Keywords: stiffened plate, existing bridge pier, retrofitting method, trial design, ..... Japanese Specifications for Highway Bridges aSHB), Part I. Generals and Part ...
Mem. Fac. Eng., Osaka City Dniv., Vol. 39, pp. 39-51 (1998)
A Seismic Retrofitting Method and Trial Design for Stiffened Plates in Existing Steel Bridge Piers Toshiyuki KITADAl, Hiroshi NAKAI2 , Taiichi KAGAYAMA3 and Masahide MATSUMURA4
(Received September 30, 1998) Synopsis: Presented in this paper are a seismic retrofitting method and a trial design for stiffened plates in an existing steel bridge pier based on the design guidelines drafted in the Hanshin Expressway Public Corporation in response to the revision of the seismic design method in the Japanese Specifications for Highway Bridges after the Hyogo-ken Nanbu Earthquake. The design guidelines are utilized in the case that the strength of their steel pier columns increases substantially and subsequently the applied seismic load exceeds more than that decided by the strength of the basement structures, if their steel pier columns are filled with concrete as a retrofitting method.
Keywords: stiffened plate, existing bridge pier, retrofitting method, trial design, seismic design
1. Introduction As steel among structural materials is very ductile in comparison with concrete, it had been considered that the steel structures, designed against an earthquake (level 1) with the maximum acceleration of 150-200 gals at the surface level of the ground, never collapse against a strong earthquake (level 2) which rarely occurs during their design life, although they may lose some of their functions. The Hyogo-ken Nanbu Earthquake (one of the level 2 earthquakes) which occurred on January 17th in 1995, however, caused various kinds of serious damage to steel bridge piers such as the collapse of bridge bearings, failure of bridge piers, local buckling, occurrence of brittle cracks, etc. Mer the earthquake, many energetic investigations have been carried out for developing the ductile steel bridge piers, which can support superstructures without increasing their elastic strength against such a strong earthquake as the Hyogo-ken Nanbu Earthquake, in research laboratories in universities, governments, public corporations, steel mill companies and bridge fabricators. On the basis of the results of these researches, the seismic design method in the Japanese Specifications for Highway Bridges OSHB) was revised in December 1996. In this seismic design method, the composite bridge piers of which steel columns are filled with concrete are recommended as one of the most effective and economical ones. No recommendable structures seem to be specified except for the rectangular cross section with the corner plates with regard to ductile steel cross sections [1]. Design guidelines [2] were drafted in the Hanshin Expressway Public Corporation (HEPC) for retrofitting the existing steel bridge piers by referring to the seismic design method in JSHB and these recent research results [3]. According to the seismic design guidelines, the HEPC is strengthening the existing steel bridge piers in which the basement structures of their pier columns are supported with anchor bolts and footing concrete and can not bear against the strong earthquakes (Types I and II of the level 2) specified in the JSHB if their steel pier columns are filled with concrete, because the strength of their pier columns 1
2 3 4
Associate Professor, Department of Civil Engineering Professor, Department of Civil Engineering Maintenance Division, Hanshin Expressway Public Corporation Master Student, Course of Civil Engineering
39
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increases substantially and subsequently the applied seismic load exceeds more than that decided by the strength of the basement structures. Demonstrated in this paper are the concept of this seismic retrofitting method in the HEPC and a trial design example according to this method by using an actual steel bridge pier of rigid framed structure with two stories and one span [4] in which several web panels around the center part of the lower beam member with box cross section buckled, the bearings supporting the lower bridge collapsed due to the Hyogo-ken Nanbu Earthquake, and subsequently the stiffened flange plate of the west column member in the second story was dented by the collision of the lower bridge. This seismic retrofitting method is almost verified through the experiment and elasto-plastic finite displacement analyses carried out by the authors. The experimental and analytical results are reported in Ref. 5. The additional investigation to verify the seismic retrofitting method completely is being executed in this year.
