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Design against progressive collapse Alexander Popoff, Jr. Vice President ABAM Engineers, Inc. Tacoma, Washington eighth and concluding paper Theof the eight-paper series on "Design Considerations for a Precast Prestressed Apartment Building" deals with progressive collapse. The paper is presented in two parts. The first part reviews existing and developing code provisions pertaining to progressive collapse. It also presents a design philosophy which is based on seismic design practices and suggests that this design philosophy is the proper approach to design against progressive collapse. The application of the suggested design philosophy is presented in the second part of the paper. It consists of a commentary and simple calculations from which a series of connection details and minimum reinforcement requirements are derived. Introduction Alexander Popoff, Jr. 44 Concern about progressive collapse has been voiced ever since unreinforced joints appeared in concrete construction. The phrase "house of cards" has been used to describe this type of construction. "Houses of cards" were built because the design engineer, precaster, or contractor could "show" that the design conformed to applicable local codes. Building codes generally give detailed requirements for the design of individual members but give little guidance for the stability design of the entire structural system. Thus, with this shortcoming in codes it was inevitable that sooner or later a major structural failure would occur. The climax came in 1968 when the 24story Ronan Point apartment tower in London, England, partially collapsed. A gas explosion on the 18th floor blew out a load-bearing wall panel, causing the chain reaction collapse of an entire corner of the building (see Fig. 1). Following the collapse, an exhaustive public inquiry was held and an impressive report describing the circumstances of the failure was prepared.' The phrase "progressive collapse" entered the engineer's vocabulary. Not surprisingly, the inquiry found that the design of Ronan Point complied with all applicable building codes and that there were no deficiencies in workmanship. However, it was revealed that the walls in the Ronan Point building were unreinforced. Thus, all forces in the joints were resisted solely by bond, friction, and gravity. British Regulations Because the Ronan Point failure so dramatically uncovered weaknesses in building codes and also because a number of similar buildings were in use at the time of the collapse, reaction to the failure was most urgent and profound in Great Britain. Six months after the Ronan Point collapse the British Government issued administrative regulations intended to prevent progressive collapses. These regulations were set forth by the Ministry of Housing and Local Government in Circular 62/68. The regulations were directed specifically to large panel precast construction. In 1970, the 5th Amendment to the 1965 Building Regulations was adopted and made law by Parliament. The provisions of Circular 62/68 and the 5th Amendment are essentially the same; they are now incorporated in the 1972 edition of the Building Regulations. These code provisions require that PCI JOURNAL/March-April 1975 Fig. 1. Aerial view of Ronan Point (London, England) after gas explosion. a building be so designed that a structural failure resulting from the removal of any portion of any one structural member be localized within a story and be limited to three stories. A local failure within a story has been defined to be 750 sq ft or 15 percent of the floor area. Alternately, all structural members 45 must be designed for an ultimate overpressure of 5 psi. These provisions were intended for any building of five or more stories. When the British Circular 62/68 was issued, it was criticized by many and applauded by a few. 2 Upon adoption in 1970 of the 5th Amendment to the British Building Regulations, the Institution of Structural Engineers felt compelled to issue a public statement expressing its concern over the stringent newly adopted requirements.3 In the ensuing years (1970 to 1972) these regulations received considerable attention from code-writing committees and were significantly modified. The modified progressive collapse provisions were incorporated in the British Unified Code for the Structural Use of Concrete. This Code was published late in 1972. The Code is presently (1974) being reviewed in Great Britain and will probably be adopted shortly. During the review stage the Unified Code received criticism;4' 5 however, none of the criticism was directed to the progressive collapse clauses. In fact, the Unified Code does not refer to "progressive collapse;" rather it refers to "stability." Unified Code requirements The following is a summary of the Unified Code requirements: 1. Unless wind loads control, the building must be designed for a horizontal force equal to not less than 1.5 percent of the building dead load. 2. All buildings must be provided with horizontal ties at each floor. These ties are peripheral and internal assuring diaphragm action and providing some ductility. The amount of tension to be resisted by ties is a function of load, span, story height, and building height. 