Design Considerations for Precast Prestressed Concrete Building Structures in Seismic Areas

Alfred A. Vee, P.E., Dr. Eng. President Applied Technology Corporation Honolulu , Hawaii

Long-time PCI Professional Member Alfred A. Vee has been in engineering practice since 1953, specializing in the design of precast and prestressed concrete structures . Most of these structures have been located in the Pacific Rim - one of the most seismologically active areas in the world. In recognition of his innovative work in advancing concrete tech ­nology, he was awarded an Honorary Doctorate Degree in engineering from Rose-Hulman Institute of Technology in 1976. That same year Dr. Yee was elected to the prestigious National Academy of Engineering. He obtained his bachelor's degree in civil eng i­neering from Rose-Hulman Institute of Technology and his master's degree from Yale University . Dr . Yee has lectured all over the world and is the author of numerous published papers including some that have appeared in the PCI JOURNAL.

40

Discusses the major design considerations necessary in the successful construction of precast, prestressed concrete building structures situated in seismic areas. The importance and interaction of stiffness, strength, toughness and, especially, fail-safe connections are emphasized. Such connections are needed not only to transfer loads but to provide continuity and overall monolithic behavior in the entire structure. Suggestions are given on how to increase the stiffness of the structure as well as advice on good and poor design practices. Examples of precast concrete buildings drawn from the author's 40 years design experience are shown and commented upon.

S ound principles, attention to detail and good judgment are necessary in designing precast,

prestressed concrete structures in seis­mic areas. These ingredients, of course, apply also to the design of all types of structures, whether they are made of concrete, steel or composite materials. However, in the case of pre­cast and prestressed concrete struc­tures , particular attention must be given to stiffness, strength and tough­ness requirements, and especially to connection detai ls. Their function is not only to transfer loads but to devel­op continuity and monolithic behavior in the entire structure.

Strength, stiffness and toughness minimize second o_rder effects result­ing from drift, deflection and rotation.

It is important that movements and ro­tations be restrained in an earthquake because they can cause both structural and nonstructural damage to buildings and in severe cases can induce col­lapse. Shear walls combined with each other through deep beams or deep wall gi rders or trusses to form giant mo­ment frames is one good method to in­crease the overall stiffness of a struc­ture.

Soft floors, seismic joints and ec­centricities (non-symmetry) in build­ing frames should be avoided regard­less whether they can be justified by mathematical calculations. If seismic joints become absolutely necessary due to unusual structure size, it is sug­gested that such joints be semi-re­strained by a system of tension ties in

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combination with the insertion of a cushioning material between building components to reduce possible dam­age from pounding action.

The purpose of this paper is to pre­sent the major design considerations that are required in the safe construc­tion of precast, prestressed concrete building structures situated in seismic areas. Another purpose of the paper is to present real life examples of build­ings that have withstood major earth­quakes and to discuss some of the de­sign features and practices that do and do not work well in seismic areas.

A list of articles pertinent to this paper is given in References 1 to 12.

This paper is drawn from the au­thor's 40 years experience designing precast, prestressed concrete struc­tures. Most of these structures are situ­ated in the Pacific Rim which is one of the most seismologically active areas in the world.

CONNECTION DETAILS The single most important ingredi­

ent in the design of precast concrete structures is the connection details. Connections between precast building elements such as columns, beams, slabs and shear walls must effectively integrate the individual structural components in full continuity with each other so that the overall building structure behaves monolithically. In this manner, the structural analysis and behavior of a building frame would be identical to that for a cast-in­place structure except that the framing system now uses precast concrete components which are assembled to­gether to act monolithically.

As an example, take the case of sim­ple precast bearing wall panels sup­porting precast concrete floor slabs. The most effective system we have found uses composite in-place connec­tions to develop full continuity be­tween adjacent slab spans as well as the precast concrete wall panel units. Fig. la illustrates a precast bearing wall panel supporting a precast floor or roof slab. The joint detail is ar­ranged so that dimensional errors in construction would have little influ­ence on the structural integrity and performance of the composite system.

May-June 1991

The precast floor slab panels are in­tegrated with a composite in-place structural concrete topping which pro­vides a uniform leveling surface and a horizontal structural diaphragm to re­sist seismic forces_ Utilities such as electric, telephone and television con­duits can be buried within this top­ping. The precast concrete slab panels are detailed with extended reinforcing bars bent in a diagonal pattern, criss­crossing over the precast bearing wall panels. Negative moment steel is added in the composite topping to de­velop continuity between adjacent slab spans. The precast concrete wall panel of the upper floor is integrated in full continuity with that of the lower floor by means of grout-filled steel sleeve mechanical connections_

This particular joint does not require a specified minimum bearing area be­tween the precast floor slab and wall panel in order to achieve adequate ver­tical support. The extended diagonal reinforcing steel from the ends of the precast concrete slab panels is de­signed to provide full shear resistance to the anticipated vertical dead and live loads on the slab span. Therefore, no reliance is placed on the compres-

GROUT-FILLED STEEL SLEEVE CONNECTION ------,

GROUT BED -------.