2. Bridge pier under consideration and damage due to the Hyogo-ken Nanbu earthquake The bridge pier [4] under consideration is located on the type III ground (weak ground) and designed according to the specifications [6] for bridges provided be HECP in 1992. Concrete is filled into the lower part of the pier columns with rectangular cross section up to the height of 1.45 m from the basement to prevent serious damage caused by the collision of a vehicle. Figure 1 illustrates the front and side elevations of the pier together with their damage. As this bridge pier is almost symmetric in the front elevation, almost the same axial force due to the earthquake in the direction of the bridge axis is applied to both pier columns. Consequentl)', treated in this report is only the west column having three types of cross section with different thickness of the component plates; cross section CD, @ and @.
She
West column
Fig.l
buckling of eb plates
----'1 Height of encased concrete
..
~
3,000
(a)Front elevation (b)Side elevation Bridge pier under consideration and damage due to the Hyogo-ken Nanbu Earthquake (unit:mm)
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The main types of damage due to the Hyogo-ken Nanbu Earthquake are summarized as follows:
i ) Shearing buckling occurred in the both sides of web plates at the center part of the lower lateral beam with box cross section.
ii ) 4 bearings and their sole plates supporting the lower bridge collapsed at Kobe side. iii) Owing to the above reason, the lower bridge slipped sideways, struck against one of the side column members and caused local buckling on it.
3. Trial check for safety and trial retrofitting of objected bridge pier The method for retrofitting the bridge pier is based on the Guidelines for Seismic Retrofitting Methods for Existing Steel Bridge Piers against Strong Earthquakes (draft) [2] (afterwards referred to as only the Guidelines) of HEPC. Mentioned in this section is the outlines of the Guidelines for retrofitting the existing steel bridge pier under consideration.
3.1 Selection of Retrofitting Method The selection of a retrofitting method is decided according to the Manual (Draft) on Restoration of Highway Bridges under Disaster by Hyogo-ken Nanbu Earthquake [7] as follows: i ) The method of filling concrete should have superior priority for economical and operational reasons. ii ) The weakest part should never be located at the pier base structures. The method of retrofitting should be, therefore, decided by comparing Mu,ane with Muc and Mp , in which Muc : the ultimate bending moment of the composite cross section consisting of the steel plates and encased concrete, which is calculated on the basis of the Manual (Draft) and the new JSHB, Part V. Seismic Design [1], M p : the fully plastic bending moment of the steel cross-section without encased concrete, Mu,ane : the ultimate bending moment at the pier base structure, which is calculated as a RC structure consisting of the anchor bolts and the paty of footing concrete subjected to compression according to Ref. 6.
3.2 Restrictions Concerning Cross-sectional Dimensions To obtain expected ductility in the pier columns, the following conditions on their cross-sectional dimensions shall be regulated [3]. i ) In order to prevent the local buckling, the following conditions are specified with regard to the plate slenderness parameters of overall stiffened plate panels RF , plate panels between longitudinal stiffeners RR , and longitudinal stiffeners and R s , respectively.
R =B F t
=!?-
R R
t
cry 12(1- ,...2) SO.4 E rc 2 • kF cry
E
12(1-~2) =O.526!?-~cry 1(;2 ·4
R s =!!L cry 12(1-~2) ts E 1(;2·0.425 where
(1)
t
E
=1163!!L~cry ts
E
SO.4 SO.5
(2)
(3)
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Il : Poisson sratio of steel material (=0.3)
B : width of stiffened plate panel b : spacing of longitudinal stiffeners (= B / n) n : number of plate panels divided by longitudinal stiffeners kF : buckling coefficient of stiffened plate panel defined by
kF· --
(1 + a/
r
+ ny s at (l+n~s) 2
(s) a t- a o
(4)a,b
k = 2~+~1+nys) F
l+n~s
at =!!:..... : aspect ratio of stiffened plate panel B a : spacing of transverse stiffeners (or diaphragms)
(5)
