Fire Protection for Structural Steel_For Dummies

8/11/2019 Fire Protection for Structural Steel_For Dummies

1/4

pr

a c

ti c

al

s o

l uti on

s

pr c

c lsolutons

pr

a c

ti c

al

s o

l uti on

s

There has been a lot of talk in recentyears about structural engineers becomingmore involved in the specifying andinspecting of reproong systems for

structural steel. The collapse of the WorldTrade Center towers has focused publicattention on some of the inadequacies inthe way reproong of structural steel hasbeen treated in the past.

There has also been a lot of debate overthe adequacy of Building Code require-ments for reproong, but the problemis not the codes, it is improper applicationand enforcement of the code provisions. When the prescriptive re protection re-quirements are correctly implemented,they result in a very re safe building.

Some re protection engineers haveproposed mandating performance basedre protection rather than prescriptive re-quirements. Although performance basedrequirements would make a lot more work for re protection engineers, they will probably result in more complicat-ed and less understood code provisions.This is not likely to improve the re safetyof buildings.

The problem is that architects whoare typically charged with specifying reprotection systems have not taken muchinterest in them. Fire protection is not

something that enhances the architec-tural design of a building and is oftenviewed as a necessary evil. As a result, reprotection specications are often vague

and inadequate.It has been argued that architects do not

often have adequate technical trainingin re protection systems, and that struc-tural engineers could do a better job. Ofcourse structural engineers dont learnmuch about re protection in school,but they are often better than architectsat guring out technical stuff when theyare presented with all of the facts. So hereare the facts this stuff is really not thathard to understand.

Fire protection systems are classi-

ed as passive, active, or manual. Pas-sive re protection consists of buildingmaterials that insulate structural ele-ments from the heat of a re, such asspray reproong, masonry encasementor rated ceilings. Active re protectionsystems are systems that are intendedto extinguish a re, such as a sprinklersystem. Manual re protection systemsare those intended to aid re ghters,such as standpipes. This article willdeal only with passive re protection ofstructural steel using spray reproong.

Fireproong thickness gauge

Fire Protection of Structural Steel...For DummiesBy Jim DeStefano, P.E.

The current politically correct termfor spray reproong is Spray-AppliedFire Resistive Material (SFRM). In thisarticle the term spray reproong will

be used there is no need for fancy language and acronyms, it only confusespeople.

What materials are available?The most common spray reproof-

ing materials are low-density ber or ce-mentitious sprays. The ber spray is lesscostly than the cementitious spray, butdoes not adhere as well to steel and is eas-ily dislodged. These materials have a gyp-sum based binder and are not suitable for wet locations or exposed locations wherethe reproong can be dislodged, such asparking garages.

There are medium-density and high-density products available which containPortland cement, and are far more durableand water resistant than the low-density products. They are also consider-ably more expensive.

For architecturally exposed steel thereare intumescent paints available. Theseusually require several coats to obtain therequired thickness. The high cost of intu-mescent paints limits their use to projects

where it is important to architecturallyexpose the structural steel.

What re rating do I need?The Building Code denes 5 general

construction classications for build-ings, with sub categories within each. You (or the Architect) must pick one ofthe construction classications for yourparticular building project. Structuralsteel framed buildings are typically eitherType 1 or Type 2 construction (Type 1and Type 2 require that the structural el-ements be non-combustible). The Build-ing Code stipulates maximum buildingheight and oor area for each construc-tion classication and use group (Table503 of the IBC). The code allows in-creases to the height and area limits if thebuilding is sprinklered, or has more than25% of the perimeter accessible to retrucks. The trick is to pick a construc-tion classication for your building thathas the lowest re ratings required for thebuilding elements.

STRUCTURE magazine November 2005continued on next page


2/4

umn will typically require thicker reproongthan adjacent beams that frame into a girder.

It is restrained or unrestrained?There is considerable confusion and misin-formation about what constitutes a restrainedassembly. Restrained assemblies require lessreproong thickness than unrestrained as-semblies with the same re rating.

In the context of re rated assemblies, the term

restrained has a different meaning frcommonly used by structural engineer

A building re is often limited to

area of a building and only heats structural steel immediately above thethere is surrounding oor or roof cotion that is capable of restraining the texpansion of the structural steel in thity of the re, the assembly will perfoter and is considered to be restrained.

There is a table contained with ASTM E119 standard (Table X3.1gives guidance in evaluating if an ais restrained or unrestrained. If steelare welded, riveted, or bolted to othbeams, the assembly is considered res

If steel beams or joists are supporbearing walls, then single spans abays are considered unrestrained. It that structural steel construction is always restrained.