NEGATIVE MOMENT CONTINUITY STEEL

EXTENDED DIAGONAL REINFORCING STEEL

sion bearing area between the floor slab panel and the top of the precast wall panel after this joint is in place.

The wall panel thickness or top of the wall need not be widened to pro­vide support tolerance for the precast concrete slab. Negative moment steel in the topping not only develops full continuity between slab spans but also serves as a tie to resist horizontal ten­sion between the slab units. In effect, the reinforcement prevents the slabs from pulling apart over the wall sup­port when the concrete undergoes vol­ume changes or if an earthquake oc­curs.

In the event that this particular joint is severely damaged during an earth­quake and the joint deteriorates such that the concrete in the top of the wall panel as well as the bearing ends of the slab panels are spalled off, the floor slab will not be in any danger of collapse. This is because the extended overlapping diagonal bars in the bear­ing ends of the slab are designed to fully resist anticipated vertical dead and live loads_ No reliance is placed on a bearing element or shear in the concrete. Therefore, this detail is prac­tically speaking fail-safe.

PRECAST BEARING WALL PANEL

TUBES FOR GROUT INJECTION

COMPOSITE IN-SITU CONCRETE TOPPING

PRECAST SLAB PANEL

PRECAST BEARING WALL PANEL

Fig. 1a. Precast bearing wall panels support precast concrete slabs with in-place composite structural topping.

41

Fig. 1 b. Precast slab panel erroneously manufactured short of wall support.

Fig. 1 c. Extended reinforcement from slab panels diagonally crossing over wall supports to provide slab shear resistance. Supplementary negative moment steel in the topping area will develop full continuity between adjacent composite slab sections.

ADDED CONTINUITY REINFORCEMENT

CAST IN PLACE CONCRETE

COMPOSITE BEAM

PRECAST CONCRETE SOFFIT-------'

PRECAST/ PRESTRESSED CONCRETE JOISTS ---'

EXTEND STRANDS INTO BEAM ---'

(a) INTERIOR BEAM CONNECTION

(b) EXTERIOR BEAM CONNECTION

Figs. 2a-2b. Connection detail for interior and exterior beam.

42

Fig. l b shows an actual project where the floor slab was erroneously manufactured short of its intended support on the precast wall panel. Al­though this mistake is undesirable, the extended reinforcing steel would pre­vent a catastrophic failure in the event of an earthquake.

Fig. lc shows how the extended slab reinforcing bars are bent in a diagonal pattern, criss-crossing over the bearing wall panel. Added negative moment continuity steel is also shown. Once the composite concrete topping is placed and cured, adequate resistance in the slab shear support is ensured even though the precast concrete slab did not initially rest directly on the bearing wall panel.

Figs. 2a and 2b show a connection detail developing full continuity be­tween the precast concrete joists and beams.

In Fig. 2a, a precast concrete soffit is utilized for a composite beam sup­porting the precast concrete joists. In­place concrete of the composite beam is also used to develop full bearing and continuity between the precast joists and beam elements. Added con­tinuity reinforcement placed in the composite structural topping is uti­lized to develop negative moment for the joists over the beam element.

In Fig. 2b, a cast-in-place beam is used to support the precast concrete joists. In all cases, reinforcing steel and/or strands in the precast concrete joists are extended into the in-place or composite beam elements to develop additional bearing and shear friction which may be necessary in the event that the precast joists are inadvertently manufactured too short. This type of joint has been used in thousands of building structures without one single instance of spalling, cracking or any other signs of distress.

For developing fully continuous splices in reinforcing steel bars, me­chanical connections specially de­signed and tested are more reliable than conventional lapped splices as commonly used in cast-in-place con­struction. Figs. 3a to 3e illustrate some of the more common types of mechan­ical connections.

Fig. 3a is a steel sleeve connection that utilizes a special high strength

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(a)

GROUT MATRIX

STEEL SLEEVE

(b)

METALLIC MATRIX

(c)

THREADED COUPLER

COUPLER WITH TAPERED THREADE

'' '1

REBAR \ill !Ill if11 il\1

(d)

Figs. 3a-3e. Typical mechanical connections .

