precast concrete construction - IFET College of Engineering

IFET COLLEGE OF ENGINEERINGDEPARTMENT OF CIVILENGINEERINGBACKGROUNDPRECAST CONCRETECONSTRUCTIONThe concept of precast (also knownas “prefabricated”) construction includesthose buildings where the majority ofstructural components are standardized andproduced in plants in a location away fromthe building, and then transported to the sitefor assembly. These components aremanufactured by industrial methods basedon mass production in order to build a largenumber of buildings in a short time at lowcost. The main features of this constructionprocess are as follows:• The division and specialization of thehuman workforce• The use of tools, machinery, and otherequipment, usually automated, in theproduction of standard, interchangeableparts and products This type of constructionrequires a restructuring of the entireconventional construction process to enableinteraction between the design phase andproduction planning in order to improve andspeed up the construction. One of the keypremises for achieving that objective is todesign buildings with a regularconfiguration in plan and elevation. Urbanresidential buildings of this type are usuallyfive to ten stories high (see Figures 1 and 2).Many countries used various precastbuilding systems during the second half ofthe 20 th century to provide low-incomehousing for the growing urban population.They were very popular after the SecondWorld War, especially in Eastern Europeancountries and former Soviet Unionrepublics. In the former Soviet Union,different precast buildings systems aredenoted as “Seria,” whereas in Romaniathey are called “Secţiunea.”In general, precast building systemsare more economical when compared toconventional multifamily residentialconstruction (apartment buildings) in manycountries. The reader is referred to theUNIDO 2 report for detailed coverage onprecast systems and their earthquakeresistance.CATEGORIES OF PRECASTBUILDING SYSTEMSPrecast buildings constitute asignificant fraction of the building stock inthe republics of the former Soviet Union andEastern European countries. These systemshave been described in the following eightWHE reports: 32 (Kazakhstan); 33, 38, and39 (Kyrgyzstan);55 (Russian Federation); 66 (Uzbekistan);68 (Serbia and Montenegro); and 83(Romania).Depending on the load-bearingstructure, precast systems described in the

WHE can be divided into the followingcategories:• Large-panel systems• Frame systems• Slab-column systems with walls• Mixed systemsLarge-Panel SystemsThe designation “large-panelsystem” refers to multistory structurescomposed of large wall and floor concretepanels connected in the vertical andhorizontal directions so that the wall panelsenclose appropriate spaces for the roomswithin a building. These panels form a boxlikestructure (see Figure 3). Both verticaland horizontal panels resist gravity load.Wall panels are usually one story high.Horizontal floor and roof panels span eitheras one-way or two-way slabs. Whenproperly joined together, these horizontalelements act as diaphragms that transfer thelateral loads to the walls.Depending on the wall layout, thereare three basic configurations of large-panelbuildings:• Cross-wall system. The mainwalls that resist gravity and lateral loads areplaced in the short direction of the building.• Longitudinal-wall system. Thewalls resisting gravity and lateral loads areplaced in the longitudinal direction; usually,there is only one longitudinal wall, exceptfor the system with two longitudinal wallsdeveloped in Kazakhstan (WHE Report 32).• Two-way system. The walls areplaced in both directions (Romania, WHEReport 83). Thickness of wall panels rangesfrom 120 mm for interior walls (Kyrgyzstan,WHE report 38) to 300 mm for exteriorwalls (Kazakhstan, WHE Report 32). Floorpanel thickness is 60 mm (Kyrgyzstan).Wall panel length is equal to the roomlength, typically on the order of 2.7 m to 3.6m. In some cases, there are no exterior wallpanels and the façade walls are made oflightweight concrete. Panel connectionsrepresent the key structural components inthese systems. Based on their locationwithin a building, these connections can beclassified into vertical and horizontal joints.Vertical joints connect the vertical faces ofadjoining wall panels and primarily resistvertical seismic shear forces. Horizontaljoints connect the horizontal faces of theadjoining wall and floor panels and resistboth gravity and seismic loads.Depending on the constructionmethod, these joints can be classified as wetand dry. Wet joints are constructed withcast-in-place concrete poured between theprecast panels. To ensure structuralcontinuity, protruding reinforcing bars fromthe panels (dowels) are welded, looped, orotherwise connected in the joint regionbefore the concrete is placed. Dry joints areconstructed by bolting or welding togethersteel plates or other steel inserts cast into theends of the precast panels for this purpose.Wet joints more closely approximate castin-placeconstruction, whereas the forcetransfer in structures with dry joints is

