In addition to the customarydesign loads for the roof struc-ture, additional criteria included:compatibility with aircraft clear-ances; ability to support a full-coverage 5-ton bridge crane;rapid speed of erection; mini-mum weight structure; architec-tural envelope that will reducewind forces; structural geometrythat would not interfere withaviation clearances; low mainte-nance requirements; and roofslope geometry that optimizedroof drainage requirements.

Alternate concepts were stud-ied using computer models. Theconcepts were evaluated using acomparative system analysisthat quantified the following:weight of steel per square foot;erection time; door deflection cri-teria; building wind envelope;building wind envelope; resis-tance to large-scale horizontaltorsion stresses due to designwind forces; and foundation sys-tem resisting uplift and horizon-tal forces. In addition particularattention was paid to accurateprediction of vertical movementof the structural element abovethe hangar door, since this wouldeffect the efficiency of the motor-ized door system.

STRUCTURAL SYSTEM

The selected concept is a two-way structural support systemfor the hanger roof and a conven-tional steel framed with concreteon metal deck floor system forthe two-story support facilitystructure. An expansion joint is

Modern Steel Construction / May 1997

THE NEW NORTHWESTAIRLINES MAINTENANCEFACILITY IN DULUTH, MN is

the airlines most important pro-ject constructed to date. Thefacility will provide heavy main-tenance “C” and “D” checks forAirbus A320 and Boeing 757 air-craft with the capability of ser-vicing wide-body aircraft if need-ed. In short, the new facility iscritical for keeping the airlinesplanes in the air.

The new facility includes: alarge hangar; shop areas; engineplant; and a materials operationthat stores and distributes mate-rials to the hangar, shop and lineoperations.

For this facility, rather thanconcentrate solely on the hangerportion of the project, the designgoal was to consider the hangararea and direct support shopactivities as a single unit. Thisapproach simulated the tech-niques of flexible manufacturingand the concept of common usework cells or work cell group-ings, with the intent of increas-ing equipment utilization, reduc-ing building square footage andincreasing efficiency of opera-tions.

The key structural element tothe project was designing a roofstructure that would resistheavy snow loads with icingeffect, large horizontal forces dueto wind exposure of the hugebuilding envelope, and upliftforces generated by the designwind speeds when the hangardoors are in the open position.

A FRESH LOOKAT HANGAR

DESIGN

A new 200,000-sq.-ft.maintenancehangar is thecenterpiece of

NorthwestAirlines support

and serviceefforts

By Sofronio C. Mendez, P.E.

provided between the hangarand support facility to minimizeadditional thermal stresses. Thesupport facility has its own brac-ing system in resisting lateralforces in the North-South direc-tion, with a portion of its East-West lateral resistance derivedfrom the hangar structure.

VERTICAL LOAD SYSTEM

The roof structure is designedas a two-way vertical load carry-ing system. A box truss spans315’ across the front of thehangar, creating a full-widthopening. Four trusses span 230’from front to rear of the hangarstructure distribution roof andcrane loading between the boxtruss at the front and the bracedbays at the rear of the structure.Structural steel weight and boxtruss deflection are design para-meters that were minimized bycreating a cantilevered action inthese four trusses. The can-tilevered action was provided bya tension tie member that trans-ferred top chord tension forcesto column in the 60’-deep bracedbay. The resulting cantilevertrusses actually behave aspropped cantilever trusses withinflection points located approxi-mately 160’ from the box truss.The tension force in the tie mem-ber creates an uplift condition onthe column of the braced bay.The tension is resisted by rockanchors installed through thepile cap and anchored 15’ intothe granite bedrock at a depth of10’ to 25’. Uplift on the columnunder normal working conditionswas minimized by framing theconcrete second floor so thatgirders are supported at theuplift columns.

Cantilever trusses aredesigned with two differentdepths to accommodate the posi-tion of the truss with respect tothe required slope for roofdrainage. Depths are 26’ and 28’.Truss panel sizes are chosen as30’-9”. The cantilever trussesare designed to accommodateloadings for snow and ice per theMinnesota Building Code,mechanical equipment located at

Modern Steel Construction / May 1997

Modern Steel Construction / May 1997

the truss bottom chord level, andtwo crane systems with inter-locking capabilities suspendedfrom the truss bottom chord.

The box truss is comprised oftwo parallel trusses 42’ deep,spaced 15’ apart and crossbraced together at the four loca-tions where the cantilever truss-es intersect. The box truss spans315’ across the main chords ofthe cantilever trusses, whichpresented a challenge to incorpo-rate the multiple functions ofsome elements. This was accom-plished by creating a doublelevel bottom chord that func-tioned coincidentally with thebottom chord lateral bracing sys-tem and the vertical box trusssystem.