STRUCTURAL TESTS ON FULL SIZE COL­UMNS WITH NMB SPLICE SLEEVES (Test Report NPD-024)

I (I

LJ r• ,, II

(e)

SWAGED COUPLER

~ n TES~ :rj !

~.._,___ __ -,~ ~ .JJ¥ 11,000mm 1620mm I 2,000mm '6~m I 1,000mm I 4-=(5~::)

(3'-3-3 / 8') (1'·3-3/ 4 ' ) (6'-6-3 / 4' ) (1' -3-3 / 4' ) (3 '-3-3/ 8 ' )

WELD

REINFORCING STEEL BAR OR STRUCTURAL STEEL PLATE OR ANGLE ------i

WELD----------~1

BUTT WELD LAP WELD

Fig. 3f. Butt and lap weld connections.

LOADING TEST IN PROCESS

r==- R( X 10 ' )

Ca8 = 345kgf / cm '

RaY= 3,810kgf/ cm '

WaY = 4,010kgf / em'

I I

40

/ I

I

/ /

/

(Concrete) (Main rebars)

(Tie hoops)

20

R( X 10· ' ) --==:J

10 20

10

Fig. 3g . Summary of results of tests on full size columns with splice sleeves.

May-June 1991

30 4or

r 40 50 60

6or

I

I

u

8

0.2801 c -[QJ~ 0

1~!-m 0 8 ~100-

8

4-D41(SD35)

/

I /

f'c = DESIGN STRENGTH

70 IOor

13

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43

non-shrink cement grout matrix which forms a grout wedge anchoring the re­inforcing steel to the external sleeve. This type of mechanical connection has been tested extensively for repete­tive loads in both tension and com­pression and is widely used through­out the world.

Fig. 3b shows a splice connection utilizing a metallic matrix which en­gages the deformations of the reinforc­ing steel bars to a serrated inner sur­face of the sleeves.

Fig. 3c shows a threaded steel cou­pler engaging threads cut into the ends of the reinforcing bars. In some cou­plers the surface deformations of the bars are used as threads which in tum engage the coupler.

In Fig_. 3d, the threaded coupler is tapered to accommodate the tapered threaded ends of the steel bars. By ta­pering the threaded ends of the steel bars, insertion into the threaded cou­pler is facilitated.

In Fig. 3e, the ends of the reinforc­ing bars are engaged by a steel sleeve hydraulically swaged under pressure to the surface deformations of the re­inforcing bars.

Typical butt and lap welded connec­tions are shown in Fig. 3f.

Fig. 3g is taken from a report on repetitive loading tests on columns with splice sleeve connections using a special grout. In this test the steel sleeves are located at the floor level of a column-to-beam specimen. The as­sembly is loaded so that bending mo­ments attain a maximum intensity in the specimen. The column-beam con­nection is then wracked repeatedly by a jacking arrangement to simulate seismic forces on a column-to-beam frame. Results of these tests show that the mechanical connections enabled the precast concrete to perform in both strength and ductility equal to that of monolithically cast columns with fully continuous reinforcing steel.

In Fig. 4 the connection details be­tween the column units and between the column and beam or girder units are illustrated. In this scheme a me­chanical connection that can develop full continuity between reinforcing bars greatly enhances the flexibility in the column-to-column joint locations. When these connections are combined

44

GROUT JOINT

MECHANICAL CONNECTION (GROUT FILLED SLEEVE)

IN-SITU CONCRETE

PRECAST CONCRETE BEAM OR GIRDER

t-----PRECAST CONCRETE COLUMN

:=;t;=~----GROUT JOINT

MECHANICAL CONNECTION (GROUT FILLED SLEEVE)

Fig. 4. Typical precast concrete frame.

FLOOR LEV EL

BOUNDARY MEMBER REINFORCE MENT

HORIZONT SHEAR REINFORC

AL

EMENT

FLOOR LEV EL ·-

ENT CONFINEM REINFORC EMENT

~F-!-!-!-!-

71~

!-!-!-~

~

~ --r--

-

-t -H ~

- 11~ ~

GROUT JOINT I A

I f-1!-H-!-!-

( I !-!---~--

-~~~

v .... ~ ~~

--~ -I -

----

I I)

\ lA

lMECHANICAL !J CONNECTIONS

Fig. 5. Precast concrete shear wall with boundary members.

with adequate grouting in the joint area, the vertical precast concrete col­umn unit can be joined at the floor line or at any other convenient location along the height of the column to de­velop full continuity.

