LONGITUDINAL STIFFENERS ON COMPRESSION PANELS Chai H. Yoo, Ph.D., P.E., F. ASCE Professor Emeritus Department of Civil Engineering Auburn University CIVL 7690 July 14, 200 9 History ● The most efficient structural form is truss
Chai H. Yoo, Ph.D., P.E., F. ASCE
Department of Civil Engineering
July 14, 2009
● The most efficient structural form is truss
with regard to its weight-to-strength ratio provided that all other conditions are equal.
Old section of NY Metro Subway system,
Tower crane post and arms,
New Orleans Super dome, etc.
Auburn University Highway Bridges, Past, Present, and FutureGeorge Washington Bridge, New YorkDesigned by Amman, opened in 1931
● For containment type structures
maintaining two or more separate pressure
or temperature zones, continuous barriers,
membranes, plates and shells, are
● When the loads (both transverse and
small→membrane, i.e., placard
topic of discussion
AASHO Standard Specifications for Highway Bridges, 9th ed., 1965 adopted for the first time the minimum moment of inertia of the longitudinal stiffener:
There was no further stipulation as to the correct value for k.
For composite box girder compression flanges stiffened longitudinally and transversely, AASHTO requites the minimum moment of inertia of the longitudinal stiffener:
It is of interest to note that the absence of a length parameter
of the longitudinal stiffener in both AASHTO equations.
A longitudinal stiffener attached to the compression flange is
essentially a compression member.
It was found that an old bridge, (curved box girder approach spans to the Fort Duquesne Bridge in Pittsburg) designed and built before the enactment of the AASHTO criteria on longitudinal stiffeners, did not rate well for modern-day traffic, despite having served for many years.
Despite the practicing engineers’ intuitive realization of the unreasonableness of the equations, they are still in force in both AASHTO Standard Specifications for Highway Bridges, 17th ed. (2002) and AASHTO LRFD Bridge Design Specifications, 4th ed. (2007) with a limitation imposed on the number of longitudinal stiffeners not to exceed “two.”
In a relatively short period of time, there were a series of tragic collapses occurred during the erection of the bridges
Danube in 1969
Milford Haven Bridge in Wales in 1970
West Gate Bridge in Australia in 1970
Koblenz Bridge in Germany in 1971
These tragic collapses drew an urgent attention to steel box girder bridge design and construction. Some of the researchers, primarily in the U.K., responded to the urgency include:
Although there were a few variations tried, such as
Effective Width Method
Effective Length Method
these researchers were mainly interested in “Column Behavior” of the stiffened compression flanges.
Barbré studied the strength of longitudinally stiffened compression flanges and published extensive results in 1937.
Bleich (1952) and Timoshenko and Gere (1961) introduced Barbré’s study (published in German) to English speaking world using the following model:
element subjected to a transverse loading
● Very thin plates depend on the membrane action as
that in placards and airplane fuselages
● Ordinary plates depend primarily on the bending action
● Very thick plates depend on bending and shear
Our discussions herein are limited to the case of ordinary plate
Elements (no membrane action, no shear deformation)
It was known from the early days that stiffened plates with weak stiffeners buckle in a symmetric mode while those with strong stiffeners buckle in an antisymmetric mode. The exact threshold value of the minimum moment of inertia of the stiffener, however, was unknown.
symmetric buckling implies column behavior and
antisymmetric buckling implies plate behavior
Critical Stress vs Longitudinal Stiffener Size
(Note: 1 in. = 25.4 mm; 1 ft = 0.305 m; 1 in4 = 0.416106 mm4; 1 ksi = 6.895 MPa)
yield zone = compact
transition zone = noncompact
elastic buckling zone = slender
It also works for horizontally curved box girders.
Tee, WT9x25: A = 7.35 in2, tf = 0.57 in
Is = 53.5+7.35(8.995-2.12)2 = 400 in4
Rectangle, d/t = 0.38(E/Fy)1/2 = 9.15 with Fy = 50 ksi for compact section:
9.15t2 = 7.35, t = 0.9 in, d=7.35/0.9 = 8.17 in
Is = 0.9(8.17)3/3 =164 in4Tee shapes are stronger than rectangles
Column Behavior Theory
Plate Behavior Theory