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Bearings ForSteel BridgesBy Charles H. Roeder, P.E.

Bearings assure that a bridge remains functional by

allowing movements while supporting vertical loads. As

a result, bearings must be designed with full considera-

tion of both movements and loads. Movements include

both tra nslations an d rotat ions, a nd t he sources of move-ment include bridge skew and curvature effects, initial

camber or curvature, construction loads, misal ignment

or construction tolerances, settlement of supports, ther-

mal effects and traffic loading. Thermal translation, , is

th e most importa nt for steel bridges and is estima ted by:

= L T

where L is the expansion length, is the coefficient of

thermal expansion, a nd T is the change in the average

br idge t e mper a t ur e f r om t he a ve r a g e t e mpe r a tur e a t

installation. A change in the average bridge temperature

causes a thermal translation, but a change in the tem-

perature gradient causes bending and deflection. Skew

and curved bridges may have more complex movementsthan suggested by the above equation, and these special

geometric effects must be considered.

Rota tions a lso must be considered in the selection an d

design of the bearing. Bearing rotation may be caused by

girder end rotations as well as initial camber of girders

and out of level support surfaces. The magnitude and

direction of a l l translations and rotations must be con-

sidered at a ll stages of the life of the bridge an d bearing.

Bearings resist forces and accommodate movements,

but t he resista nce of a force and a ccommodation of move-

ment in the same direction are normally mutually exclu-

sive events. Restraint forces occur when any part of am o v e m en t i s p r e v e n t e d . F o r c es d u e t o d i r e ct l oa d s

include th e dead load of the bridge a nd loads due to tra f-

fic, earthquakes, water and wind. Temporary loads due

to cons t r uc t ion e quipment a n d s t a g ing a l so occur . I t

should be noted that the majori ty of the direct design

loads are reactions of the bridge superstructure on the

bearing. The engineer must consider the worst possible

combination of loads and movements without designing

for unr ealistic or impossible combinations or conditions.

B earings are ty pical ly locat ed in an a rea t hat collects

dirt and moisture, and as a result , bearings should be

d e s i g n e d t o h a v e t h e m a x i m u m p o s s i b l e pr o t e ct i o nagainst the environment and to allow access for inspec-

Reprinted from Modern Steel Construction

BRIDGECROSSINGSPractical Information For The Bridge Industry

No. 7, June 1997

Figure 1: Bearing Types

ElastomericBearing

ShpericalBearing

PotBearing

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tion. Furt her, allowa nces for bearing replacement should

be part of the design process, since the expected life of

most bearings is shorter than for other bridge compo-

nents.

Bearing SelectionAfter the design requirements a re establ i shed , the

bearing type must be selected. The selection should be

made wi th the goa l of achieving the most economica l

solution th a t supports a ll required loads w hile accommo-

da ting all requ ired movements . The economic evalua tion

should consider both the initial cost as well as the long

term maintenance . A wide range of bear ing types are

possible (see Figure 1), including: elastomeric bearings;

bearing pads; PTFE (polytetrafluorethylene) or lubricat-

ed bronze sliding surfaces; pot bearings; disk bearings;

rocker or rol ler bearings; and cylindrical or spherical

bearings.

Many engineers misjudge the capabilities of individ-

ual bearing types, or they improperly evaluate the loads

and movements. Either error leads to a poor selection of

bearing type, poor bearing performance and increased

maintenance and construction costs. More information

on these topics is conta ined in the St eel B ridge B earing

Selection and Design Guide, which was recently pub-

lish ed by t he NS B A (ca ll 800/644-2400). It is impor ta nt

that the engineer initially select the most viable options

for further design consideration (see Table 1). This table

i s no t a de s ig n docume nt , howe ve r . I t doe s pr ov ide

approximate practical limits to help the engineer select

the most viable options. Examination of this table andcomparison of the bearing capabil i t ies with the design

load an d movement requirements for steel bridges shows

tha t e last omeric bear ings or e last omeric bear ing pads

will often be the lowest maintenance and most economi-

cal solution for steel bridges. Unfortunately, this finding

is opposed to the preconceived notions of some engineers

and these preconceptions often lead to a bearing system

that is well below the optimum. The engineer should

keep his or her options open during the selection process

and Table 1 clearly identifies the options that should be

carried forwa rd into lat er sta ges of the design process.

Bearing DesignA discussion of bearing design exceeds the space limi-

tations of this art icle . However, for more information,

pleas e consult t he AASH TO Load a nd Resista nce Factor

D e s i g n S p e c if i ca t i on s o r t h e N S B A S t e el B r i d g e

Bearing Selection and Design Guide.

Attachment andInstallation of Bridge BearingsL a t e r a l f o r c e s m a y a r i s e f r o m w i n d , t r a f f i c o r

hydr a ul i c l oa ds a n d th e y a r e induced by t he be a r ing

resista nce due to imposed displacements or seismic load-

ing. For stream crossings, hold downssuch as anchor

boltsare r ecommended if th e elevat ion of t he bottom of

th e superstructur e is wit hin 2 of th e design flood eleva-

tion. The potential for upli ft under gravity load exists

only in bridges that are continuous with a high live load-

to-dead load ratio, very uneven span lengths, curved, or

heavily skewed. The engineer must consider uplift and

lateral forces when designing an chorage a nd a tta chment

deta i l s . The deta i l depends upon the la tera l load and

Load Translation Rotation Costs

Bearing Types Min. Max. Min. Max. Limit Initial Maintenance(kN) (kN) (mm) (mm) (rad.)