It should be noted that the mechani­cal connection between column units can be "blindly" executed; that is, ac­cess openings are not needed, thus eliminating the need for patching and grouting after the connection is in­stalled. The grout-filled steel sleeve

connection shown in Fig. 4 is classi­fied as a "blind" connection that can achieve full continuity in the steel. A thin layer of grout inserted between the column units, together with the continuity provided in the reinforcing steel by such connections, will result in connected column units that act monolithically as a fully continuous member.

In a "blind" connection, the me­chanical connection is basically a ta­pered steel sleeve which envelops the

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ends of the connecting reinforcing bars as shown in Fig. 3a. This sleeve is made large enough to allow reason­able tolerance for erection and is com­pleted by filling with a special grout to develop the required anchorage of the reinforcing steel.

Threaded couplers or sleeves filled with metallic matrix can also be used but these require access pockets to ex­ecute the connection and therefore cannot be considered as "blind" con­nections. These threaded couplers or metallic matrix sleeves have been used mainly for cast-in-place construction since the tolerances required wouio be extremely close and access to the sleeves must be provided before con­crete pouring.

The joint location between the pre­cast concrete beam and column can be at a point of highest moment adjacent to the column or at some distance away for convenience in precasting and erection. In the case of a horizon­tal joint, the joint area is cast-in-place to develop full continuity. The me­chanical connection is totally exposed before installation with in-place con­crete and, therefore, the reinforcing bars can be spliced by grout-filled sleeves, welding or other methods.

, ... , .. -,, ... tlfl , •••• .... , ... -·-· ..... . ···" .,,..... . .. ······ ,........ . ,,.... .. ..... .... . .... .

, • .r .. •• . ........ ' ... , ..... .. ....... ······ ....... ........ .. ..... ......... ...... ······ ...... ••••• •••••• •••••••••• •

•• ••••••••• •••• ················I •••••••••••••••• ••••••••••••••••

Fig. 6a. Overall view of Ramon Magsaysay Building, Manila, Philippines.

May-June 1991

Welding is labor intensive and time consuming. The heat generated from welding can cause damage to bond in the steel bars and cracking in the adja­cent precast concrete. Furthermore, high quality welding requires close su­pervision and inspection whereas me­chanical connectors using sleeves with grout can be installed quickly and reli­ably without the need for special skills and supervision.

Grout-filled sleeves have sufficient built-in tolerances and can easily be installed at normal temperatures, thus avoiding any damage to the reinforc­ing steel or adjacent concrete. In situa­tions where epoxy coated reinforcing steel is used, grout-filled sleeves are particularly advantageous because no heat or damage to the epoxy would re­sult. Sleeves with a metallic matrix or threaded couplers are difficult to apply in precast concrete work since they would require extreme precision in bar placement to have all corresponding reinforcing steel properly aligned.

To Clevelop continuity between the beam and column elements, the longi­tudinal steel in the connection must have sufficient anchorage in the col­umn. Equally important, confinement steel must envelop both the column

vertical steel and the horizontal bars of the beam or girder in the high moment region adjacent to the column. High strength concrete sufficiently plasti­cized to completely fill all void areas between the reinforcing steel and pre­cast concrete contact surfaces will pro­duce a fully continuous joint integrat­ing the beam and the column in mono­lithic frame action.

Fig. 5 shows the connection be­tween precast concrete shear wall pan­els with boundary members. The grouted joint between the precast panel elements in combination with properly spaced mechanical connec­tions to develop continuity in the rein­forcing steel can produce a shear wall frame acting monolithically through­out its height.

The precast concrete shear wall re­inforcement is detailed in exactly the same way as a cast-in-place shear wall. Vertical wall steel spaced regu­larly throughout the length of the wall panel is required to develop shear fric­tion along the entire contact face be­tween the precast components. Bound­ary elements are detailed to provide the required tension and compression resistance as would be analyzed in a monolithic cast-in-place structure.

Fig. 6b. Lower portion of Ramon Magsaysay Building, Manila, Philippines.

This 18-story (including basement) reinforced concrete structure utilizes precast, prestressed concrete joists and composite in-place floor slabs. The structural system to resist lateral forces due to seismic or wind loads is a shear wall system. The shear walls are symmetrically clustered about the center of the building, thus eliminating eccentric forces. This building experienced severe earthquake forces in 1968 and 1972 (Richter Scale 7.2) and in July 1990 (Richter Scale 7.7) without suffering any structural damage.

