In Ma rch 1995, as superi n t e n-dents for Blake Co n s t ru c t i o nCo., we had to tackle one of thebiggest challenges we’d eve r

faced: creating 33 re i n f o rc e d - c o n-c rete raker beams for a 13,000-seat,single-tier arena that’s part of theSan Diego State University StudentActivity Center.

The engineering drawings for the

105-foot-long, 2-foot-wide, 9-foot-deep raker beams we re the mostcomplex we’d ever seen. Be c a u s ethe beams needed to be finished byMa rch 1996, it was our job to devisea forming system that could accom-plish the task quickly and efficient-l y. We came up with a plan that per-mitted the raker and concoursebeams to be poured as a single unit.

Conventional Forming Techniques Rejected

Our approach called for assem-bling a 20,000-pound rebar cage onthe ground, lifting the cage ontoc o l u m n s, and then enclosing itwithin a single 55,000-pound self-spanning steel form. Because of thes y s t e m’s complexity, we constru c t-ed a 4-foot scale model to help pro-

Raker-Beam Construction Requires Rugged Steel Forms

BY JEFF STEELE AND MARK LARSEN

System devised by California arena builder permits bothraker and concourse beams to be poured as a single unit

Self-spanning steel formwork makes the grade at San Diego State University: Arena construction required two custom formsto create 33 reinforced-concrete raker beams.

ject engineers, carpenters, iro n-w o rk e r s, and managers understandh ow it would work .

Site and equipment limitationsmade this the most practical sys-tem for the project. We considere dother forming techniques, but hadto reject them for seve ral re a s o n s.

First, we could not assemble acomplete rebar cage in the self-spanning steel form because ourc rane wouldn’t have been able tohandle the combined 75,000pounds of rebar cage and form-w o rk. Lifting each 55,000-poundf o rm re q u i red approximately 90%of the capacity of the cra n e. Theraker beam re i n f o rcement was ex-t remely dense to meet Zone 4seismic re q u i re m e n t s, and iro n-w o rkers re q u i red full access to allsides of the cage.

Second, assembling re i n f o rc e-ment within a steel form after it hadbeen lifted in place would not havebeen economical. Mo re form unitswould have been re q u i red to main-tain a reasonable construction cy-c l e, and attempting to install re i n-f o rcement within a sloped form ,sometimes as high as 50 feet above

g ra d e, would have re q u i re de ven more iro n w o rk e r sand exc e s s i ve amounts ofc rane time.

T h i rd, using job-builtwood forms would have re-q u i red extensive shori n g ,but the soil could supportonly limited shoring we i g h t .The project is located onthe site of the former Az t e cBowl football stadium, ori g-inally constructed in the1930s on as much as 40 feet

of fill. We had to sink more than 30050-foot-deep caissons to supporta rena columns and slab work. Wec o n s i d e red it unsafe to pour heavybeams supported on shoring thatrested on filled and often slopingg round. It could have resulted inbeam shifting and sagging.

Formwork Assembly

Because there we re raker beamsof two different slopes, the unusu-al construction technique re q u i re dtwo different ra k e r-beam form s,e n g i n e e red and manufactured bySymons Corp., Des Plaines, Ill. Theforms were 105 feet long and 2 feetw i d e, one with a slope of 25.2 de-g rees and the other with a slope of24.8 degre e s. Both could accom-modate field modifications.

The raker and concourse beamf o rms we re constructed pri m a ri l yof Sy m o n s’ standard steel form-

Figure 1. The photoat left shows acompleted rakerbeam (left) and a20,000-pound rakerbeam reinforcingcage (right). Eachcage was assembledon the ground andthen lifted intoplace.

Figure 2. A 105-foot-long, 2-foot-wide custom raker formis flown into place over thereinforcing cage. Lifting each55,000-pound form requiredapproximately 90% of thecrane’s capacity.

w o rk manufactured with integra ls t i f f e n e r s. These standard pieceswe re combined with the form i n gs y s t e m’s major custom pieces, lo-cated at the raker nose, the pie-shaped intersection of the ra k e rand concourse beams, and the col-umn support points (columns sup-p o rted the rebar cage at its centerand each end).

The integral soffit of the ra k e r-beam form consisted of a series of ahinged doors that could be openedfor setting and stripping. Each doorwas small enough and light enoughto be opened and closed by hand.Wo rking platforms we re mountedon opposite sides of the form, onep l a t f o rm at the top and the other atthe bottom.

The riser components within thera k e r-beam form we re drilled andinstalled on-site. They we re setalong a slightly ascending para b o l-ic curve rather than on a slopings t raight line. This ensures that thesightline distance for any spectatora b ove the head of a spectator in therow below is identical thro u g h o u tthe are n a .

The concourse support beamconsisted of three separate pieces,including a soffit with working plat-

f o rms and individual sidewalls. Thesoffit was jacked, stripped, andm oved to the ground by crane us-ing a C-hook picking unit that cra-dled the form .