3. External load-bearing walls and columns must be restrained horizontally at each floor with a force equivalent to not less than 3 percent of the ultimate 46 wall and column vertical load. 4. In buildings over five stories in height, effective vertical ties must be provided in all columns and walls. The area of this tie is equal to the minimum main reinforcement. 5. In buildings over five stories where the vertical ties do not meet the required minimum: (a) The building must be designed so that any single vertical loadbearing element can become incapable of carrying its load without causing collapse of the structure or any significant portion of it except that (b) Any vertical load-bearing element, which cannot be allowed to be ineffective, must be designed, together with its connections and horizontal members providing lateral support, to withstand a load of 5 psi applied from any direction. The Unified Code departs from the immediate post-Ronan Point preoccupation with internal blast and instead directs attention to ductile performance. The member removal or 5 psi design (Item 5 above) is required only when the building lacks vertical continuity. The Unified Code provisions represent the latest British thinking on progressive collapse. Development in United States The British Code provisions on progressive collapse are of importance because they were the first provisions to deal explicitly with progressive collapse and thus initiated numerous discussions in engineering fraternities. 6-9 (see also references cited in Reference 9). The British provisions are also important because they evolved from a most restrictive position (5th Amendment, 1970) to a sound engineering approach (Unified Code). In the United States, the Department critical features. Similar design apof Housing and Urban Development proaches have been presented in (HUD) has been the prime instigator in Europe. 12.13 They endeavor to prethe development of progressive col- sent a design philosophy rather than a lapse criteria. In 1971, HUD circulated rigid prescription. The PCI Committee on Precast Con"Provisions to Prevent Progressive Collapse."10 These provisions were in- crete Bearing Wall Buildings is currenttended for buildings requiring Federal ly developing recommendations for the financing. The criteria presented by design of large panel buildings. In its HUD were almost identical to the Bri- current draft, the report presents design tish Fifth Amendment 1970 regulations. recommendations with particular attenThe HUD document was reviewed tion given to internal floor and wall ties. In mid-1974, HUD commissioned the by various private agencies and professional and trade associations. The Pre- Portland Cement Association to develop stressed Concrete Institute Committee standard criteria for the design and conon Precast Concrete Bearing Wall struction of large panel structures. The Buildings and the American Concrete PCA study is a 3-year undertaking; it is Institute Committee 356 on Industrial- intended to provide criteria necessary ized Concrete Construction reviewed to achieve overall structural integrity the document in depth. The reviews considering all normal and abnormal agreed that progressive collapse must loads. Concurrently, the National Bube prevented but generally disagreed reau of Standards is continuing a with the methods proposed by HUD. detailed study of abnormal loadings on For example, the HUD-suggested de- buildings.9 signs of 5 psi overpressure or random removal of structural members were Design Against felt to be too radical and inappropriate. Progressive Collapse The 1971 HUD document was revised several times. The current version, Progressive collapse in reinforced issued in May 1974, requires a nominal diaphragm and wall ties and also re- concrete buildings became a matter of quires either a 5 psi overpressure design serious concern with the introduction or random removal of member design. of large load-bearing precast units in This approach is counter to the present buildings. To some degree, building tendency towards more generalized cri- codes are responsible for this concern. teria which would be relevant to any Building codes have emphasized member design: column design, flexural type of accidental damage." While the HUD-initiated guidelines member design, wall design, footing deemphasize design procedures which sign, and other individual elements. deal with a failure (removal of mem- The design of joints, even in cast-inbers) or incipient failure (5 psi over- place structures, has been neglected by pressure), other methods emphasizing the engineer.14 Cast-in-place joints and member systhe interdependence of elements and the ductile joining of these elements tems have been given importance only into a coherent single assembly are in in buildings designed for seismic resisthe development stage. tance. 