2298/FUACE0802193R

THE ENGINEER HRISTA STAMENKOVIC PROPOSAL FOR TRIANGULAR REINFORCEMENT OF SUPPORT WALLS

UDC 692.1/.2:624.042.7(045)=111

Vladimir Radojičić

University of Kosovska Mitrovica, Faculty of Technical Sciences, Serbia

Abstract. This paper presents and a brief overview of Mr Hrista Stamenkovic's proposal of the triangular reinforcement in supporting walls exposed to horizontal forces (earthquake, wind). The reinforcement triangular allows total exploitation of reinforcement in compression and tension, is that, the most rational and the most economical design with the highest degree of safety will be achieved.

Key words: wall, earthquake, wind, shearing, triangular reinforcement.

In July, 2007, an elderly engineer, Mr Hrista Stamenkovic died abroad, far from his ancestorś graves and his birthplace and Serbia, leaving the legacy of his achievements in the world building construction.

He moved to America having finished his studies at the Faculty of Civil Engineering, University of Belgrade. And he stayed there forever.

With the great knowledge from his studies and his experience gained at many Ameri-can building sites, he achieved the pioneering results in Building practice of the 20th cen-tury, which became property of the mankind.

The engineer Mr Hrista Stamenkovic solved many issues. The most important ones are those related to the area of separation and congregation of lightweight concrete and basic concrete. It was published in Japan. He also created a unique system for control of proper quantity of aggregates, cement and water for the definite concrete unit depending on the accuracy (given) of the specific gravity of ingredients, published in ACI (USA). He is the author of the most precise system of absorbed, particularly free water in aggregate, published in ACI (USA) as well as of the system of defining specific gravities of light ag-gregates, more precise from standard concrete, published in France (RILEM).

Special attention must be paid to his solution for the design calculation of bracket walls against seismic loads by the system of diagonal wall reinforcement where shear stresses act along the reinforcement length, published at the international Congress, San Francisco (1984), Washington (1980) and in France (1990).

Received August 21, 2008

V. RADOJIČIĆ 194

His complete knowledge of mechanical principles introduced him to the area of avia-tion technology where he achieved outstanding results. He explored the causes of material fatigue in helicopter driveshafts and airplane wings and his authentic solutions were pat-ented in Canada and the USA.

Mr Hrista Stamenkovic was a member of two Academies: American Academy for Mechanics and New York's Academy of Science. He also worked for numerous building institutions: The Institute for Concrete (ACI), American Institute for civil Engineering (ASCE) American Institute for Pre – Stressed Concrete, International Union for examin-ing materials, Committee for finishing off concrete (ACI) and at some symposiums.

IApart from his rich inheritance, I would like to add a detail to this rich legacy of Mr Hrista Stamenovic, illustrating his last days.

He used to phone me once or several times a month. His questions were short and usual and nothing indicated it was his last conversation. His way of communication did not lead me to think differently.

However, during the last months his speech was slower and quieter and it appeared he wanted to conceal decrepitude and bad hearing. He asked me questions he did not expect answers to, because he did not have either strength or patience.

And then, a month and more passed without any calls, and the sinister thoughts came haunting me. There were no more calls and no more dear Mr Hrista Stamenkovich.

THE ANALYSIS OF AUTHORS PROPOSAL

The theory of Mr. Hrista Stamenkovic is based on the theory of deformation of a member exposed to pure shear action, and it represents an answer to a question how to overcame the defects of the classical theory, and the answer is "A wall (an element) ex-posed to shear forces, which is reinforced by triangular rebars, can withstand much stronger seismic forces, than a common wall exposed to the same forces."

Triangular reinforcement of a wall exposed to shear forces increases his resistance to lateral load.

A triangular rigidity in a shear wall can greatly increase its resistance to lateral loads. During the 1964 Alaskan earthquake, there occurred spectacular diagonal tension fail-

ure. The structures entirely complying with the UBC showed diagonal tension failure in such a way that escaped any reasonable explanation. The following quote illustrates it "The relative contribution to shear strength provided by vertical and horizontal web rein-forcement is not fully understood."