45

Fig. 7. Hotel Liwayway, Manila, Philippines. Wide columns and deep spandrel beams make up this shear wall type structure capable of maintaining rigidity even after cracking under repeated loads. This picture, which shows that the hotel suffered only nonstructural damage on building's exterior, was taken shortly after the August 1968 earthquake which reached a magnitude of 7.0 on the Richter Scale.

STRUCTURAL FRAMING SYSTEMS

In the overall performance of a structural concrete frame, primary consideration should be given to the development of stiffness, strength and toughness.

In the past, some design engineers believed that pure moment resistant frame systems were more desirable than shear wall systems for high rise buildings in seismic regions. The justi­fication for this philosophy is that mo­ment resistant frames are flexible and therefore can move, deflect and absorb more energy, thus reducing the re­quired calculated base shear forces . It was also thought that, in the case of shear wall structures, the base shear forces would be greatly increased due to the stiffness of such framing and therefore would be considered unde­sirable for seismic areas.

However, our experiences during

46

the past 20 years have shown that flex­ible building structures that incur ex­cessive deflection or rotation will suf­fer considerable damage in an earth­quake. This damage could result from repetetive stresses at the joints be­tween framing members, which causes progressive deterioration in stiffness. This, in tum, would cause a further in­crease in deflections and rotations, re­sulting in considerable damage to non­structural partitions, facades, utility lines, building contents and other building elements - including damage to the structural frame itself. With pro­gressive joint deterioration, such building structures can become stati­cally unstable and collapse complete­ly. Increasing the lateral stiffness of the structure will result in a decrease in detrimental second order (P-Delta) effects and thus increase the critical load capacity and overall stability of the building.

In the case of shear walls, these ele-

ments produce stiffness and, although they may crack under repeated loads, they are able to maintain a large de­gree of their stiffness without the sig­nificant deterioration displayed by moment frame structures. Earthquakes in the Philippines, Chile, Mexico and elsewhere have repeatedly confirmed the advantages of the stiff shear wall building frame concept.

Figs. 6a and 6b show a high rise of­fice building in Manila which utilizes precast, prestressed concrete joists in­tegrated with a cast-in-place structural topping. Connections of the joists are similar to that shown in Fig. 2b. The main lateral resisting element is a cen­tral concrete shear wall core which was cast-in-place. Main vertical rein­forcing steel in the shear wall core was joined by mechanical connections using a metallic matrix.

In August of 1968, this building ex­perienced a major earthquake with epicenter immediately east of Manila

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-~·

I

Fig. 8. Cast-in-place reinforced concrete structure with stiff upper floors and a soft moment frame ground floor showed partial collapse of the flexible floor columns. The photograph on left shows the overall building, while the view on the right shows a closeup of the column failure.

at a magnitude of 7.0 on the Richter Scale. The only damage observed was cracking in the nonstructural concrete hollow block partitions of the toilet and stairwell areas . Some loosening of screws attaching metal partitions in the toilets were also noticed.

The exterior surface of the building is clad with marble slabs embedded to precast panels which in turn are com­positely attached to the building frame by extended reinforcing bars . No cracks or damage occurred on this fa­cade despite reports from the night se­curity personnel that the building did undergo substantial accelerations and movement during the earthquake.

Other nearby buildings suffered se­vere damage to both structural and nonstructural elements. In some in­stances, partial or total collapse oc­curred, resulting in a loss of more than 300 lives. Virtually all moment frame buildings of both structural steel or re­inforced concrete suffered major

May-June 1991

structural or nonstructural damage during the earthquake. Nonstructural elements, such as marble or stone claddings, nonbearing partitions, ceil­ings and utility ducts, were badly dam­aged or collapsed and required exten­sive repairs.

During this earthquake in Manila, a hotel building structure (shown in Fig. 7) utilizing wide columns and deep spandrel beams demonstrated its capa­bility of maintaining stiffness even after cracking under repeated loads. The inherent stiffness of the framing system prevented the collapse of the building.

In another instance, a reinforced concrete structure with stiff upper floors and a soft ground floor moment frame (shown in Fig. 8) suffered a par­tial collapse due to the flexibility of the column beam framing system on the lower floor.

A similar magnitude earthquake oc­curred in Manila in 1972 and the high

rise office building shown in Fig. 6 again experienced only minor damage to the nonstructural concrete block walls.

In the devastating earthquake of July 16, 1990 (magnitude 7.7 on the Richter Scale epicenter north of Mani­la), at least 25 major buildings were heavily damaged in Manila, with a loss of 34 lives. Again, this high rise office building experienced only minor damage to the nonstructural concrete block walls in the toilet and stairwell areas . No other structural damage was observed.

