ACI Building Code Research Articles

Flexural Strength of Prestressed Concrete Members With Unbonded Tendons

T he current design practice when calculating the flexural strength of prestressed concrete members with unbonded tendons is to use an equation for predicting the stress in the prestressing steel f 8 at the flexural strength which takes into account a number of parameters. These are: p, = A,8/bd, the ratio of prestressing reinforcement; f^ = compressive cylinder strength of concrete; and f8e = stress in prestressing steel due to prestress alone after losses. Note that in the formula for p: A„$ = area of prestressing steel b = width of concrete compression face d = depth of centroid of prestressing steel from concrete compression face The equation used in the 1977 ACI Building Code' is:

Cyclic Shear Transfer and Type of Interface

Cyclically reversing and monotonic shear transfer tests are reviewed for companion intitially cracked specimens of normal weight and lightweight reinforced concrete. Both composite and monolithic specimens were tested. The interfaces of all composite specimens were roughened as required by sec. 11.7.9 of the ACI Building Code, ACI 318-77. In some cases the bond at the interface was deliberately broken. In the case of monolithic specimens and composite specimens in which good bond is obtained at a roughened interface, the strength under cyclically reversing shear is about 80 percent of the shear transfer strength under monotonic loading.

Behavior of Concrete Columns Under Controlled Lateral Deformation

This paper reports tests of nine model cantilever columns. The objectives of this phase of a study of the deformation capacity of columns in structural systems were to experimentally determine and analytically predict the complete axial load-moment-curvature (PM-4) relationships for columns with large axial loads and minimal ties. The controlled lateral deformation loading method used maintained stability after the maximum moment resistance was reached and allowed the descending branch of the P-M-+ relationships to be measured. This bracing against instability is similar to the lateral resistance which stronger members in an unbraced frame can provide. The measured P-M-+ relationships were not as brittle as previously indicated, but were nearly flat-topped and exhibited significant ductility. An analytical procedure was developed which closely agreed with the relationships observed. Concrete crushing strains ranged between 0.0096 and 0.0160 in./in. Predictions of an ultimate ltrain equation indicated that the values greater than 0.003 in./in. ,here due to the appreciable moment gradients. The results indicate ;,at the ACI Building Code limiting strain criteria can be safely inn-eased to 0.004 in./in. for columns with moment gradients which produce reversal of moments in the column length. Such an increase could have substantial benefits by allowing economical usage of higher strength reinforcement (f, of 80 to 90 ksi) in many columns.

Equivalent Frame Analysis for Lateral Loads

The equivalent frame method for analyzing reinforced concrete buildings described in the ACI Building Code Requirements (ACI 318-77) is suitable for analyzing single stories for gravity loading only. A new procedure that permits using the equivalent frame concepts for the analysis of multistory buildings for both lateral and gravity loads is described. Limited verification studies show the method predicts lateral deflections that generally agree with measured deflections. Comparisons of the new analytical procedure with conventional plane frame analyses made using effective slab widths are given. Problems and limitations in applying the method in plane frame and space frame analyses are reviewed.

Tests of One-Way Post-Tensioned Slabs With Unbonded Tendons

T provisions in the ACI Building Code (ACI 318-77)1 for prestressed concrete recognize a difference in the amount of mild steel required for ultimate strength and crack control between bonded and unbonded members. However, the ACI Code equations are based mainly on experience with and tests on beams with relatively low span/depth ratios compared to those common with post-tensioned slabs. Previous research efforts concerning members with unbonded tendons and various span/depth ratios have shown that the ACI• Code expressions for steel stress at ultimate may be unconservative for members such as slabs with span/depth ratios as high as 40 or more, 2' 3 while reinforcement required for crack control may be on the conservative side.

