foundations for metal building systems.pdf

8/10/2019 Foundations for Metal Building Systems.pdf

1/4July 201312

solutions for the practicingstructural engineer

PRACTICALSOLUTIONS

Alexander Newman, P.E., F. ASCE,is a forensic and structuralconsultant in the Boston area. Heis the author, most recently, of theFoundation and Anchor DesignGuide for Metal Building Systems(McGraw-Hill, 2013). He can bereached at [email protected].

By Alexander Newman, P.E.,F. ASCE

Finding a Practical

Solution for Your Project

Foundations for MetalBuilding Systems

Metal building systems (MBS), alsoknown as pre-engineered metalbuildings, are proprietary struc-tures designed and manufactured

by their suppliers. Metal buildings are extremelypopular and they account for a substantial per-centage of low-rise nonresidential buildings in theUnited States. Te design of foundations for these

structures often involves

special challenges. Tedesign procedures areoften not well under-stood, because they arenot specified in the build-ing codes and technical

design guides. Until the recent publication ofFoundation and Anchor Design Guide for MetalBuilding Systems (McGraw-Hill, 2013), there havebeen no authoritative books on the subject. Asa result, the foundation designs produced bydifferent engineers for the same metal buildingstructure could range from those that cost a trivial

amount to those that are quite expensive to con-struct. Tis article discusses the reasons for suchdisparity and misunderstanding and examinesthe available design options.

The Main Challenges

Several challenges make foundations for metalbuilding systems different from those used inconventional buildings:

Single-story MBS are extremelylightweight. Te total weight of thestructure could be between 2 and 5 pounds

per square foot (psf), which means that astrong wind results in a net uplift loadingon the foundations.

Te most popular types of the primaryframes used in MBS gable rigidframes exert significant horizontalcolumn reactions on the foundations.Such reactions could be present in someconventional building foundations as

well, but rarely at every column, and incombination with uplift.

Because MBS are proprietary structures,the manufacturers often report slightly

different column reactions for the buildingswith identical loading and configuration.In the construction projects that use publicfunding and require competitive bidding,the MBS manufacturers cannot be selectedprior to the foundations being designed.

Accordingly, the column reactions mustbe estimated by the foundation designers,running the risk that the final reactions

will exceed those used in the design. Unfortunately, in many situations the ownerof the building decides to procure the metalbuilding superstructure first and designthe foundations later, as an afterthought.

Without a structural engineer involved inestablishing the design parameters for theMBS, some manufacturers might choose toprovide the cheapest design possible. Onesuch example is a building with fixed-baseframe columns, which might result inminor cost savings for the manufacturer butin major cost increases for the foundation

vis--vis pin-base columns. Te lack of clear design proceduresnaturally results in uneven designsolutions. Some foundation designsfor metal buildings have been overlycomplicated, and some have been barelyadequate for the imposed loads (or notadequate at all).

Uplift and Horizontal

Column Reactions

In single-story MBS, the dead load is generally

insufficient to counteract the effects of wind-gen-erated uplift. In addition, building codes requirethat no more than 60% of the dead load likelyto be in place be used in combination with winduplift (the International Building Code (IBC)basic load combination for the allowable stressdesign method). Tus the weight of the ballastmust be substantial. For a typical shallow foun-dation, such as an isolated column footing, theballast consists of the footing, column pedestal(if any), and the soil on the ledges of the footing.Some engineers also include a contribution of thesoil frictional resistance.

(b)

V

H

LV

L

L

H

RH

R

WIND

H

LV

L RH

VR

H

L

(a)

Figure 1: Te direction of horizontal column reactions in a single-span rigid frame: (a) from gravity loads;(b) from wind or seismic loads.


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Quite often, the minimum size of columnfoundations is dictated by the minimumamount of ballast, not by the soil-bearingcapacity for downward loads. Tis oftencomes as a shock to the foundation design-ers unfamiliar with MBS specifics. Te designexample in the sidebar illustrates the processof sizing an isolated column footing for uplift.Gable rigid frames exert horizontal column

reactions on the foundations. Tis occursunder gravity loading, when the reactionsare numerically the same but act in opposingdirections (Figure 1a), as well as under windor seismic loading, when the reactions usuallyact in the same direction (Figure 1b).

Some Available

Foundation Systems

Te vertical and horizontal column reactionscan be resisted by a variety of foundation sys-tems, such as those listed below and illustrated

in Figure 2 (page 14). Properly designed, eachsystem can resist the required level of hori-zontal and vertical frame reactions. However,experience shows that some systems couldbe more or less applicable in various circum-stances. Each system has advantages anddisadvantages, as summarized in able 1.