Plain Elastomeric Pad 0 450 0 15 0.01 Low LowCotton Duck Pad 0 1400 0 5 0.003 Low LowFiberglass Pad 0 600 0 25 0.15 Low Low

Steel ReinforcedElastomeric Bearing 225 3500 0 100 0.04 Low Low

Flat PTFE Sliding 0 >10000 25 >100 0 Low Moderate

Spherical Lub. Bronze 0 7000 0 0 >0.04 Moderate Moderate

Pot Bearing w/o Sliding 1200 10000 0 0 0.02 Moderate High

Pot Bearing w/ Sliding 1200 10000 25 >100 0.02 Moderate High

Rocker Bearing 0 1800 0 100 >0.04 Moderate High

Single Roller 0 450 25 >100 >0.04 Moderate High

Spherical PTFE w/oSliding Surface 1200 7000 0 0 >0.04 High Moderate

Spherical PTFE w/FlatPTFE Sliding Surface 1200 7000 25 >100 >0.04 High Moderate

Table 1: Summary of Bearing Capabilities

Note: 1 kip = 4.45 kN and 1 inch = 25.4 mm

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often the most economical alternative wit the least main-

te na nce for s te e l b r idg e s a nd the g r e a te r s impl i c i t y

inherent in the instal lat ion, a t tachment and anchorage

of the se bea r ing s i s a no the r r e a son for the i r g r e a te r

economy. Because of the greater stiffness and the result-

ing forces expected with stiff bearing systems, it is often

advantageous to use a s t ructura l key way ra ther than

the bearings to restrain unwanted movements in these

systems.

ConclusionsB r i d g e b e a r i n g s a r e n o t w e l l u n d e r s t o o d b y

many engineers. The materia ls used in them aredifferent fr om those encount ered in oth er stru ctur-al systems and the behavior and modes of fa i lurealso are quite di f ferent . As a result , bridge bear-ings are a contributing factor to a large portion ofthe long-term maintenance cost of steel bridges.This article presents a breif overview of the bridgeb ea r i ng i ssue . For a more i n-depth d i scuss i on ,plea se refer t o Steel Br idge Bea ring S election AndDesign Guide, published by the NSBA. The publi-ca tion ca n be ord ered by callin g 800/644-2400.

Cha r l es H. Roeder, P.E., i s a pr ofessor of civi l

eng in eer i n g a t t he U n i ver si t y of Wash in g ton i n

Seattle.

uplift resistance required at the bearing as well as the

st i f fness of the bear ing system. In past years , br idge

de s ig n spe c i f i ca t i ons r e qui r e d tha t s t e e l b r idg e s be

anchored aga inst upl i f t in a l l cases; today a t tachment

and anchorage requirements depend upon the load and

r e s t r a in t r e qui r eme nts r a t he r tha n a r b it r a r y r u le s or

restrictions.

A variety of a t tachment detai ls are possible. Lateral

for ce s a r e sm a l l i n e l a s t omer i c bea r ing s or be a r ing s

equipped with a PTFE sliding surface. Therefore, these

flexible bearings often require little or no lateral resis-

ta nce an d fr iction ma y provide adequat e lateral restraint

(see figure 2a). Uplift r estra int is needed only in special

condi t ions, s ince gravi ty wi l l provide adequate upl i f t

resista nce in most ca ses. However, flexible bear ings per-

mit simple details such as that of Figure 2b when uplift

resista nce is required.

St i f f bear ing systems such as pot bear ings develop

l a r g e l a t e r a l f o r c e s w i t h v e r y s m a l l d e f o r m a t i o n s .

Therefore , a t tachment and anchorage are l ike ly to be

r e qui r e d , a nd the l a r g e r for ce s must be a n t i c ipa te d .

Str onger a nd st i f fer a t t achment de ta i ls (see Figure 3)

are used for pot bearings and other stiff bearing systems.P rior discussion has noted tha t elast omeric bearings a re

NATIONAL STEEL BRIDGE ALLIANCEOne East Wacker Dr., Suite 3100Chicago, IL 60601-2001ph: 312/670-2400 fax: 312/670-5403

The mission of The National Steel Bridge Alliance (NSBA),

which was formed in 1995, is to enhance the art and

science of the design and construction of steel bridges. Its

activities include organizing meetings, conferences and

national symposia, conducting the Prize Bridge Awards

competition, supporting research, developing design aids,

and providing assistance to bridge owners and designers.

The NSBA membership includes representatives from all

aspects of the steel bridge industry.

Figure 2a & 2b: Attachment and anchorage of flexible

bearings

2b:withupliftrestraint

2a:withasmalllateralload

Figure 3: Minimum attachment detail for a stiff bearing