15.16 In precast concrete buildThe second approach attempts to ings economic pressures and difficulties identify particular features of precast in execution further contributed to the structures which are vulnerable to ac- neglect of joints. At times, except for cidental instability and to suggest gen- bearing stresses, joints became more of eralized criteria for the design of these a "nuisance" which was filled with PCI JOURNAL/March-April 1975 47 grout rather than an important struc- etc.), must freely borrow from its castin-place counterpart to avail itself of a tural detail. Nevertheless, cast-in-place concrete ductile performance. Presently, for precast concrete buildstructures designed and constructed according to accepted codes and prac- ings the solution to progressive collapse tices, and especially those designed rests with the application of good engieven for only nominal seismic resis- neering judgment and common rulestance, are not generally susceptible to of-thumb in conjunction with applicable progressive failures. Even with only ACI and PCI standards. The developsecondary attention given to joints, ment of effective wall and floor diathese structures, if properly detailed, phragms cannot be overemphasized.17 have a great amount of ductility, redundancy and an ability to absorb sudden loads. Structural Review A cast-in-place concrete building will contain numerous time-honored ductile Calculations are made to determine features: minimum slab reinforcement loads and to design structural elements of 0.0018 bt, minimum column rein- and systems. There are no progressive forcement of 1 percent, walls anchored collapse calculations. There is, in lieu to floors with at least #3 bars at 12 in., of calculations, a structural review. two #5 bars around openings, wall rein- Normally this review is in fact part of forcement in two directions, tensile the design process itself and proceeds strength in columns, and other struc- concurrently with the design. The retural features. These features enable the view is the art of structural design; it cast-in-place concrete bearing wall is a series of checks against reasonablebuilding to absorb even artillery shells ness; it is the exercise of engineering without precipitating total failure. judgment. Time-honored and arbitrary criteria Pertinent parts of this structural realso exist in structural steel buildings: view which normally would be distrilimiting slenderness ratios, semi-empiri- buted throughout the design calculacal design of stiffeners, connections de- tions are assembled in this part of the signed for a minimum force of 6000 lbs, eight-paper series on "Design Considminimum connections of truss members, erations for a Precast Prestressed Apartminimum size and length of welds, and ment Building." other structural details. The reservoir of detailing practices, Lateral loads All buildings should be designed for which for structural steel and cast-inplace concrete buildings is at a com- some minimum lateral loads. Some fortable level, is not yet full for precast European codes specify the minimum concrete buildings. It is this void that lateral load as a percentage of the buildthe PCI Committee on Precast Concrete ing dead load. This is an arbitrary reBearing Wall Buildings, the Portland quirement which seldom governs a Cement Association, and HUD are now design; wind or seismic forces are usualtrying to fill. However, until these and ly higher. This arbitrary lateral load other works are completed, the precast does, however, insure some overall structure, to withstand "non-calcu- stability, particularly in a Iight, narrow lated" loads (loads arising for misalign- building. V,,,,i,,, _ (1.5/100) W ment, from imperfect member toler= 0.015 (20,000) = 300 kips ances, from settlement, vibrations, Wind design loads for this building shrinkage, creep, temperature variations, blast, aircraft or vehicle impact, are higher than the minimum above. 