Structure Magazine - Multi-story Tilt-up Building

8/12/2019 Structure Magazine - Multi-story Tilt-up Building

http://slidepdf.com/reader/full/structure-magazine-multi-story-tilt-up-building 1/4STRUCTURE magazine July 2008

d i s c u s s i o n a n d

u p d a t e s o n s t r u c t u r a l m a t e r i a l s

B U I L D I N G

B L O C K S

22

By Mark E. Remmetter, P.E., Shane A. Walters,P.E., and Joseph J. Steinbicker, P.E., S.E.

Tilt-up construction is continuing togrow at a record pace. Over the pastthree years, about one billion squarefeet of tilt-up wall panels have beenconstructed to provide approximately2.3 billion square feet of usable floorspace. While a significant percentage ofthis growth has come from single-storybuilding construction, the multi-storytilt-up market has really begun to flour-ish. The design of these multi-story tilt-upbuildings is significantly more involvedthan that of single story buildings.

EOR versusSpecialty Engineer While many multi-story tilt-up buildingsare designed in whole by the engineerof record (EOR), quite often the designof the tilt-up panels is left up to a tilt-up specialty engineer. This delegationof duties need not be a stressful eventfor either professional, as long as eachis aware of his or her specific responsi-bilities. The tilt-up specialty engineer’sscope can be restricted to only panelthickness and reinforcing design, or can

include many other items such as paneloverturning stability analysis and de-sign of panel-to-structure and panel-to-panel connections.It is very important for the EOR to

make it clear in the design documents what is expected from the tilt-up spe-cialty engineer. Floor and roof loads to

sist the lateral loads. In this case, interiobracing or shearwalls must be provideto resist the lateral loads in the short di

rection of the building. The stiffness othe bracing or shearwalls should be compared to the stiffness of the endwalls tensure that the latter do not actually enup resisting the lateral loads.The design of panels with many open

ings is much more complicated thathe design of simple solid panels. Whilthe in-plane shear is usually provideas a load per foot of wall length, thdistribution of this load among adjacent panels is typically not uniformThe lateral loads are distributed to th

wall panels based on the relative stiffness of the concrete column sectionbetween panel openings. This methoof load distribution applies not onlbetween panels, but also between strip

within a panel. For multi-story buildings with many panels and openings, iis most efficient to model each wall as series of concrete frames, including botcolumn strips between openings anbeams strips above and below openingfor each panel. The panels are linkedtogether at each floor level in the mode

so that the lateral loads are distributeacross them. Vertical loads are also included in this analysis, providing complete picture of the axial loadsshears and bending moments in the column strips and beam strips.It is not unusual to find that th

resultant of the vertical loads will bshifted towards one side of the panel duto the overturning effect of the lateraloads. Because of the frame action o

Multi-Story Tilt-Up BuildingsA Design Approach

the wall panels should be listed on theframing plans, including both beamreactions and distributed loads, such

as those along an endwall. These grav-ity loads should be broken down intodead and live load components so thatan efficient design can be achieved forthe concrete panels. The wind upliftloads should also be specified for theroof framing members.It should be obvious from the design

drawings whether the tilt-up panels areexpected to resist in-plane lateral loadsfrom the floor and roof diaphragms.Design values for both wind and seis-mic loads must be provided, if they

are applicable. Each floor plan shouldindicate the loads to each individual

wall that participates in resisting lateralloads. Note that these loads might varyfrom floor to floor, especially in highseismic areas.

In-Plane Loadsand Lateral

StabilityMany of the multi-story

tilt-up structures currently

being constructed are of-fice buildings. These rangefrom three to six stories,are typically rectangular inplan, and have tilt-up pan-els that are full of punchedopenings for windows. Ona rectangular office build-ing, the endwalls may nothave enough capacity oroverturning stability to re-

Five-story tilt-up building. Courtesy of Tilt-Up Concrete Association, © John Gillan Photography.

Mu

Vu

GRAVITY LOADS

LATERAL

LOAD

LATERAL

LOAD

LATERAL

LOAD

Computer model for in-plane load analysis.



the panel, the beam strips above and belowthe openings can be subject to relativelylarge shears and moments that must beaccounted for in the design of the panelreinforcing. Design of the column stripsshould account for the axial loads, the in-plane moments from the frame analysis,and the out-of-plane moments caused by

wind or seismic ef fects .