The sequence of tighteningthe connections between the boxtruss and supporting columnswas a critical erection concernbecause the box truss is support-ed on slender columns. To pre-vent secondary stresses in thesupporting columns created bythe initial rotation of the boxtruss ends, bottom chord connec-tions were not tightened until allgravity loads were applied to thestructure.

Sub-framing in the form ofjoist and joist girder were usedto support the roofing system.Bottom chord framing was pro-vided to support moving cranesand mechanical catwalks. Thediagonal bracing at the bottomchord level of the trusses servesas a diaphragm, reacting withthe steel roof deck in resistinglongitudinal and transversewind forces. The diaphragmtransfers all the lateral forces tothe vertically braced frames atthe perimeter and interior of thebuilding.

The maximum vertical deflec-tion is reflected in the mechani-cal connections at the top of thehangar door. These deflectionsare accommodated by a piston inthe door mechanism that accom-modates vertical movement.

Of utmost importance to thedesign and constructibility of thetruss system was the connectiondetail at the top and bottom of

HANGAR COSTSAVINGS

By Larry Kloiber, P.E.

THE LOW BID OF $4.5 MILLION

FOR THE STEEL PACKAGE ON THE

NORTHWEST AIRLINES MAINTE-NANCE AND ENGINEERING FACILITY IN

DULUTH, MN, WAS $500,000 OVER

THE PRELIMINARY BUDGET. As a result,the airline opted to reduce thescope of the project and also to askLeJeune Steel Company to submitcost reduction proposals to the pro-ject’s construction manager,McClier.

The scope reduction, whichdeleted provisions for a future sec-ond floor, along with 14 proposed

modifications, generated $300,000 worth of savings.• Change roof deck finish. The roof deck finish was changed from a G90

galvanized to standard grey paint. Since the entire roof interior was fieldpainted white, there was no change in final appearance.

• Special design of joist and joist girders. The joists and joist girders wereconservatively sized as standard uniformly loaded SJI designs. McClierwas able to supply the special bridge crane and snow drift loads so it waspossible to custom design for exact load conditions, which saved 30 tons.

• Eliminate sub-framing in trusses. The roof trusses were designed withdouble angle braces midway between panel points to reduce minor axisslenderness and save overall weight. LeJeune did a labor-vs.-material coststudy and determined it was less expensive to increase the sizes of someof the main members, thereby eliminating the shop and field labor associ-ated with the braces.

• Revise vertical bracing. The vertical bracing for the hangar was designedas tension members in a X pattern. Cost studies indicated single diagonalsdesigned for both tension and compression would be less costly to fabri-cate and erect.

• Use of oversize holes for trusses. The roof trusses were specified to behigh-strength bolted with standard holes. This would have required fullshop assembly of trusses up to 30’ deep and over 200’ long. The use ofoversize holes resulted in a significant savings compared to the cost ofassembling, drilling the holes and then disassembling to ship—along withthe impact of this procedure on the project schedule. It was determinedthat by using 1”-diameter slip critical A390 bolts in oversize holes alongwith special dimensional checks and limited sub-assembly checks, all ofthe truss members and gusset plates could be drilled on the CNC drill line.

Larry Kloiber has been with LeJeune Steel for more than a quarter centu-ry. He is currently employed as a structural engineer and fabrication consul-tant. This article is based on a paper from this year’s NSCC.

the exposed tension struts. Thedesign force in each tension strutis 1000 kips, which must betransmitted to the verticallybraced frames inside the supportfacility.

Of equal importance is thedetailing of the box truss gird-er—specifically, making surethat distortion of the truss cross-sectional geometry is minimal totake advantage of its full verticalload-carrying capacity. Two-waybracing was utilized to achievethe required stiffness. Because ofthe possibility for cumulativejoint movement contributing tothe truss deflection, all of thebolts were fully tensioned usingload indicator washers.

LATERAL LOAD SYSTEM

The hangar lateral bracingsystem is symmetric in theNorth-South (front-to-rear)direction and asymmetric in theEast-West (side-to-side) direc-tion. Lateral bracing bays arepositioned on three sides of thestructure, the rear wall and thetwo side walls, plus the fourbraced bays that are part of thecantilever truss system. Thissystem allows the front wall ofthe hangar to be free of bracesand maximizes the availablefloor space for aircraft position-ing. The side walls are bracedwith two-bay-wide vertical brac-ing systems. The rear wall con-tains two single-bay bracing sys-tems connected at the top by avertical truss along the top of therear wall. Together, the systemacts as a rigid frame in transfer-ring lateral loads to the founda-tion.