Te table compares cost, reliability anddegree of versatility of the selected founda-tion systems used in pre-engineered buildings.Here, reliability refers to the probability of thefoundation system performing as intendedfor the desired period of time under variousfield conditions. Te most reliable systems cantolerate inevitable irregularities in construc-

tion, loading and maintenance. Te overallreliability of a foundation system depends onthree factors that define the systems ability tofunction in adverse circumstances:

Simplicity of installation. Tefoundations that are difficult to installor require a perfect installation tend to

be less reliable, because some placementerrors are common and perfection infoundation construction is rare.

Redundancy. Redundant systemshave more than one load path fortransferring the column reactions tothe soil. If one load path is blocked,another path takes over.

Survivability. Can the system maintainits load-carrying capacity after some ofthe adjoining building elements havebecome damaged? For example, whathappens if the slab on grade is cut orpartly removed?

Foundation System Cost Reliability Versatility

ie Rod Low to high Low to high Low to medium

Hairpins and Slab ies Low Low Low

Moment-Resisting Foundation High High High

Slab with Haunch Medium Low to high Low to medium

rench Footing Medium to high High HighMat High High Low

Deep Foundations High High High

able 1: Comparative cost, reliability and degree of versatility of selectedfoundation systems for metal building systems.

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Versatility, as noted in Table 1, is possessedby the systems that can be used with variousfloor and soil conditions (e.g., floor trenchesand pits).Some foundation systems commonly used

in MBS are: ie rods(Figure 2a). In this intuitively

appealing solution, the foundationsat the opposite building columns aretied together, extinguishing both

horizontal column reactions. ie rodconstruction ranges from the cheapestand least reliable, such as a couple ofreinforcing bars placed in a thickenedslab, to the relatively expensive andmuch more reliable, such as concretegrade beams. Te survivability of theformer is low, because there is a distinctpossibility that the slab on grade willbe cut or partly removed at somepoint, while the grade beams placedbelow the slab will likely survive sucha scenario. Tere are also the issues

of elastic elongation of the tie rodunder load and whether the tie rod isconsidered a tension- tie memberunder the provisions of the AmericanConcrete Institute standard ACI 318.Te versatility of this system is at thelower end of the spectrum, since tierods cannot be used in buildings withdeep trenches, depressions, and pits.

Hairpins with slab ties (Figure 2b). Tegeneral idea behind this design is thesame as in the tie-rod system, but thetension force is resisted by distributed

steel reinforcement in the floor slab (slabties) rather than by discrete tie rods. Tisis the least expensive method of resistinghorizontal column reactions and, forthis reason, hairpins have been widelyused in the past. But the system suffersfrom multiple disadvantages. Amongthem is a total reliance on the floor slab,

which makes the system vulnerable to

the slab being cut or partly removed.Other issues include construction

joints in slabs on grade, where the slabreinforcement generally stops, and eventhe fundamental issue of treating slabson grade as structural elements. Slabson ground are excluded from the scopeof ACI 318, except when they transmitlateral forces from other portions of

the structure to the soil. If the designerintends to have the slab on grade comply

with ACI 318, the slab must be designedand constructed with greater care thanthe prevalent practices. At the very least,it should be reinforced more substantiallythan with a layer of light welded-wirefabric, to provide for a minimumpercentage of shrinkage reinforcement.

Moment-resisting foundations(Figure2c). Tese foundations work similarlyto cantilevered retaining walls: the

weight of the foundation, and any soil

on top of it, resists overturning andsliding caused by external horizontalforces. Because it does not depend ona contribution of the slab on grade,the moment-resisting foundationrepresents one the most reliablesystems available. It also is one of themost versatile, since deep trenches,depressions, and pits in the floor or no floor at all do not affect itsfunction. Te system can even be usedin hillside installations, where oneend of the building is lower than the

other. However, the design proceduresfor moment-resisting foundations arerelatively lengthy and the constructioncosts could be high.

Slab with haunch(Figure 2d). Tissystem has been widely used inresidential construction, and somehave tried to use it for supportinglarge pre-engineered buildings as well.

Te slab with haunch, also known asa downturned slab, works similarlyto the moment-resisting foundation,and a rigorous design would resultin the haunch of the size similar tothe footing of the moment-resistingfoundation. Needless to say, this is notthe size the proponents of this systemhope for. Te reliability and versatilityof the slab with haunch dependson whether the design relies on thecontribution of the slab on grade. Ifit does, both reliability and versatility

would be at the low end of thespectrum, similar to the hairpin system.

rench footing(Figure 2e). In this design,a deep trench is excavated and filled

with concrete. Te resulting foundationcould be made heavy enough to resistuplift and deep enough to developpassive pressure of the soil. Sincethe design does not depend on thecontribution of the slab on grade, bothreliability and versatility of this systemare high. Obviously, the trench footings(also known as mass foundations or

Tie rod

(a)

FH FH

Hairpin

Slab tes(b)

(c)

FH

FH

FH

(d)

Column pedestal

Figure 2: Common Foundations Used in Metal Building Systems: a) ie rod; b) Hairpins with slab ties;c) Moment-resisting foundation; d) Slab with haunch.