It is not uncommon for a productman to encourage architects and ento treat all assemblies as unrestraineallows them to sell considerably moproong material for a project. Som

Flutes of metal deck have not been completely lled over beam

Fireproong should be applied to bare unpain

Once you know your construction class-ication, Table 601 of the IBC denes there ratings required for each building ele-ment, oor construction, roof construc-tion, columns, etc.. This part is real easy.

There are no re ratings tabulated forbrace elements that resist only wind orseismic lateral loads. This is based on theassumption that it is unlikely that a hurricane

or earthquake will strike while the buildingis on re.The code does require that beams which

brace a column must have the same rating asthe column that they are bracing. Similarly,beams that support a wall around a stairmust have the same rating as the wall.

Is it an assembly or just a beam?Fire ratings are listed as restrained assem-

blies, unrestrained assemblies and unre-strained beams. If you are attempting toachieve a re rated oor or roof, you are deal-

ing with an assembly. If you are re rating a beam that braces a column or supports a rated wall, you are dealing with an unre-strained beam. Since an unrestrained beamrequires a thicker application of reproof-ing than a restrained oor assembly with thesame re rating, beams that frame into a col-

F o r

A d v e r t i s e r I n f o r m a t

i o n , v i s i t w w w . s

t r u c t u r e m a g . o

r g

STRUCTURE magazine November 200518


3/4

building codes such as the California codeactually stipulate that all assemblies must betreated as unrestrained.

Which UL design should I use?The Underwriters Laboratory (UL) pub-

lishes a directory that lists the re rated assem-blies, beams, columns and walls that they havetested. UL is not the only testing laboratorythat performs re tests, but they are the mostprolic. There are other laboratories such asFactory Mutual that also list re test results.

Fire tests are dened in ASTM E119. Afull size mock-up is placed in a test furnaceand subjected to a re with a prescribed time-temperature curve. The time period to failureis recorded for the test. Since actual buildingre conditions are different from an E119test, an assembly with a 2 hour rating willnot necessarily survive a real re for 2 hours.The E119 test is a good method of rating therelative re resistance of different buildingelements, but it is not a good predictor of anelements actual duration in a real re.

Each listing in the UL directory describes ingreat detail all of the signicant componentsof the test specimen, such as beam size, typeand thickness of reproong, type, size andgage of metal deck, thickness of concrete slaband type of concrete aggregate. Your buildingconstruction must match all of the componentsof the test specimen for the UL test that youreference. This requires some tedious readingthrough the UL directory to nd a test that issimilar to the construction of your project.

What thickness should thereproong be?

The UL test will indicate the reproongthickness that is required for a given re rat-ing. This thickness is valid only for the beamsize that was used in the test. For instance, ULtest D739 was based on a W8x28 steel beam.Unless your project has been designed with allof the beams being W8x28s, you will need toadjust the reproong thickness for each beamsize used.

There is a simple formula that is used tocalculate the required reproong thicknessbased on the ratio of beam weight to heatedperimeter. This is referred to as W/D, where W is the weight per foot in pounds and Dis the heated perimeter in inches (not beamdepth). You dont actually have to do thiscalculation yourself, instead you shouldrequire the reproong contractor to submit W/D calculations for approval along with aschedule of reproong thickness for eachbeam and column size on the project.

Just to keep everybody confused, some ref-erences will base the thickness calculations onthe ratio of cross sectional area to heated pe-rimeter. This is referred to as A/P, where A isthe area in inches squared and P is the heat-ed perimeter in inches (note that they havechanged from D to P).

What about surface preparation?Spray reproong is intended to be ap-

plied over bare, unpainted steel. A lightcoating of rust will actually improve the ad-

Application of reproong

STRUCTURE magazine November 2005

hesion of the reproong. If for somethe steel has been painted, things gecomplicated. Unless you can substthat the reproong will have adequhesion to the paint, you must take addal steps such as applying a bonding athe steel or securing metal lath to theprior to reproong.

Many re rated assemblies require the

side of the metal deck to be spray repalong with the beams. Often galvanizedeck will have a light lm of oil on itsthat needs to be removed by solvent clFireproong manufacturers specicatioften say that the cleaning of the meta will be by others . Your spec should in who is responsible for the solvent csince it is not likely that the Ironworkbe doing it.

What Special Inspectionare required?

Spray reproong is subject to Inspections under chapter 17 of theTesting is required of the reproongness, bond/adhesion and density. Threquires that one thickness test be perfor every 1,000 square feet of rated


4/4

Documents

Fire Protection for Structural Steel_For Dummies