In 1970, a 38-story building using precast, prestressed concrete floor slabs and precast concrete column-tree frames was built in Hawaii (see Figs. 9a-9d) . The lateral resisting framing system comprised a cast-in-place shear core and precast concrete col­umn-tree frames. In addition to sup­porting vertical loads, the column-tree frames functioned as a main lateral re-

47

Moana Hotel, Honolulu , Hawaii . This 38-story, 1260 room hotel was constructed using cast-in-place shear walls with precast concrete moment frames supporting precast prestressed concrete floor slabs. Built in 1970, today the building is in very good condition and has required minimal maintenance over the past 20 years.

Fig. 9b. Ala Moana Hotel, Honolulu , Hawaii . Precast moment frame unit being erected.

48

UPPER COLUMN REINFORCING BARS

LOWER COLUMN SPLICE SLEEVES

Fig . 9c. Details for splicing upper and lower columns used in building Ala Moana Hotel, Honolulu , Hawaii.

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I I I I I I I I

1 '

' ' I 1 I 1 I I

Fig. 9d. Ala Moana Hotel, Honolulu, Hawaii. Solid precast concrete walls were applied to transverse and longitudinal facade to minimize drift between floors .

May-June 1991 49

' (b) MOMENT FRAME '"""~/ , tc) COMBINATION

OF (a} & (b) (a) CANTILEVERED

SHEAR WALL :::::<

l\111\i

m:iiil~ I/ :::;:;:;

[:/ t t ~:::: ::;:: [:=:: t =:

MOMENT FRAMES ~ [ : :::=:::

~ r/////r: ~'?.' ~ MOMENT FRAMES

... -'\ ~:-:::ARW ALL

(d) SHEAR WALL - MOMENT FRAME COMBINATION

Fig. 1 0. Schematic diagrams showing the interaction of shear wall and moment frames.

CONCRETE SHEAR WALL

SEISMIC FORCE

AT REST

SOFT FLOOR

SOFT FLOOR

SEISMIC FORCE

Fig. 11 . Schematic concept of soft floor action.

sisting element in the longitudinal di­rection of the building while the shear core primarily resisted the lateral forces in the transverse direction. This structure, designed for Seismic Zone II, experienced seismic forces of mag­nitude 6.0 on the Richter Scale in April of 1973 without any visible damage.

The precast concrete column tree frames were spliced at midheight with grouted steel sleeves (see Figs. 9b and 9c). To minimize the drift and torsion­al effects in the longitudinal and trans-

50

verse directions, solid precast concrete panels were inserted at the end walls and extreme ends of the longitudinal facade (see Fig. 9d).

In most high rise buildings, shear walls combined with moment frames are generally used to resist lateral forces. As shown in Fig. 10, a pure shear wall is stiffer at the lower levels while a moment frame is stiffer at the upper levels; the combination of these two elements can result in minimizing large deflections in both the upper and lower floor levels.

Fig. 11 illustrates that a soft floor can be particularly hazardous if it is located in the lowest floor of a build­ing where maximum lateral loads could occur under seismic conditions. An actual example of this type of fail­ure is shown in Fig. 8. However, soft floors have been known to fail under seismic conditions even if located on upper story levels.

If only the lower floors were to be stiffened with shear walls and then not continued at the upper levels, abrupt change in stiffness could result in the failure of the soft floor at the upper level where a sudden change of stiff­ness occurs. Therefore, where practi­cal, both shear walls and moment frames should run continuously throughout the entire building height without allowing such soft floors to occur.

A structural arrangement utilizing shear walls connected by deep girders and columns (where they are permit­ted architecturally) can create signifi­cant additional stiffness in the building frame.

In Figs. 12a and 12b, a giant mo­ment frame system can be created by utilizing shear walls in combination with wall girders that are floor-to-floor in height. These wall girders could be used wherever permissible, such as at

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MECHANICAL ROOM ELEVATOR PENTHOUSE

(a) GIANT MOMENT FRAME SYSTEM

(b) GIANT MOMENT FRAME SYSTEM

Fig. 12. Schematic elevations of giant moment frame system.

Ph3 Ph3 -~6_2=( 1 I 4) SEI --++--e:-_3~( 1 I 16) SEI

p -- P-iP..~: '!'!'::!'!'!:~: - . ~ .. "': ·:.

{::·:: :i ..

·Fig. 13. Relative stiffness of shear walls connected with deep wall girders (giant moment frames).

mechanical room and elevator room floor levels. As shown in Fig. 13, when shear walls are coupled together with deep wall girders the resulting deflections are minimized consider­ably .