Consistent Construction Sequence

The ra k e r-beam re i n f o rc i n gcages we re assembled (usually twoat a time) in jigs adjacent to a ply-wood wall. This wall was mark e dp recisely with the riser locationsand the rebar ties’ 65-degree anglere l a t i ve to the longitudinal steel.Once the cages we re lifted intop l a c e, the ties we re ve rtical. Iro n-w o rkers used a T- s q u a re device top roject the riser locations on thewall out to the jig.

The construction sequence re-q u i red that the ra k e r’s rebar cage beset first (Fi g u re 1). It was picked us-ing a spreader bar with attachmentse ve ry 10 to 15 feet. To minimize sag-ging, light tempora ry shoring wasplaced under spans as needed.

The concourse support beamf o rms we re moved into place next,a l l owing for subsequent installa-tion of the beam’s re i n f o rc e m e n t .

The 55,000-pound ra k e r- b e a mf o rm was then set over the entire

raker cage (Fi g u re 2) and bolted tothe columns. The long ra k e r- b e a mf o rms we re flown using eight at-tachment points—four matchinglocations on each side—approx i-mately 21 feet apart. To maintainthe slope during the lift, we usedexact-length chokers manufac-t u red for the job.

Once the ra k e r-beam form hadbeen set over the rebar cage andbolted in place, the crane was out-fitted with slings to lift the slight sagin the rebar cage so that the formsoffit doors could be fully closed.The raker form was rigid enough toe n s u re negligible deflection.

Bolsters within the raker formmaintained consistent cage-to-formwall spacing. The cage was assem-bled to much tighter dimensionalt o l e rances than typical for concre t ebeam work so that spacing and cov-er specifications could be met.

Precise Concrete Slump Required

Po u ring each integral ra k e r- c o n-course beam re q u i red 80 cubicy a rds of 5000-psi concre t e. A super-p l a s t i c i zer was added to the con-c rete at the jobsite to produce thed e s i red slump of 5 inches. (Al-though specifications re q u i red on-ly a 4000-psi material, we chose thish i g h e r- s t rength concrete to achievethe two-day, 3000-psi strength re-q u i red for stripping.) Pl a c e m e n tbegan at the bottom of each ra k e rbeam and took four to five hoursusing either a 3-cubic-yard bucketor a 28-meter boom pump.

As concrete placement pro c e e d-ed, we had to be sure slump washigh enough to allow concrete tocompletely fill the rebar cage, butnot so liquid as to ove rf l ow the ri s-er form s. Ready mix truck delive ri e swe re carefully timed to allow thep receding lift to stiffen enough top re vent riser ru n ove r.

Connector Beam Form

The three self-spanning steelf o rms used to construct the con-nector beams between adjacentraker beams (Fi g u re 3) also fea-t u red an unusual design, with

Figure 3. One of the three self-spanning steel forms used to construct theconnector beam and lock adjacent raker beams together. Lowered into placeanywhere from 15 to 50 feet above grade, these connector-beam forms could bepositioned and stripped as a single piece.

hinged doors at the soffits, allow i n gthe form to be lowe red into placeand stripped as a single piece. Wechose this type of self-spanning steelf o rm for both its efficiency and safe-ty and expect to use it again in the fu-t u re.

Since the are n a’s connector beamswe re only 3 feet wide and locateda n y w h e re from 15 to 50 feet aboveg ra d e, iro n w o rker safety was imper-a t i ve. To help meet this need, eachf o rm was 6 feet wide. The extra 3 feetcontained a working platform withinthe form to improve iro n w o rker safe-ty and increase materi a l - s t o rage ca-p a c i t y. A wood bulkhead was con-s t ructed and braced inside the steelf o rm to establish the beam dimen-s i o n s.

Successful Outcome

We we re happy to see the beamssuccessfully completed on schedulelast Ma rch. At the height of constru c-tion, crews finished as many as fourra k e r-concourse beams per we e k .And thanks to our unique form i n gsystem, Blake Co n s t ruction Co. isstill scheduled to complete the are-na by Ma rch 1997.

CreditsOwner: San Diego State UniversityContractor: Blake Construction Co. Inc.,San DiegoReinforcing steel contractor: Pacific Re-inforcing Steel, Santee, Calif.Ready mixed concrete supplier: CalMatConcrete & Aggregate, San DiegoArchitect: Carrier Johnson Wu, SanDiegoStructural engineer: Martin/Martin,Wheat Ridge, Colo.

J e ff Steele is assistant superinten-dent and Mark Larsen is concrete su-perintendent for Blake Constru c t i o nCo. Inc., San Diego. Larsen has 25years of public-works construction ex-perience, and both Steele and Larsenhave worked on many large public-works projects in the San Diego area.

PUBLICATION #C960738Co py right © 1996, The Ab e rdeen Gro u pAll rights re s e rve d