Fatigue Strength of Signal Mast Arm Connections A Summary by tzv97744


									                                              CENTER FOR TRANSPORTATION RESEARCH
                                                THE UNIVERSITY OF TEXAS AT AUSTIN

                                                             Project Summary Report 0-478-S
    Center for                                           Project 0-478: Structural Fatigue Details
    Transportation Research                                         Authors: K.H. Frank
          The University of Texas at Austin
                                                                       August 2005

                                         Fatigue Strength of Signal Mast Arm
                                               Connections: A Summary
                Introduction                             The details of the typical       serving as a seal weld.
                   This project investigated          unstiffened, socketed con-             The hole cut into the end

                the fatigue characteristics           nection are shown in Figures        plate to accept the mast arm
                of traffic cantilever signal          2 and 3. This is the most           significantly reduces its bend-
                mast arms shown in Figure             common type of connection           ing stiffness. A typical service
                1. The cantilever mast arm is         between the mast arm and            fatigue failure is shown in
                cost-efficient and relatively         the connection box on the up-       Figure 4. The failure occurs
                simple to design. The overall         right mast. The term socketed       when there is cracking at the
                structure is more aesthetically       comes from the socket hole          toe of the fillet weld connect-
                pleasing than a cable structure       cut into the end plate to ac-       ing the end plate to the arm.
                or a truss cantilever structure.      cept the mast. The hole allows      Galloping of the mast arm
                The same characteristics that         the drainage of the molten          from steady winds blowing

                make this traffic signal struc-       zinc from the interior of the       on the back of the signal or
                ture desirable also represent         arm during the galvanizing          gusts from trucks passing
                the most negative factors             process. An unequal leg fillet      under the arm produce the
                of the structure; it is a non-        weld is used on the outside         repeated fatigue stresses that
                redundant structure and the           to connect the arm to the end       cause the cracking.
                mast arm is very flexible.            plate with the interior weld

                                               Figure 1. Typical Cantilever Signal Support

                 Project Summary Report 0-4178-S                        ––
                                      Figure 2. Typical Mast Arm Connection

What We Did…                             not as significant as the speci-        These finite element analyses
                                         mens treated with Ultrasonic            extended the range of geometries
    The purpose of this study was        Impact Treatment (UIT). Increase        beyond those included in the ex-
to determine if the fatigue pro-         in the end plate thickness was          perimental project.
visions of the recently adopted          also found to provide an inex-             Based on the results of the
2001 edition of the AASHTO               pensive means of increasing             tests performed during this test
Highway Signs, Luminaires and            the fatigue performance of the          program, the following conclu-
Traffic Signal Specifications are        connection. An extensive finite         sions can be made:
accurate. Most existing structures       element analysis generated stress
do not meet the fatigue provision                                                • The test results confirm the
                                         concentration factors for a va-           classification of the unequal
of the new specifications. Also,         riety of connection geometries.
the specifications indicated that                                                  leg fillet welded socket con-
the addition of stiffeners to the                                                  nection detail as an E’ category
connections would greatly in-                                                      detail.
crease fatigue performance. The                                                  • The design provisions for the
                                                          Exterior Fillet-Weld
study examined the accuracy of                                                     stiffened connection details do
the specification with full-scale                                                  not predict the actual behavior.
fatigue tests of the mast arm and                                                  The research shows that a
its connections.                                                                   longer stiffener is better than
                                                       Interior Fillet-Weld        a shorter stiffener, contrary
What We Found…                                                                     to the design provisions. The
                                                                                   design equations also did not
   During this project, fifty-five
                                                                                   accurately predict the location
full-size mast arm connection
                                                                                   of first crack initiation.
specimens were tested to deter-
mine their fatigue resistance. The                                               • The UIT weld treatment pro-
present specifications were found                                                  cess provided significant fa-
to overestimate the fatigue life of                                                tigue life improvement when
connections with stiffeners. Sev-                                                  the treatment was performed
eral connection details exhibited            Figure 3. Cross Section               under certain conditions. Spe-
improved fatigue performance;                  of Typical Socketed                 cifically, when the difference
however, the improvements were                     Connection                      between the maximum stress