48 .,_se ,.,...Y 34n INSERT &THREADED COIL ROD AT 36"O.C. TYP. 4 SIDES Fig. 2. Diaphragm connections. Refer to the calculations in the second paper of this series ("Analysis of Lateral Load Resisting Elements" by John V. Christiansen). Were the design wind or seismic forces less than 300 kips, the 300 -kip lateral load, uniformly distributed over the height of the structure, would be used. Slab reinforcement The following three items should be provided. 1. Furnish a nominal percentage of reinforcement in the topping using ACI 318, Section 7.13. 0.0018 bt = 0.0018(12)2.5 = 0.054 sq in. Use 6 x 6 — 6/6 mesh (A8 = 0.058 sq in.) The tension capacity of the mesh is A8 f, = 0.058(0.90)60 = 3.1 kips per ft Provide the same tension or shear capacity in the slab-to-wall connection. PCI JOURNAL/March-April 1975 Use 3/4-in, insert with threaded coil rod at 3 ft in all four sides of the building. 2. Also, provide two #5 bars in the topping along all the exterior walls and around all openings. 3. Further, provide nominal connections between the flanges of the double tee and from the double tee flange to the wall. The results are shown in Figs. 2 and 3. The above are minima. The roots of Items 1 and 2 are found in ACI 318. The above reinforcement not only provides ductility but is necessary for diaphragm action. It is a coincidence that the arbitrary minimum reinforcement suggested above is approximately the same as that computed. Refer to Sheets 9 and 10 of the fourth paper in this series ("Design of Secondary Floor Members" by Barrett, Dunbar, and Gillaspie). The flange-to-flange and flange-to49 wall connections suggested above are also arbitrary. They add some strength to the diaphragm and also tend to align the double tees compensating for differential camber between elements. Another important function of the mesh and the topping-to-wall ties is to anchor the exterior walls to the entire building. The topping-to-wall ties provide lateral stability to the wall. While it is convenient to think of shear flowing from the slab to the wall, bracing of the vertical load carrying elements cannot be taken for granted. Vertical wall reinforcement and horizontal joints In this building the exterior walls are both bearing and shear walls; they carry high vertical loads and are designed accordingly. In some cases shear walls carry relatively low vertical loads and high shear loads; these, in particular, must be checked for minimum horizontal and vertical reinforcement. In the building at hand the exterior walls may be designed as "walls" or as "columns." Both assumptions must be reviewed. 1. Loads transmitted through columns Column area for 12-ft wall segment. A=2[(27x5.5)+(17x9)] = 603 sq in. Minimum reinforcement in columns (1/100) 603 = 6.03 sq in. Minimum reinforcement through horizontal joint (see ACI 318, Section 7.10.5) (25/100)6.03 = 1.50 sq in. 2. Loads transmitted through wall Wall area in 12-ft segment (5.5x144)+2(17x9) = 1098 sq in. Minimum wall reinforcement 0.0015(1098) = 1.65 sq in. The review for "wall" reinforcement is somewhat fictitious. As designed, the mullions act as columns and in this sense there are no "walls." This review, nevertheless, is useful to give another parameter for the minimum steel. 3. Minimum vertical reinforcement For edge mullion use four #6 bars = 1.76 sq in. For center mullion use four #8 bars = 3.16 sq in. Reinforcement provided in 12-ft wall panel = 6.68 sq in. > 6.03 (ok) Reinforcement provided in 6-ft wall panel = 3.52 sq in. > 3.02 (ok) Note that four bars are provided in each mullion. These are placed in the corners and tied. 4. Horizontal wall joint Provide two #8 bars in all mullions. The minimum reinforcement thus provided (3.16 in. in 6-ft pan- 1 'a" G ROOV E Bearing mullions in wall. 50 JDS 2- #4x I'-6' WELD TO, LAT45° L12xI'Ft 6x6 - W 2.9x W2.9 WWF CONNECTIONS AT '13 POINTS (8' MAX.) — Fig. 3. Auxiliary diaphragm connections. els and 4.74 sq in. in 12-ft panels) is well above the minima calculated using ACI 318, Section 7.10.5. A reasonable argument may be advanced for two #6 bars in the edge mullions and two #8 bars in the center mullions. The author prefers two #8 bars in each mullion. To anchor the bars use grouted tube connections. For details, see the PCI Design Handbook, p. 6-25 [see Eq. (6-28)] . (1200)] le = A ,, f,/ [t For #8 bars le = 0.79 (60,000)/ [0.85 (3.1) 1200] = 15 in. Determine the confinement reinforcement [see PCI Design Handbook, Eq. (6-7)] . A., h = A go f/(µ f,․) = 0.79(60,000)/ [ 1.4(60,000)] = 0.56 sq in. PCI JOURNAL/March-April 1975 The confinement reinforcement is small. Therefore, use column ties (see Fig. 4). Horizontal wall reinforcement and vertical joints The minimum reinforcement is 0.0020 bt. A ,, = 0.0020 x 12 x 6 = 0.144 sq in. per ft Use #4 bars at 16 in. (f, = 60 ksi) Design the vertical wall-to-wall joint for this minimum reinforcement. Use two connections per floor. Each connection, A,=5 ft x 0.15 0.75 sq in. Use two #6 bars (A.,, = 0.88 sq in.) See Figs. 5, 6, and 7. Note the length of the studs and bars. Other reinforcement in wall panel Provide two #5 bars along top and bottom of panel. 