"Even though any wall exposed to shear resembles cantilever its behavior during de-formation and its stress state do not bear the slightest resemblance to the behavior and stress state of any bent member.

In fact, its behavior and its stress state is designed as a bent element and we may call it "cantilever" or a part of bent member, but in essence all shear walls, in general, are ex-posed to pure shear.

In a shear wall, one diagonal is exposed to stretching (tension) and another to short-ening (compression).

In a real cantilever, as a part of a simply supported beam, such deformation of diago-nals does not exist. Also, a beam supported on two points is never loaded, that way, and resemble an element exposed to pure shear, where one diagonal will be elongated and the other shortened as is the case with the shear wall.

The Engineer Hrista Stamenkovic Proposal for Triangular Reinforcement of Support Walls 195

In a shear wall, shear strain (elongation of a given unit) can be measured directly and, by multiplying such strain with its modulus of elasticity, stresses are determined.

The need to understand this phenomenon in shear walls for future structures can be illustrated by the words: "The use of shear walls or shear wall equivalents becomes an imperative at certain high rise buildings at mutual load induced displacements."

Any shear wall provides much more lateral stiffness by triangular reinforcement than a moment resisting frame and its ductility exceeds that of a moment resisting frame (fig 1). This is because any diagonal failure will be controlled by truss reinforcement and not by the concrete itself.

Fig. 1. Shear wall reinforcement in section and

elevation as per ACI 318-95 guidelines Fig. 1.1 Graphical guide for reinforcing a

shear wall by a single wire (bar); start with corner I and follow the pattern (arrows) from bar I to bar 12. Bars 7 and the last bar, 12 or 24 (depending on the number of layers), should be welded against each other, or against bolt I.

MAIN CHARACTERISTICS AND ADVANTAGES OF THE AUTHOR CONCEPT OF SHEAR WALL

A shear wall is a wall designed so as to resist lateral forces parallel to the wall. For the sake of the safety of a building walls must in both directions.

The concept of a shear wall is rigidity, or prevention of diagonal elongation within the shear wall.

Application of the horizontal haunches in present-day practice prevents diagonal fail-ure and creates rigidity.

By placing stronger vertical reinforcement at the edges and stronger horizontal rein-forcement at the top and bottom of the wall and with diagonal reinforcement, the maxi-mum possible rigidity will be achieved by formation of four rigid triangles connecting the top and bottom of the wall.

V. RADOJIČIĆ 196

Thus, such diagonal reinforcement (including horizontal and vertical) could be the same or less than that applied in the classical concept but with a much higher resistance to failure.

The triangular reinforcement allows total usage of reinforcement in compression and tension, is that, the most rational and the most cost-efficient design with the highest de-gree of safety will be achieved.

This design makes the wall act as a truss while resisting deformation under wind or earthquake loading, and simultaneously act as a ductile flexural member in the failure.

The foundation and walls (in both directions) will react as one structural element in the same way and the dynamic impact will be transferred directly to the ground. The re-sistance of a shear wall to possible failure could become unlimited, because the addition of triangular reinforcement is possible.

The author comes to a conclusion: "it appears to be correct that all punching shear cracks in concrete are developed, generally at 45o … and it becomes evident that the cracking follows a straight line between the external force (horizontal of a shear wall) and the support (here the overturning point or hinge point) up to 45o."

When the angle of possible cracking in a shear wall exceeds 45o, that is, when the compressive stress distribution of the support and compressive stress distribution of the external force cannot influence each other, then the direction of cracks towards an over-turning point (support) will change and become more erratic.

REINFORCEMENT FOR AUTORS SHEAR WALL

The entire shear wall is reinforced by continuous reinforcement, applying only one bar (wire) and installing two or four bars at any cross section of any chord of the truss. Such reinforcement will be bent in a shop or in the field. (figure 2.).