Fig. 14 shows a coupled shear wall system utilized in the building shown in Fig. 6. The vertical shear walls of the elevator cores are joined together at the roof penthouse level to enhance the stiffness of the building.

In Fig. 15, shear walls located at the end walls of the building are joined to­gether with deep wall girders at the roof level to form a giant moment

May-June 1991

frame. The deep wall girders at the roof level are also used to support me­chanical equipment rooms.

An effective and cost beneficial lat­eral shear resi sting framing system could be attained by the use of super diagonals as shown in Fig. 16. These super diagonal s couple the internal shear walls with the exterior column frame to generate maximum resistance and stiffness against lateral and over­turning forces. The super diagonal can extend several floors vertically from each level in order to generate the most efficient height to span ratio. Super diagonals can be used effective-

ly where deep wall girders are inap­propriate because of occupant access problems.

Architectural and functional consid­erations will determine the most ideal locations of these super diagonal ele­ments. Generally, the roof area is free from any functional restraints and super diagonal frames easily could be placed in this location to enhance the lateral stiffness of the building.

Eccentricities .in the framing system should be avoided wherever possible regardless of justification by structural calculations and detailing. Eccentrici­ties generally produce exaggerated movements that can be damaging. In­deed, many buildings with even mod­erate eccentricities have suffered total collapse during earthquakes. Deflec­tions, drift and rotations should be minimized in every instance since they are the prime cause of damage and failure in building frames and non­structural elements. Furthermore, a symmetrical building would favor economical precast concrete construc­tion due to the uniformity in produc­tion and erection of the precast con­crete components.

Seismic joints (Fig. 17a) between building unit s should be avoided wherever pos si ble since separate building units have different periods of vibration and under seismic activity tend to pound against each other at the joint areas. Considerable damage in seismic joint areas due to pounding of separate building units was witnessed in the 1985 Mexico City earthquake (Fig. 18). In many cases, total building collapse was directly caused by thi s pounding.

By eliminating seismic joints and developing full continuity between building frame units, pounding can be prevented. However, in eliminating seismic joints, floor areas now become relatively large and the question of shrinkage effects and floor slab re­straints causing problems of slab and wall cracking is of major concern. In most instances, these problems can be solved by the addition of continuous reinforcing steel in both directions to handle the shrinkage effects.

To reduce shrinkage stresses, these large floor areas can also be cast in smaller sections to establish temporary

51

shrinkage control joints which are then connected structurally after a long pe­riod of curing and drying time during construction. One method of connect­ing the smaller cast sections at the shrinkage control joints is shown in Fig. 19.

In other instances, however , the floor area may be so large that seismic (expansion) joints become absolutely necessary. In this instance, the width of the joint should be sufficient to pre­vent contact between the separate framing units. However, in the case of an extended period of seismic activity , resonance may build up considerable movement beyond the magnitude of the joint separation provided and se­vere damage may result. Installation of a tension tie system in combination with a suitable cushoning material be­tween building units could be consid­ered a partial solution in preventing potential damage from pounding ac­tion (Fig. 20).

In this regard, it is again important to emphasize the need for structural stiffness in the separate framing units since higher stiffness coefficients will reduce the degree of lateral movement and thus reduce the difficulty in devel­oping a manageable seismic joint. The stiffness can be increased substantially by using shear walls , giant moment frames or super diagonal systems.

CONCLUDING REMARKS For precast concrete construction in

seismic areas, all joints between pre­cast concrete units should develop full continuity and toughness. Floor slab and beam units utilizing composite in­place concrete topping and jointing can develop horizontal diaphragm ac­tion and fail-safe connections to resist seismic forces .

With the availability of reliable me­chanical splicing devices, precast con­crete shear wall , slab, column, beam and girder components can be in­stalled at any convenient location in a structure to develop full continuity and monolithic frame action . The struc­tural analysis for precast concrete con­struction would be identical to that of a cast-in-place concrete structure.

Stiffness in the overall framing sys­tem is necessary to insure against

52

PENTHOUSE MACHINE ROOM I~

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ELEVATOR CORES ~

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Fig. 14. Giant moment frame utilized in Ramon Magsaysay Building, Manila, Philippines. The structure is 18 stories high (including the basement) .

structural and nonstructural building damage by minimizing drift , deflec­tion and rotation. To develop framing stiffness, shear wall elements are ex­tremely important. When these shear wall panels are integrated with deep wall girders to produce giant moment frames , the stiffness in the structure is greatly enhanced . Super diagonal frames can also be an effective method to develop stiffness in the building frame. These frames can be used where access or functional re­quirements require more open space in a floor plan.