Load-Bearing versusNon-Load-Bearing PanelsMulti-story tilt-up building wall panels can

be either load-bearing or non-load-bearingfor the interior framing members. For non-load-bearing panels, perimeter columns andedge beams are provided to support thevertical floor and roof loads. Load-bearingpanels eliminate the need for this perimeterframing, which in most cases leads to a moreeconomical design. In addition, the use ofnon-load-bearing panels as shear walls issomewhat difficult, since the connectionbetween the panel and the structure mustaccommodate the vertical deflection of theperimeter framing. In fact, it is desirableto use load-bearing panels as shear wallsbecause the added vertical loads increase theoverturning resistance.

Some recent multi-story office buildingshave used a combination of load-bearingpanels and perimeter framing. The typicalfloor beams at the sidewalls and the deck atthe endwalls are supported by the panels,

while the girder reactions are supported onperimeter columns. This arrangement actually

works quite well when the girder reactions arelarge and would otherwise require a heavilyreinforced concrete wall section or pilaster.

The increased vertical forces on load-beapanels can have an effect on the panel thness, leg width between openings, and pareinforcing. However, this effect is not ususignificant, because for multi-story panels, cstruction loading and lifting configuration affect the design. In many cases, the constrtion loads are actually more critical for the deof the panel thickness and reinforcing than final, in-place design loads, although obvioboth should be accounted for in the design.

Second tier tilt-up panel lifted into place.

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Structural Integrity –Full-Height versus

Stacked PanelsTilt-up buildings up to three stories

tall are generally constructed usingfull-height panels. For taller buildingswith more floors, it may be moreeconomical to construct the wall using

a stacked panel arrangement. A stackedpanel configuration is constructed bylocating a horizontal panel joint a fewfeet above one of the supported floors,typically at a reveal location so that thejoint can be hidden. The height of theindividual panels in the stack can varydepending on design and constructionissues. The bottom building panels areerected first, followed by constructionof the supported floors up to the toplevel of that panel. The second tier ofpanels is then erected on top of the

lower panels and braced down to thealready constructed floor. The selectionof panel heights and sequencing ofthe stacked panels should involvethe general contractor that will beperforming the construction. A variety of factors can influence the

decision to use a stacked panel arrangement.These include available panel casting area,crane size, lifting hardware capacity, panelthickness, panel bracing configuration andconstruction sequence and timing. ACI 318 has structural integrity provisions

that must be incorporated into the design ofmulti-story tilt-up buildings. These require-ments address longitudinal and transverse tierequirements for supported slabs, as well asvertical tension tie requirements for the wallpanels, including stacked panel configura-tions. For bearing wall structures three ormore stories in height, there are specific loadand spacing requirements for the structuralintegrity connections.The connections between stacked pan-

els are designed to transfer all design loadsacross the joint. These include in-plane

shear from diaphragm loads, out-of-planewind and seismic forces, tension forces dueto panel overturning, and in-plane momentsacross the horizontal joint. The structuralintegrity requirements are not additive tothese design loads. Connections that relysolely on friction to transfer loads are notallowed. For stacked panels, a minimumof two connections between panels capableof providing a nominal tensile strength of3000 pounds per horizontal foot of wall

are required. Each connection must have anominal tensile strength of 10,000 pounds.These connections are typically placed at amaximum spacing of four feet on center,

with some concentration at each end of thepanel. One commonly used type of con-

nection that meets the above design require-ments uses reinforcing bars extending fromthe lower panel that are inserted into a groutsleeve in the upper panel.

Erection Bracing OptionsThere are three basic options for bracing

multi-story panels. The simplest and mostcommon configuration is bracing to theslab on grade on the inside of the building.The slab must be checked to ensure thatit can resist the brace forces. This includesverifying that it meets not only the stability

requirements for the uplift and slidingforces from the brace reaction, but also thatthe slab has the strength (i.e., thickness andreinforcing) to resist these loads. Panelscan also be braced to the outside of thebuilding using a cast-in-place deadmanor helical ground anchors. The use of thehelical ground anchors has only recentlybeen applied to tilt-up construction. Theyhave the advantage of fast installation andremoval, as well as a verifiable capacity. The

Grout sleeve connection between stacked panels.

anchors are field tested to ensure thathey meet the load requirements othe panel braces. With stacked panels, bracing the uppepanels to the building floor structurbecomes a concern. The braces are atached to the supported floor below thhorizontal panel joint between stackepanels. The supported floor slab capac

ity should be checked to verify thait has the strength to resist the bracforces. Similarly, the floor framing mushave the strength to support the bracloads. For a precast concrete floor system, the bracing loads and locationshould be provided to the precast supplier for incorporation into the desigof those elements.