Lateral loads in the North-South direction transfer to thetwo side bays and the fourbraced bays through the girt sys-tem, roof deck diaphragm, andthe bottom chord bracing system.

Lateral loads in the East-Westdirection are transferred to thebraced truss-frame at the rear ofthe hangar through a transla-tional mode, and to the sidebraced bays through a torsionalmode. The load pathflows fromthe girt system of the side walls

By Larry Kloiber, P.E.

THE PROJECT SPECIFICATIONS REQUIRED THE FABRICATOR

TO BE RESPONSIBLE FOR THE DESIGN OF THE CONNEC-TIONS. However, LeJeune Steel Company’s (LSC)

policy is to develop connections and submit thesesketches and calculations to the engineer of record forapproval, as per the AISC Code of Standard Practice.Because the airline would not waive the connectiondesign requirement, LSC retained ComputerizedStructural Design (CSD) to design connections based onthe provided forces.

The detailing phase of the project began with a meet-ing attended by the owner, structural engineer, connec-tion design engineer, steel detailer, steel erector, joistsupplier, deck supplier and LeJeune project manage-ment, engineering and operations personnel. Basic con-nection concepts, design and approval processes, mate-rial specifications, and construction sequences were alldiscussed and agreement was reached.

Connection design and detailing was a team effortinvolving CSD, Computerized Detailing Inc. (CDI) andLSC. The process started with the project structuralengineering preparing the design forces for all of theconnections. LSC then proposed various types of con-nections for each member. CDI prepared connectionsketches showing the geometry and preliminary details.LSC reviewed these sketches for constructability andeconomy and forwarded their comments to CSD, whosized all elements of the connection and made anyrequired structural modifications. CDI then used thesedesign sketches to detail all of the connections, whichCSD then reviewed and approved. These connectionswere also submitted to McClier for approval as the engi-neer-of-record.

In developing connections, in addition to the normalgoals of structural integrity and fabrication economy, aspecial effort was made to simplify erection. Because ofthe complexity of the framing, and the anticipated condi-tions at the site, it was determined that the major slipcritical connections should be made on the ground. Anyconnections that needed to be made in the air requiredthe use of 90’ aerial lifts operating on rough ground. Itwas decided to make all major truss member bolts inde-pendent of any sub-framing connections. This wasaccomplished by field bolting sub-framing such as brac-ing to connection plates shop welded to the main trussgusset plates. The bolts in the gussets could be ten-sioned and inspected before the trusses were erected.All of the main roof trusses also were detailed to mini-mize bolt requirements. Chord members were upsizedto make it possible to splice at alternate panel points.Splices were located at panel points to utilize gussetplates and bolts already at these points. Compressionchords were finished to bear to reduce bolts needed forthese splices.

The bottom chord bracing system consisted of W14struts and WT diagonals in an X pattern. The struts weredetailed in 60’ lengths to eliminate splices. The WT diag-onals were originally oriented with the stems up. Theengineer was able to review crane clearances and allowthe WTs to be detailed back-to-back and eliminate thecenter splices.

The most critical and complex connections in thehangar are the connections in the main roof trusses tothe supporting jack trusses at the front and rear of thehangar. The large connection forces at the front boxtruss assembly made it necessary to increase the size ofthe truss verticals in order to utilize double gage lines toinstall all of the required bolts.

The roof trusses were designed with fixed supports atthe rear of the hangar. In order to transfer the large ten-sion forces through the supporting columns to the ten-sion tie, the columns were rotated 90 degrees. Thisallowed the use of gusset plates on the flanges to trans-fer the tension to diagonal while also transferring thevertical reaction to the column flanges.

The continuous truss at the rear supports joists atboth the top and bottom chords. Because of the continu-ity and the short spans, the truss member forces wererelatively light. It was decided to rotate both chords toallow the joists to bear on the top flanges.

The side wall columns were W36 sections over 90’high. These columns were mill ordered full length andfabricated without splices to facilitate erection.

CSD sized all detail material and connectors alongwith checking members for net section, block shear,bearing, etc. Gusset plates were checked for tear outand buckling. Complete design calculations were fur-nished to the engineer for review and approval.

The project specifications also required the erector toprepare a detailed erection plan showing the assembly,hoisting and bracing of the hangar steel. L.H. SowlesCompany, the erector, prepared a detailed assemblyand hoisting plan and retained Clark Engineers todesign the shoring and guying system. CSD checkederection stresses for the roof trusses.

Details To Simplify Erection