Castellated beams havebeen used as struc-tural members in theEurope and theUnited States since

the early 1900’s. The theory behindthe castellated beam is to increasethe beam’s depth and strengthwithout adding additional mate-rial. Prior to automated cutting andwelding technology, the manufac-turing process used to make castel-lated beams was to cut the beamapart and weld them back togethermanually. This was labor intenseand costly. With rising labor costs,expanded production, gained effi-ciencies and advancement in man-ufacturing of rolled shapes, the useof castellated beams had beenabandoned until recently. SMIbegan manufacturing their versionof the castellated beam, the Smart-BeamTM, about 18 months ago witha single target market in mind;Composite Floor Beams.

Manufacturing ProcessSmartBeams are manufactured

by cutting a WF beam in a zigzagpattern along the web with an au-tomated computer-controlled

plasma torch, separating the twohalves, then welding the web postback together at the high pointswith an automated submerged arcwelding process as shown in Fig. 1.This process increases the depth ofthe beam by approximately 50%,therefore increasing the strengthand stiffness by 40%. An originalW18x35 after castellation becomesa CB27x35 with openings spaceduniformly along the web. Thebeams are welded back togetherwith camber build in, thereforeeliminating a costly offline camber-ing process.

SmartBeam DesignThe SmartBeam design is very

similar to that of Vierendeel trussdesign in that the shear componentcreates bending through the webopenings that has to be accountedfor in the bending stress calcula-tions. The design procedure forcastellated beams is shown in de-tail in a time tested and respectedpublication, The Design of WeldedStructures, by Omar Blodgett.There are 3 basic design checks indesigning castellated beams.

Modern Steel Construction / May 2001

Billy Milligan, P.E.

Billy Milligan, P.E.,

is Vice President of

CMC Steel Group

dba SMI Steel Products

in Rockwall, TX.

Vierendeel BendingVierendeel bending is a combi-

nation of true bending and vieren-deel bending created by sheartransfer across the web openings asshown in Fig. 2. The primary bend-ing stress is generated by momentor a flange force in the beam, eithertension or compression, and is ap-plied uniformly over the area of thetee. The secondary stress, or veiren-deel stress, is caused by the transferof shearing forces over the webopening. The resulting overallstress is the sum of the primary andsecondary stresses. The tee sectionis then analyzed by traditionalmethods in accordance with AISC’sASD 9th edition or LRFD 2nd edi-tion steel design codes.

ShearThe shear in a castellated beam

has to be checked both verticallythrough the opening and horizon-tally through the web post. Thevertical shear check is checked inthe same manner that one wouldcheck shear in a WF beam. The hor-izontal shearing stress, more com-monly called vq/It stress, has to bechecked at incremental intervalsvertically along the web post be-cause of the variability of the crosssection.

Web Post BucklingThe web post design procedure

was developed by Drs. Dick Red-wood and A. A. Aglan of McGillUniversity in Montreal, Canada.

Dr. Redwood has spent severalyears testing and developing de-sign procedures for both castel-lated beams and beams with webopenings. Their tests include bothfull scale destructive as well as fi-nite element model tests on a fullrange of opening heights, openingwidths, web post widths, and webthicknesses. Their published tablesgive the ultimate capacities of thefull range of the previously men-tioned variables. The published ca-pacities are simply compared to therequired resistance for the geome-try of opening selected with a fac-tor of safety or load factor applied.

Cost Effective ApplicationsAlthough there are numerous

different applications for which aSmartBeam can be economicallyused, they all have common char-acteristics. These include compos-ite application and longer thantraditionally used spans. As a re-sult of the deeper section, theSmartBeam has better long spancapacities and vibration character-istics than other structural floorframing materials.

Office Buildings/MezzaninesSmartBeams are ideally suited

for long-span composite floor con-struction. Today owners are look-ing for more flexibility in spaceplanning which leads to more col-umn-free space. This economicallygives them floor flexibility withoutthe intrusion of interior columns.By incorporating the HVACthrough the web openings, about1’-0” of floor-to-floor height can be

taken out of the height of the build-ing compared to other long-spanfloor systems.

Parking GaragesWith increasing real estate costs,

developers have determined that itis more cost effective to buy lessproperty and to maximize theirspace by using multi-level parking.SmartBeam parking decks havebeen constructed for as little as$4500 per space. This is a savings ofapproximately $1500 per spaceover a comparable pre-cast parkinggarage. For an average sizedgarage of 500 cars, this is a savings

of $750,000 per garage. The long-span nature of parking garageslends these types of structures tothe use of SmartBeams. This alter-nate gives owners a clear-span, col-umn free parking area with thequality of a continuous slab floordeck and the efficiencies and costcompetitiveness of a steel framedparking deck.

Summary and ConclusionThe castellated beam is a time

proven composite structural floor-framing member that with thepower of the computer and auto-mated manufacturing equipmenthas become a more economicalfloor framing system. With Smart-Beams excellent long-span capabil-ity, vibration characteristics andcost effectiveness it will become thelong-span composite structuralmaterial of choice for floor con-struction.

May 2001 / Modern Steel Construction

figure 1

figure 2