Project Summary Report 0-4178-S                      ––
                                          Figure 4. Mast Arm Fatigue Failure
              Fi gure 4. Mas t A rm Fa tigue F ail ur e

   in the treatment area and the             category E’ to category D.            develop recommendations for
   stress when treated is low (less          This improvement is not rep-          larger size arms.
   than approximately 10 ksi),               resented in the current speci-     2. UIT treatment is an effective
   the UIT weld treatment dra-               fications, as the base plate          means of increasing the fatigue
   matically improves the fatigue            thickness is not a variable in        life of service mast arms. Im-
   life.                                     the fatigue design provisions.        plementation of these recom-
   The test results also indicate            The finite element analysis           mendations will increase the
that the galvanizing process                 confirmed the experimen-              fatigue life to at least Category
influences results of the UIT                tal result and indicated that         D, which should be sufficient
weld treatment. The test speci-              the improvement in fatigue            to prevent fatigue fractures in
mens that were treated prior to              strength with increasing base         most applications.
galvanizing did not benefit from             plate thickness diminished
the treatment, indicating that the           with plate thickness beyond 2
galvanizing process negated any              inches.
improvement due to the weld               • The full penetration weld
treatment process. Although the             exhibited a significant fatigue
UIT Retrofit procedure was the              life improvement; however,
only retrofit solution tested, the          this was in part due to the
results of these tests indicate a           reduced stress caused by the
very significant fatigue life im-           thick backing bar.
provement through the use of
this treatment method. At this            The Researchers
time, the UIT Retrofit procedure          Recommend…
provides the best method for
improving the fatigue life of a           1. The end plate thickness for 10-
connection already in service.               inch arms should be increased
                                             to 2 inches. This simple change
• Increase in the end plate thick-           in the design provides the sim-
  ness for a 10-inch mast arm                plest method of increasing the
  from 1.5 inches to 2 inches                fatigue life of this connection.
  increased the fatigue life from            Further study is required to

Project Summary Report 0-4178-S                           ––
      For More Details...
     Research Supervisor:                                            Karl H. Frank, P.E., (512) 471-4590
                                                                     email: kfrank@uts.cc.utexas.edu
     TxDOT Project Director:                                         Heather Gilmer, Construction Division, (512) 506-5921
                                                                     email: hgilmer@dot.state.tx.us
     TxDOT Research Engineer: Tom Yarbrough, RTI, (512) 465-7685
                              email: tyarbro@dot.state.tx.us

     The research is documented in the following reports:
     • 0-4178-1, Fatigue Strength of Fillet-Welded Transverse Stiffeners with Undercuts, April 2005
     • 0-4178-2, Fatigue Strength of Signal Mast Arm Connections, July 2004

     To obtain copies of a report, contact: CTR Library, Center for Transportation Research,
                              (512) 232-3126, email: ctrlib@uts.cc.utexas.edu

                                                               Your Involvement Is Welcome!

       This research was performed in cooperation with the Texas Department of Transportation and the
U. S. Department of Transportation, Federal Highway Administration. The contents of this report reflect
the views of the authors, who are responsible for the facts and accuracy of the data presented herein. The
contents do not necessarily reflect the official view or policies of the FHWA or TxDOT. This report does
not constitute a standard, specification, or regulation, nor is it intended for construction, bidding, or permit
purposes. Trade names were used solely for information and not for product endorsement. The engineer in
charge was K. H. Frank, P.E. (Texas No. 48953).
                                          The University of Texas at Austin
                                          Center for Transportation Research Library
 Center for                               08 Red River #5
 Transportation Research
      The University of Texas at Austin
                                          Austin, TX 78705-650

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