Before any casting of concrete, one bolt will be installed at any corner around which reinforcement is bent (figure 3.).

Fig. 2 Vertical position of Fig. 3 Horizontal position of reinforcement reinforcement of shear wall of shear wall with both forms

Such reinforcement could fail only after reaching own plastic stage and could never be e problem of the so – called bad detailing.

The Engineer Hrista Stamenkovic Proposal for Triangular Reinforcement of Support Walls 197

REFERENCES 1. Berg, v.b. and Stratta, J.L. "Achorage and the Alaska Earthquake od March 27, 1964", American Iron

and Steel Institute, New York, 1964, p.66. 2. Felix Barda, John M. Hanson and W. Gene Corley, "Shear Stretgh of Low-Rise Walls with Boundary

Elements", Reinforced Concrete Structures, Publication SP-53, American Concrete Institute, detroit MI, 2 nd printing, 1978, pp. 150-151, 154.

3. Stamenkovic, Hrista, "Suggested Revision to ACI Building Code Clauses Dealing with Shear Friction and Shear in Deep beams and Corbels", discussion, ACI Journal, May 1978, pp. 222-224, Fig. D.

4. Stamenkovic, Hrista, "Comparison of Pullout Strength of Concrete with Compressive Strength of Cylinders and Cores, Puls Velocity and Rebound Hammer", discussion, ACI Journal, March-April 1991, p. 154, Fig. B.

5. Stamenkovic, Hrista, "Mechanism of Crackings in RC Beam and Their Prevention", Journal of Building Industry "Nase gradjevinarstvo" (Our Construction), Beograd, Yugoslavia, September-October 1950, pp. 491-495.

6. Stamenkovic, Hrista, "Suggested Revision to Shear Provisions of Building Codes" discussion, ACI Journal, October 1978. No. 10, Proceeding V. 75, pp. 565-567.

7. H. Gallegos and R. Rios, "Earthquake-Repair", Reinforced Concrete Structures in Seismic Zones, ACI Publication SP-53, detroit, MI, 1978, p. 476, Fig. 4.

8. Stamenkovic, Hrista, "Sheard and Torsion Design of Prestressed and non Prestressed Concrete Beams", discussion, PCI Journal, Chicago, Ilinois, November-December, 1981, pp. 106-107.

9. Aleksander G. Tarics, "Conference Higlights Earthquake Effects", Journal of the Civil Engineering, ASCE, October, 1984, p. 10.

10. ACI-ASCE Committee 326, "Shear and Diagonal Tension", Proceedeing, ACI, Vol. 59, January-February-March 1962, Detroit, MI.

11. Timoshenko and Young, Elements of Strength of Materials, 5th edition, D. Van Nostrand Company, New York, 1968, p.62, Fig.3.11, p.194.

12. R. Saliger, Praktische Statik, Leipzig and Wien, Franz Deuticke 1927, Sweiste Auflage, S. 148, Abb. 198 *p. 148, Fig. 198).

PREDLOG DIPL. ING. HRISTE STAMENKOVIĆA ZA BEZBEDNO ARMIRANJE ZIDOVA - PLATNA

Vladimir Radojičić

Ovaj rad je posvećen inženjeru, g-dinu Hristu Stamenkoviću i njegovom predlogu trouglastog armiranja zidova izloženih smicanju kojim se ostvaruje krutost i sprečava dijagonalni lom.

Trouglasto armiranje omogućava potpuno iskorišćavanje armature pri pritisku i zatezanju, a to znači da se postiže najracionalniji i najekonomičniji projekat sa najvišim stepenom sigurnosti. Ceo zid izložen smicanju armira se kontinualnom armaturom koristeći samo jednu šipku (žicu) i postavljanjem dve ili četiri šipke na svakom poprečnom preseku svakog pojasa rešetkastog nosača. Pre nalivanja betona postavlja se po jedan klin u svaki ugao oko koga se savija armatura. Ovakva armatura može da se savija u radionici ili na licu mesta.