Application of the 1977 ACI Building Code to Determine End Hook Embedment for Full Value

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Proposed Revisions By Committee 435 to ACI Building Code and Commentary Provisions on DefIections*

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Suggested Revisions to ACI Building Code Clauses Dealing with Shear in Beams

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Suggested Revisions to ACI Building Code Clauses Dealing with Shear Friction and Shear in Deep Beams and Corbels

Suggested Revisions to ACI Building Code Clauses Dealing with Shear Friction and Shear in Deep Beams and Corbels

Shear-Bond Analysis of Steel-Deck-Reinforced Slabs

A brief review of steel-deck-reinforced floor systems and resulting failure modes is presented, followed by the development and evaluation of design equations for one-way shear-bond capacity. A linear regression of pertinent strength parameters was utilized to fit potential design equations to experimental data. Results of three of several possible equations are given; these are based on parameter grouping from 353 specimens of various types tested at Iowa State University and 151 proprietary tests conducted by steel-deck manufacturers. Example regression plots pertaining to slab elements for six different steel deck configurations were developed. Effects of shoring and gage thickness are considered. The preferred design equation was obtained from a relationship for shear design in the 1971 ACI Building Code.

AISC Column Design Logic Makes Sense for Composite Columns, Too

Structural engineers of North America have regarded the composite column generally as a curious stepchild who does not quite belong to a family, yet appears to behave very well. Fathers of the American Concrete Institute have provided shelter by permitting their own ACI routines to be applied to "concrete compression members reinforced longitudinally with structural steel shape, pipe, or tubing." As a "concrete compression member" the stepchild is restrained from doing its own thing as effectively as its structural steel cousins are allowed to do. The ACI Building Code forbids consideration of axially loaded columns by insisting that all columns also function as beams. Within the structural steel shape and tubing family, compression members can be assembled for design as axially loaded columns. Composite columns that are incorporated into structures with connections identical to those used for steel shapes or tubes ought to be considered analytically to behave exactly the same as the shapes or tubes.

POST-TENSIONED FLAT PLATE TO COLUMN CONNECTION BEHAVIOR

Three post-tensioned flat plate specimens (9 ft square) with a single column stub in the center and reinforced with various amounts of bonded reinforcement were loaded to failure. Test data were obtained on shear capacity, flexural strength, and general behavior in the column connection area. The results were compared with the current ACI Building Code (318-71) design equations and previous flat plate test results. The supplementary bonded reinforcement enhanced overall behavior providing crack control and increasing shear and flexural strength. This study represents only the first phase of a continuing research program on post-tensioned flat slabs. The results of the two later nine-panel model tests will be reported upon in the near future.

Computer program developments by the Portland Cement Association

At the present time less than half of the design and design/construction firms use computers. Although the cost of a terminal or even a small computer installation is within reach of every such firm, very few can afford to develop a program which will make the computer do their work. Hence, PCA decided about five years ago to undertake the programming effort needed for the design of concrete structures and pavements. A group of programs for the IBM 1130 was released in 1967-68 and included analysis and design programs for airport pavements, concrete columns, flat plates, continuous frames, bridges, shearwalls, central core and exterior column supported floor systems, and wall-beam frames. A new group of programs was released in 1971. These programs, to be discussed in more detail' include: an updated and enlarged version of the analysis and design of flat-plate floors; analysis and design of concrete columns and column stacks; quantity and cost estimating for building frames; analysis of frameshearwall structures; analysis and design of footings and foundation mats; and preliminary analysis of multi-center arch dams. All these programs were developed for the IBM 1130 and some will be converted to other machines. The provisions of the 1971 ACI Building Code were carefully followed. A comprehensive maintenance service is provided for the 1971 PCA programs, providing any assistance or information that may be needed by the users and also providing for the distribution of information obtained from users.

Automatic design of sloped spread footings

The design of sloped concrete spread footings, conforming to the proposed requirements of the new ACI Building Code 318-71 is presented. A computer program for automatic design is explained. Results are compared with flat concrete spread footings, proving sloped footings to be considerably more economical for low soil pressures.