51 PLAN COLUMN REINF. - I % MIN. 8 x 4'-0" DOWELS DRY PACK io 2/2" TUBE, GROUT FULL TIES SECTION Fig. 4. Horizontal wall joint. L3x2x3/e m DESIGN TI DEVELOP 2— #6 V.I. I „VI. Fig. 5. Wall-to-wall vertical 52 joint. 2- 3/4" 0 x 0'-8" STUDS AT 8OC. I II i^ I COLUMN TIE 1^ %II L3x3x 3/8x 1'-0" Fig. 6. Alternate wall-to-wall vertical joint. Provide two #5 bars on each of four ("Design of Load Bearing Wall Panels" sides of each window. by Charles H. Raths). This reinforcement is arbitrary, it follows the spirit of ACI 318, Section Frame reinforcement 14.2(h). The beam moments are relatively Note how this arbitrary reinforcement very small. See Figs. 10 and 11 of the coincides with the calculated reinforce- second paper in this series ("Analysis ment required for creep: Refer to Sheet of Lateral Load Resisting Elements" by 5/5F of the third paper in this series John V. Christiansen). For all practical Fig. 7. Wall-to-wall horizontal joint location. PCI JOURNAL/March-April 1975 53 COLUMN THREADED COIL ROD 2- 3/4"® INSERT &COIL ROD AT EA. BEAM OR 2- 06 COIL TIE ALTERNATE AT COLUMN L L/4 L/4 2-#6 THRU SLEEVES IN COLUMN -TOPPING SLEEVE # 3 AT 12 - I'-10° FOR 30 & 36' SPANS, 1'-9" FOR 24" SPANS PROVIDE 50 SQ IN.(±) OF NON-SHRINK GROUT Fig. 8. Minimum negative beam reinforcement. purposes these moments may be neglected. Refer to the "Commentary" in the fifth paper of this series ("Design of Frame" by Gensert, Peller, Parikh, and Fujita). Nevertheless, it should not be forgotten that some moments do exist. Frame action, even if only nominal is very desirable. Hence, some negative beam reinforcement should be provided. To judge the amount of negative reinforcement consider two approaches: 1. See ACI 318, Section 10.5.1. /min = 200 /f 5 = 200/60,000 = 0.0033 bd = 22(18) = 396 sq in. (approx.) pbd = 0.0033(396) = 1.32 sq in. Note that this reinforcement is not required. This section of ACI 318 54 is used only as a frame of reference for judgment. 2. The approximate negative live load moment is w12/10. w = 40psf X 12 = 0.480 kips per ft on 33-ft average spans, or w = 40 psf X 18 = 0.720 kips per ft rn 24-ft spans M LT (max) = w12 /10 = 52.3 ftkips M„ = 1.7(52.3)12 = 1070 in.-kips A, = M ,U/ [f 54( d — a/2)] = 1070/[60(0.90)21] = 0.95 sq in. Since all the beams were designed as simple spans, this reinforcement is not required. The calculations are done to provide a gage for judgment. Consider two #6 bars at each column. Two #6 bars plus mesh in topping equals say 1.0 sq in. This reinforcement is close to the "calculated" reinforcement above. Note that the reinforcement provided amply satisfies frame wind moments. To develop the moment couple, near the bottom of beams, provide grout for approximately 0.88 sq in. x 60 ksi = 50 kips force Note also that the reinforcing steel connected to the exterior walls further stabilizes the wall at points where the load of the walls is concentrated (see Fig. 8). Column splices ACI 318, Section 7.10.5, requires tensile continuity through column joints. The Commentary to ACI 318 states that this continuity is required regardless of other design requirements.' This is simply a provision intended to provide some ductility. In cast-in-place columns this requirement is easily achieved; in precast columns it is not. To minimize column joints, first consider precasting columns in three, four, or more story heights. The number of troublesome column joints is thus reduced considerably (check columns for handling stresses). One method of providing tensile continuity through the columns is shown in Fig. 9. The dowels project through the base plate at the bottom of the column and through the plate at the top of the column into a tube. The diameter DOWELS SLEEVES FOR DOWELS CONFINEMENT REINFORCEMENT Fig. 9. Column-to-column connection. PCI JOURNAL/March-April 1975 55 of the tube should be two bar diameters. Confinement around the tubes is provided with two to four additional column ties. The length of the tube is computed by Eq. (6-28), p. 6-25 of the PCI Design Handbook. In the upper part of the building (say upper third) use four #8 dowels. In the lower part of the building use four #11 dowels. In the upper part of the building, where gravity loads are small, the dowels thus furnished will provide more tensile capacity than required by ACI 318, Section 7.10.5; in the lower part, the tensile capacity will probably be somewhat less than required. At the base of the columns any number of