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formless footings) can only be used inthe soils that allow for the excavatedtrench to be stable during construction.Tis typically requires clayey soils.

Mats(Figure 2f). Using mats mightbe advantageous in metal building

foundations bearing on poor soils.According to one rule of thumb, whenisolated column footings cover morethan 50% of the buildings footprint,mats become economical. Mats aretypically reinforced in two directions,

both at the top and at the bottom.Heavyweight mats work well in resisting

wind uplift, and their continuousreinforcement solves the problem ofextinguishing the horizontal columnreactions at the opposite ends of theframes. One challenge of using mats in

metal buildings with multiple-span rigidframes is the placement of anchor boltsfor interior columns. Tis often requiresplacing a separate mud slab, which canbe used to temporarily support anchorbolts. Mats possess high reliability they are unlikely to be cut casually buta low versatility, because they do not

work with deep trenches, depressions,and pits. Teir cost is relatively high.

Deep foundations(Figure 2g). Tere aretwo main types of deep foundations:deep piers (also called caissons, or

drilled shafts) and piles. Deep pierstypically possess enough dead loadto counteract moderate wind uplift.If additional ballast is needed, acontribution of the perimeter gradebeams could be considered. Tegrade beams also engage the passivepressure-resistance of the soil andthus help resist horizontal columnreactions. Piles can resist both upliftand horizontal forces in a variety of

ways, including friction in cohesivesoils and flexure. Because deep

foundations generally do not dependon a contribution of the floor slabs,these foundations are both reliable andversatile. But they are also costly andare typically used in only poor soils,particularly those where weak strataare underlain by competent materials.

By understanding the advantages and disad-vantages of various foundation systems used inpre-engineered buildings, designers should beable to select the foundation design that mostclosely matches the expected use, configurationand performance of the building as a whole.

Figure 2 (continued): Common Foundations Usedin Metal Building Systems: e) rench footing;

f ) Mat; g) Deep foundations.

A Simplified Design Example for Sizing an Isolated ColumnFooting for Downward Forces and Uplift.

Given:Select the size of an isolated column footing to support an interior column of asingle-story multiple-span rigid frame. Te spacing of the interior columns within theframe is 60 feet; the frames are 25 feet on centers. Te following loads act on the roof:3 psf dead load, 30 psf design roof snow load, and 14 psf wind uplift. Te depth of thefooting must be at least 3 feet below the floor. Te column is supported by a 20 inch by 20inch concrete pedestal extending to the top of the floor. Use allowable soil bearing capacityof 4000 psf. Assume the average weight of the soil, slab on grade and foundation is 130lbs/ft3. Te building is not located in the flood zone. Use IBC basic load combinations.

Solution.Te tributary area of the column is 60 x 25 = 1500 (ft2). Te design loads on

the column are:Design dead load D= 4.5 kips Design snow load S = 45 kipsDesign wind uplift load W= 21 kipsotal downward load D+ S = 4.5 + 45 = 49.5 kipsotal uplift load on foundation (0.6D + W) = 0.6 x 4.5 21 = 18.3 kips

Weight of the soil, slab on grade and foundation is 0.130 kips/ft3x 3 ft. = 0.39 kips/ft2(ksf )Net available soil pressure is 4.0 0.39 = 3.61 (ksf)Required area of the footing for downward load is 49.5/3.61 = 13.71 (ft2)For downward load only, the sign of the footing is 3.7 feet by 3.7 feet at a minimum.

Check stability against wind uplift. Minimum required weight of the foundation, soil onits ledges and tributary slab on grade (Dmin, found) can be is found from:

0.6Dmin, found+ W= 0Dmin, found= 18.3/0.6 = 30.5 (kips)

Tis corresponds to 30.5/0.130 = 234.62 (ft3) of the average weight of ballastWith the depth of footing 3 feet below the floor, this requires the minimum square foot-ing size of (234.62/3)1/2= 8.84 (ft.)o reduce the footing size, try lowering the bottom of the footing by 1 foot. Ten the

minimum required square footing is (234.62/4)1/2= 7.66 (ft.).o arrive at a nominal size, use 8.0 by 8.0-foot footing, with a depth of: 234.62/(8)2=

3.67 (ft.).Te final footing size is 8.0 ft. x 8.0 ft. x 3 ft. 8 in. deep, as controlled by uplift.

Te complete version of this design example, including concrete design for various loadingconditions, can be found in the new book, Foundation and Anchor Design Guide for MetalBuilding Systems (McGraw-Hill, 2013).

Widen trench

footing atcolumns

Slab on grade

(e)

FH

(f)

FH F

H

(g)

FH

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