Download Design against progressive collapse

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