main column bars can be easily projected into 6 or 8, or 10-in, diameter corrugated metal pipes set in the footing. Dowels equal to 1 or 2 percent of the pile area should project from the piles into the footing. Summary In this structural review various connections and reinforcement minima criteria were suggested. Other types of connections and reasonable departures from the suggested reinforcement minima are certainly possible. All connections were designed for the minimum reinforcement provided. The minimum reinforcement provided is essentially the same as that which would be provided in a cast-in-place building designed according to ACI 318. Should calculated forces require larger amounts of reinforcement and stronger connections, obviously, these should be provided. This review provides nominal ties throughout the structure to effect a sound structure. ACI 318 provided the guidelines for the judicial provisions of ties. Concluding Remarks To date, the incidents of progressive collapse failures in the United States 56 has been low. However, with the constantly changing patterns of building construction there is reason to believe that, without timely intervention, the number of buildings susceptible to progressive collapse will increase.7,9 Ad-hoc and studious reviews of the frequency of occurrence of abnormal loads, prompted by the Ronan Point failure, suggest that abnormal loads have a predictable and significant probability of occurrence.7,9,18 The frequency of occurrence of abnormal loads may, under some circumstances, be reduced. Total elimination of abnormal loads is not possible. However, it is possible to significantly reduce the susceptibility of a building to progressive collapse by providing ductility in members and joints. References 1. Griffiths, H., Pugsley, A., and Saunders, D., Collapse of Flats at Ronan Point Canning Town, Her Majesty's Stationary Office, London, 1968. 2. Collins, A. R., et al., "The implications of the Report of the Inquiry Into the Collapse of Flats at Ronan Point, Canning Town," The Structural Engineer (London), V. 47, No. 7, July, 1969. 3. Institution of Structural Engineers, "The Resistance of Buildings to Accidental Damage," The Structural Engineer (London), V. 49, No. 2, February, 1971. 4. Farebrother, J. E. C., "A Quizzical Look at CPI10," Concrete (London), September, 1973. 5. "United Code Symposium," Concrete (London), May, 1974. 6. Ferahian, R. H., "Design Against Progressive Collapse," Technical Paper No. 332, National Research Council of Canada, Division of Building Research, Ottawa, 1971. 7. Allen, D. E., and Schriever, W. R., "Progressive Collapse, Abnormal Loads, and Building Codes," Structural Failures: Modes, Causes, Responsibilities; American Society of Civil Engineers, 13. Creasy, L. R., et al., Report on StabilNew York, 1973. ity of Modern Buildings, The Institu8. Popoff, A., "Stability of Precast Systion of Structural Engineers, London, tems," Proceedings of the Interna1971. tional Conference on Planning and De- 14. Jirsa, J. 0., "Cast-in-Place Joints for sign of Tall Buildings," Volume III, Tall Concrete Buildings," Proceedings, ASCE, New York, 1972. International Conference on Planning 9. Somes, N. F., "Abnormal Loading of and Design of Tall Buildings, Volume Buildings and Progressive Collapse, III, ASCE, New York, 1972. Publication NBS 1R 73-221, National 15. ACI Committee 318, "Building Code Bureau of Standards, Washington, Requirements for Reinforced Concrete D.C., 1973. (ACI 318-71)," American Concrete In10. U.S. Department of Housing and Urstitute, Detroit, 1971, Appendix A. ban Development, Federal Housing 16. International Conference of Building Administration, "Provisions to Prevent Officials, Uniform Building Code, 1973 Progressive Collapse," Washington, Edition, International Conference of D.C., 1971. Building Officials, Pasadena, Califor11 Mainstone, R. J., "Internal Blast," Pronia, Sections 2625, 2626, 2627. ceedings, International Conference on 17. Lewicki, B., and Pauw, A., "Joints, Planning and Design of Tall Buildings, Precast Panel Buildings," Proceedings, Volume IB, ASCE, New York, 1972. International Conference on Planning 12. Comite Europeen du Beton, Internaand Design of Tall Buildings, Volume tional Recommendations for the DeIII, ASCE, New York, 1972. sign and Construction of Large-Panel 18. Sanders, P. H., "Evaluation of the Risk Structures, CEB, Paris, 1967. (Transof Vehicle Impact on Structures," lated from the French by C. V. AmerStructural Failures: Modes, Causes, ongen, Cement and Concrete AssociaResponsibilities, ASCE, New York, tion, London.) 1973. Discussion of this report is invited, Please forward your discussion to PCI Headquarters by August 1, 1975 PCI JOURNAL/March-April 1975 57