Stirrup Effectiveness in Reinforced Concrete Beams with Axial Force

A study of the effectiveness of web reinforcement in reinforced concrete beams subject to axial forces is reported, involving the testing of 60 simply supported reinforced concrete T-beams and eight restrained reinforced concrete T-beams. The primary variables were axial force, the amount of web reinforcement, the concrete strength, and the shear span to depth ratio. Secondary variables were the web reinforcement yield point and the amount of flexural reinforcement. Measurements were made of applied loads, deflections, web reinforcement strain, and diagonal tension crack width. The effectiveness of the web reinforcement was found to be unaffected by the presence of axial forces. The design procedures for beams with web reinforcement contained in the ACI Building Code, ACI 318-63, were found to be conservative. Relationships are proposed which more closely reflect the variation of ultimate shear strength with variation of relevant parameters.

Experimental Behaviour Of Reinforced ConcreteContinuous Deep Beams

Test results of eight reinforced concrete continuous deep beams are reported. The main parameters considered were the shear span to depth ratio, the amount and type of web reinforcement, and the amount of the main longitudinal reinforcement. The vertical web reinforcement had more influence on the shear capacity than the horizontal web reinforcement. The failure is initiated by a major diagonal crack in the intermediate shear span between the edges of the load and intermediate support plates. Comparisons between test results and the ACI building code (318-89) show little agreement.

Diagonal Tension Cracking in Concrete Beams with Axial Forces

Study made of available data on diagonal tension cracking in reinforced concrete beams with and without axial load. Increment of shear between calculated flexural cracking and observed diagonal tension cracking was found to be a function of flexural reinforcement ratio, p , and modular ratio, n . In the study, 31 reinforced concrete beams were tested under combined transverse load and axial load, including both tension and compression. Results obtained confirm the conclusions reached as result of study of previously available test data. Two modified forms of Equation 26-12 of the ACI Building Code, AC1 318-63, are proposed as applicable to both prestressed concrete and reinforced concrete with or without axial load.

Test of a Reinforced Concrete Flat Slab

The behavior of a flat slab, the second of the 5 test structures built and tested in the course of an investigation of multiple-panel reinforced concrete floor slabs, is described and analyzed. The test structure, which comprised 9 5-ft square panels, was designed essentially on the basis of the ACI Building Code (ACI 318-56). Test data included deflections at 33 locations, reinforcement strains at 335 locations, and all three components of each of the 16 column reactions. Moments computed from measured reinforcement strains were compared with the ACI design moments and indicated that the total amount of reinforcement was sufficient but was in some cases inefficiently distributed. Although the overall factor of safety of the structure was adequate, the tests clearly indicated that the service-load behavior was marginal and that excessive cracking and deflections can be expected in such structures designed for and subjected to high permanent live loads.

Efectos del recubrimiento de los cables sobre el comportamiento de las vigas pretensadas

This deals with an investigation of plastic sheathing applied to prestressing cables near the end of the beams, to prevent their direct bond with the concrete. Tests on five T beams, of 10.3 m length showed that the length of bonding required by section 2611 of the 1963 ACI Building Code is inadequate in the case of sheathed cables. The necessary length so that maximum bending moment and shear force can take place, when sheathing is adopted, is twice the anchorage length that the above specification requires.

THE DYNA FRAME SYSTEM OF STRUCTURAL BUILDING ELEMENTS

Dyna Frame is a patented structural system that makes use of individual structural members which are field assembled to make the structural whole. Precast concrete beams and columns are used in conjunction with any of the standard cored-slab systems to comprise a structural frame in the conventional sense. The design of the members generally follows the ACI Building Code (ACI 318-63) . Dyna Frame is in current production in the Baltimore-Washington area, and has been used in a variety of building projects over the past three years, including multi-story motels, office buildings, parking garages, warehouses and schools. It has proved itself to be highly competitive with other framing systems, such as cast-in-place concrete in its various forms and structural steel and open-web joists. COLUMNS