Connection Design atFloors and Roof

Connections of tilt-up panels to thmain structure are required to resisgravity loads, wind (in- and out-ofplane), seismic (in- and out-of-plane)and thermal bowing. Non-load-bearinpanel-to-structure connections musbe designed to allow vertical deflec

tion of the perimeter load-bearing steel oconcrete framing. Slotted inserts are usuallsufficient to achieve this, and are availablfrom several manufacturers. In addition tthe wind and seismic out-of-plane forcesthe connections should also be designed t

resist thermal bowing. The method of calculation for these forces can be found in thPCI Design Handbook .For buildings constructed with load-bearin

panels and cast-in-place floor slabs, joist obeams bearing on the sidewalls are usuallconnected to panels using framing angleor seated connections attached to embeplates cast into the panel face. The use opocketed connections is not recommendedas it can interrupt the vertical reinforcing inthe structural strips between openings. Obuilding endwalls, the panels will be directl

connected to the cast-in-place slab using reba wall ties or a similar system. These ties witransfer diaphragm loads, out-of-plane windseismic forces, and gravity loads. For concretslabs over metal deck at endwalls, the gravitloads are supported on a continuous edgangle, which is supported by embed platecast into the panel. Wall ties should also bused on the sidewalls to transfer lateral loadinto the panels and out-of-plane loads fromthe panels into the floor system.



Shane Walters, P.E. is a Project Engineer with Steinbicker & Associates, Inc., in Dayton,Ohio. He has been in the tilt-up design field for 6 years. Shane may be reached via email [email protected].

Mark Remmetter, P.E. is Vice President and managing principal of Steinbicker & Associates,Inc. He has been designing tilt-up structures for over 16 years. Mark may be reached via emailat [email protected].

Joe Steinbicker, P.E., S.E. is President of Steinbicker& Associates, Inc. He is a founding

member of the Tilt-Up Concrete Association and currently serves on its Board of Directors.He is also a founding member and past chairman of ACI Committee 551 – Tilt-Up ConcreteConstruction. Joe may be reached via email at [email protected].

For buildings with precast concrete floor androof systems, coordination between the precastsupplier and the tilt-up panel engineer isrequired to determine the locations and typesof connections that are needed. For gravityload support of the precast systems, a corbel isoften incorporated into the wall panel design.This can be a continuous corbel for hollow-core planks or individual corbels for double

tee stems. A positive connection to the panelsis still required to transmit the in- and out-of-plane forces between the floor system andthe wall panels while accommodating verticaldeflection of the members at the endwalls.Roof connections for steel-framed multi-

story tilt-up buildings are very similarto those used for single-story structures.Lateral diaphragm loads on the sidewallsare transmitted to the wall panels throughthe framing member connections or directlyinto the wall panel through a continuousperimeter angle, which can be connected to

the wall panel with bolts or embed plates.Out-of-plane loads are transferred directlyinto the roof framing members. At buildingendwalls, the perimeter angle connectionto the wall panels transfers wind uplift andgravity loads, diaphragm shears, and out-of-plane loads between the tilt-up paneland roof structure. Panel-to-roof-structureconnections for precast concrete roof systemsare similar to those used for precast concretefloor systems.

Conclusions

While the design of multi-story tilt-upbuildings shares many similarities to the de-sign of single-story tilt-up buildings, there area number of additional engineering issues toaddress. The design engineer must understandthe load path and how the applied loads canaffect the wall panel design, which may haveportions affected by torsion, uniaxial bend-ing, biaxial bending, tension and compres-sion. In addition, a thorough understandingof the design tools (i.e. computer programs)used to engineer the tilt-up wall panels isequally important.

Building design and construction is a teameffort. Cooperation and coordination betweenthe design professionals (i.e., the EOR andtilt-up specialty engineer) is critical to thesuccessful design of a tilt-up building. Theyshare a common goal: the safe engineeringdesign and construction of a building thatmeets the needs of its owner.▪

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Structure Magazine - Multi-story Tilt-up Building