accomplished at discrete points. A plan of alarge-panel building from Kazakhstan withthe connection details. In this system,vertical wall panel connections areaccomplished by means of groove joints,which consist of a continuous void betweenthe panels with lapping horizontal steel andvertical tie-bars. Horizontal jointreinforcement consists of dowelsprojected from the panels and the hairpinhooks site-welded to the dowels; the weldedlength of the lapped bars depends on the bardiameter and the steel grade. Vertical tiebarsare designed for tension forcesdeveloped at the panel intersections.Lateral stability of a large-panelbuilding system typical for Romania isprovided by the columns tied to the wallpanels (WHE Report 83). Boundaryelements (called “bulbs” in Romania) areused instead of the columns as “stiffening”elements at the exterior. The unity of wallpanels is achieved by means of splice barswelded to the transverse reinforcement ofadjacent panels in the vertical joints.Longitudinal dowel bars placed in verticaland horizontal joints provide an increase inbearing area for the transfer of tensionacross the connections. Wall-to-floorconnection is similar to that.Frame SystemsPrecast frames can be constructedusing either linear elements or spatial beamcolumnsubassemblages. Precast beamcolumnsubassemblages have the advantagethat the connecting faces between thesubassemblages can be placed away fromthe critical frame regions; however, linearelements are generally preferred because ofthe difficulties associated with forming,handling, and erecting spatial elements. Theuse of linear elements generally meansplacing the connecting faces at the beamcolumnjunctions. The beams can be seatedon corbels at the columns, for ease ofconstruction and to aid the shear transferfrom the beam to the column. The beamcolumnjoints accomplished in this way arehinged. However, rigid beam-columnconnections are used in some cases, whenthe continuity of longitudinal reinforcementthrough the beam-column joint needs to beensured. The components of a precastreinforced concrete frame.Precast reinforcedconcrete frame with cruciform and linearbeam elements (Seria 106) is an example ofa frame system with precast beam-columnsubassemblages (Kyrgyzstan, WHE Report33). The system was developed inKyrgyzstan in 1975. The load-bearingstructure consists of a precast reinforcedconcrete space frame and precast floor slabs.The space frame is constructed using twomain modular elements: a cruciform elementand a linear beam element (Figure 8). Thecruciform element consists of the transverseframe joint with half of the adjacent beamand column lengths. The longitudinal framesare constructed by installing the precastbeam elements in between the transverseframe joints. The precast elements are joinedby welding the projected reinforcement bars(dowels) and casting the concrete in place.Joints between the cruciform elements arelocated at the mid-span of beams andcolumns, whereas the longitudinal precastbeam-column connections are located closeto the columns. Hollow-core precast slabs

are commonly used for floor and roofstructures in this type of construction.Slab-Column Systems with Shear WallsThese systems rely on shear walls to sustainlateral load effects, whereas the slab-columnstructure resists mainly gravity loads. Thereare two main systems in this category:• Lift-slab system with walls• Prestressed slab-column systemLift-slab systems were introduced inthe last decade of the Soviet Union (period1980-1989) in some of the Soviet Republics,including Kyrgyzstan, Tadjikistan, and theCaucasian region of Russia, etc. This type ofprecast construction is known as “SeriaKUB.” The load-bearing structure consistsof precast reinforced concrete columns andslabs, as shown in Figure 10. Precastcolumns are usually two stories high. Allprecast structural elements are assembled bymeans of special joints. Reinforced concreteslabs are poured on the ground in forms, oneon top of the other, as shown in Figure 11.Precast concrete floor slabs are lifted fromthe ground up to the final height by liftingcranes. The slab panels are lifted to the topof the column and then moved downwardsto the final position. Temporary supports areused to keep the slabs in the position untilthe connection with the columns has beenachieved. In the connections, the steel bars(dowels) that project from the edges of theslabs are welded to the dowels of theadjacent components and transversereinforcement bars are installed in place.The connections are then filled withconcrete that is poured at the site.Most buildings of this type havesome kind of lateral load-resisting elements,mainly consisting of cast-in-place or precastshear walls, etc. In case lateral load-resistingelements (shear walls, etc.) are not present,the lateral load path depends on the abilityof the slab-column connections to transferbending moments. When the connectionshave been poorly constructed, this is notpossible, and the lateral load path may beincomplete. However, properly constructedslab-column joints are capable oftransferring moments as shown by severalfull-scale vibration tests performed inKyrgyzstan on buildings of this type.Another type of precast system is a slabcolumnsystem that uses horizontalprestressing in two orthogonal directions toachieve continuity. The precast concretecolumn elements are 1 to 3 stories high. Thereinforced concrete floor slabs fit the clearspan between columns. After erecting theslabs and columns of a story, the columnsand floor slabs are prestressed by means ofprestressing tendons that pass through ductsin the columns at the floor level and alongthe gaps left between adjacent slabs (seeFigure 12). After prestressing, the gapsbetween the slabs are filled with in situconcrete and the tendons then becomebonded with the spans. Seismic loads areresisted mainly by the shear walls (precastor cast-in-place) positioned between thecolumns at appropriate locations. Thistechnology has been used in Yugoslaviaduring the last 40 years under theproprietary name, “IMS Building System,”and it can be found in all major Yugoslavcities, including such as Cuba, the