In seismic areas, the existence of

soft floors in the framing system can lead to severe damage or collapse. Ec­centricities (non-symmetry) in the framing system should be avoided, re­gardless of mathematical justifications and structural detailing.

Sei smic joints between building units should be avoided wherever pos­s ible in order to prevent pounding damage. If such joints become neces­sary due to the overall size of the structure, tension ties in combination with a cushioning material can be in­cluded in the seismic joint design to reduce possible damage due to pound­ing action.

PCI JOURNAL

GIANT MOMENT FRAME

ELEVATION

moment frames utilized in the Municipal Office Building, City and County of Honolulu, Hawaii.

Fig. 16. Super diagonal frame system.

May-June 1991

SUPER DIAGONAL

CORE WALLS

-- ft--SEISMIC JOI NT

'/ ///// ///////,

(a) BUILDING WITH SEISMIC JOINTS

//// '// '/ '/ '/ '/ /.

(b) BUILDING WITHOUT SEISMIC JOINTS

Fig. 17. Building with seismic joints (top) and building without seismic joints (bottom) .

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Fig. 18. Damage and collapse of buildings due to pounding action at seismic joints during Mexico City earthquake of September 19, 1985.

PCI JOURNAL

TUBES FOR GROUT INJECTION

SLABS OR BEAMS

---~----~o.f--2" TEMPORARY JOINT

WIRE LATH "STAY IN PLACE" FORM

STEEL SLEEVES AND JOINT TO BE GROUTED AFTER SHRINKAGE OF SLABS AND BEAMS TAKE PLACE

Fig. 19. Cast-in-place joint detail for shrinkage control.

-·+-~-1-~-------SEISMIC JOINT

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TENSION TIES IN COMBINATION WITH A

CUSHIONING MATERIAL TO ABSORB IMPACT

Fig. 20. Proposed detail for reducing damage due to pounding action at seismic joints.

May-June 1991

REFERENCES I. Englekirk, Robert E., "Overview of

PCI Workshop on Effective Use of Precast Concrete for Seismic Resis­tance," PCI JOURNAL, V. 31 , No. 6, November-December 1986, pp. 48-58.

2. Englekirk, Robert E., "An Analytical Approach to Establishing the Seismic Resistance Available in Precast Con­crete Frame Structures," PCI JOUR­NAL, V. 34, No. I, January-February 1989, pp. 92-101.

3. Englekirk, Robert E., "Seismic Design Considerations for Precast Concrete Multistory Buildings," PCI JOUR­NAL, V. 35, No.3 , May-June 1990, pp. 40-51.

4. Fintel, Mark, "Ductile Shear Walls in Earthquake Re sis tant Multistory Buildings," ACJ Journal, Proceedings V. 71 , No.6, June 1974, pp. 296-305.

5. Fintel, Mark, "Performance of Precast Concrete Structures During Rumanian Earthquake of March 4, 1977 ," PCI JOURNAL, V. 22, No. 2, March-April 1977, pp. 10-15.

6. Fintel, Mark, "Modem Concrete Struc­tures Survive Romanian Earthquake," Civil Engineering -ASCE, V. 48, No. 10, October 1978, pp. 80-81.

7. Fintel, Mark, "Performance of Precast and Prestressed Concrete in Mexico Earthquake," PCI JOURNAL, V. 31, No. I, January-February 1986, pp. 18-42. (See also Discussion in November­December 1986 PCI JOURNAL, pp. 143-144.)

8. Hawkins, Neil M., " State of the Art Report on Seismic Resistance of Pre­stressed and Precast Concrete Struc­tures," PCI JOURNAL: V. 22, No. 6, November-December 1977, pp. 80-110, and V. 23 , No. 1, January-Febru­ary 1978, pp. 40-58.

9. Hawkins, Neil M., "Precast Concrete Connections," Proceedings, U.S.-PRC Workshop, 1981 , pp. 28-42.

10. Iverson, James K., "First Impressions of Earthquake Damage in San Francis­co Area," PCI JOURNAL, V. 34, No. 6 , November-December 1989, pp . 108-124.

II. Scanlon, Andrew, and Kianoush, Reza M., "Behavior of Large Panel Precast Coupled Wall Systems Subjected to Earthquake Loading," PCI JOUR­NAL, v .. 33, No. 5, September-Octo­ber 1988, pp. 124-151.

12. Seckin, M., and Fu, H. C., "Beam-Col­umn Connections in Precast Rein­forced Concrete Construction ," ACI Structural Journal, V. 87, No. 3, May­June 1990, pp. 252-261.

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