Philippines, and Egypt. A typical buildingunder construction.EARTHQUAKE PERFORMANCEThere is a general concern among theearthquake engineering communityregarding the seismic performance ofprecast construction. Based on experience inpast earthquakes in Eastern European and inCentral Asian countries where these systemshave been widely used, it can be concludedthat their seismic performance has beenfairly satisfactory. However, when it comesto earthquake performance, the fact isthat “bad news” is more widely publicizedthan “good news.” For example, the poorperformance of precast frame systems ofSeria 111 in the 1988 Spitak (Armenia)(M7.5) earthquake is well known (seeFigure 14). However, few engineers areaware of the good seismic performance (nodamage) of several large-panel buildingsunder construction at the same site, asshown in Figure 15; these large-panelbuildings were of a similar seria as the largepanelbuildings described in the WHEReport 55 from the Russian Federation(Seria 464). The buildings of Seria 111 weresimilar to the precast concrete frame systemof Seria IIS, described in the WHE report 66(Uzbekistan). The precast prestressed slabcolumnsystem (IMS Building System)described in WHE Report 68 (Serbia andMontenegro) has undergone extensivelaboratory testing that predicted excellentresistance under simulated seismic loading.These building have been subjected toseveral moderate earthquakes withoutexperiencing significant damage. Due totheir large wall density and box-likestructure, large-panel buildings are very stiffand are characterized with a rather smallfundamental period. For example, a 9-storybuilding in Kazakhstan has a fundamentalperiod of 0.35 to 0.4 sec (WHE Report 32).In general, large-panel buildings performedvery well in the past earthquakes in theformer Soviet Union, including the 1988Armenia earthquake and the1976 Gazlyearthquakes (Uzbekistan). It should benoted, however, that large-panel buildings inthe area affected by the 1976 Gazlyearthquakes were not designed with seismicprovisions. Most such buildings performedwell in the first earthquake (M 7.0), butmore damage was observed in the secondearthquake that occurred the same year (M7.3), as some buildings had been alreadyweakened by the first earthquake (RussianFederation, WHE Report 55). Large-panelbuildings performed well in the 1977Vrancea (Romania) earthquake (M 7.2)and in subsequent earthquakes in 1986 and1990 (Romania, WHE Report 83).SEISMIC-STRENGTHENINGTECHNOLOGIESAccording to WHE reports, no majorefforts have been reported regarding seismicstrengthening of precast concrete buildings.However, seismic strengthening of precastframe buildings was done in Uzbekistan(WHE Report 66). The techniques usedincludethe installation of steel straps at the columnlocations (see Figure 16) and reinforcing thejoints with steel plates to provide additionallateral confinement of the columns.Conclusion:

Overall, the precast concrete industryhas come a long way since its inceptionduring the Ancient Roman Era. It has comethrough many design innovations andupgrades. Today’s precast concrete is amongthe strongest and structurally sound buildingmaterials on the market, allowing for theoverall innovation of architecture tocontinue to thrive as well. In addition to itsstructure and limitless design options,precast concrete is among one of thegreenest building materials on the markettoday as it pertains to the scale in which it isused. Because of all of the aforementioneduses and benefits of precast concrete, thereis good reason to see its usage continue andexpand moving into the future.