Engineering Problem Statement: 2-Way Prying Action in Structural Steel Flanges
Cobey Alderden
Milwaukee School of Engineering
CAE 7810
Professor Gary Shimek
September 11, 2023
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When designing a steel connection, there are many checks an engineer must perform on the
design to ensure that it is strong enough to resist all of the possible loadings that could be
imposed on it. These checks are called limit states. Limit states are, in essence, ways in which a
designed member could potentially fail based on years of previous observation and testing. The
limits states that apply to a steel member or steel connection rely on a multitude of different
variables; some of those being the type of force being applied to the member, the structural role
of the member in the overall building, and the type of connection or member being designed.
One limit state that occurs in structural steel that is connected with bolts and subject to tensile, or
pulling, forces is prying action.
The American Institute of Steel Construction (AISC) defines prying action as:
“The phenomenon (in bolted construction only, and only in connections with
tensile bolt forces) whereby the deformation of a connecting element under a
tensile force increases the tensile force in the bolt above that due to the direct
tensile force alone” (2017).
This definition can be simplified down to the understanding that when steel members pull on a
group of bolts that connect it to another member, there is a potential increase in the force that the
bolt must resist, and a decrease in the force that the steel member needs to resist, depending on
the stiffness of the steel member that is being connected. This is due to the fact that if two
connected members are subject to loading, the force will be resisted more in a stiffer element and
less in the less stiff element. This principle is what creates the need to check for prying action
due to the high degree of variability that exists between the stiffnesses of steel members.
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The AISC Steel Construction Manual provides guidelines on how to design a connection that
meets the before mentioned prying action limit state. However, the design guidelines in the AISC
Manual are only applicable to steel W-Shapes, or “I-beams” and angles that have prying action
in one direction, along the cross section of the member. This is due to the nature of the shapes
cross section and the interaction of the flanges of the members and the web. The web serves as a
stiffening element, which means that it will attract more force than the flanges, which are
significantly less stiff. This creates the prying action mentioned earlier that must be checked.
My Capstone research will investigate the issue of 2-way prying action, that is prying action in
the longitudinal plane of a member, or looking at the member from its side, rather than the crosssection. This problem occurs when in addition to the common prying action orientation
previously described, w-shape members have a stiffener that attaches at a ninety-degree angle to
the web with bolts that are in tension on either side of the stiffener in the member flange. In this
configuration, it is reasonable to assume that there would be prying action in the plane of the
stiffener, as it is essentially acting as a web and increasing the members stiffness in this plane.
The AISC currently does not have any design guidelines for when this two-way prying action
likely occurs.
This issue is important because there could be structures out there today that were designed and
built without taking into account this additional check. If two-way prying action is present and
not accounted for, there is an increase in the amount of load that goes into the connecting bolts.
This additional load could be enough to make a connection fail. The assumption of two-way
prying leads to an increase in the capacity of the connecting member. If two-way prying action
does not have a significant impact on the distribution of forces in a connection, this assumed
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material strength increase could also be incorrect. Both of these design considerations, if done
incorrectly, could lead to the failure of a connection.
This problem is interesting to me because it has the potential to be implemented into the AISC
steel manual if enough research and testing can be done to prove that it is viable. This testing
will be difficult due to the complex nature of the issue of prying. The results of prying action
cannot be easily measured in a physical lab environment. It will take multiple iterations of a
computer model in conjunction with physical test results to be able to gather enough data to
accurately assess whether two-way prying should be a limit state that is considered. The
challenging part then comes in the development of a design equation. The goal of this project is
to 1.) assess whether two-way prying exists, and 2.) develop a design equation for two-way
prying. To create an equation, there are multiple conditions that need to be assessed, such as
member dimensions, member strengths, bolts strength, bolt configurations, and the path of the
load through the member. These factors will have an impact on whether the two-way prying
action will occur and how it will act within the connection.
Through my initial research I have developed the following questions that I intend to answer by
the end of my capstone research.
1. Does a stiffener perpendicular to the web of a structural steel W-Shape cause a prying
action perpendicular to the plane of prying action described in the AISC Steel
Construction Manual?
2. Does two-way prying action result in a significant increase in the amount of tensile load
resisted by the connecting bolts?
3. What is the relationship between the thickness of the web stiffener and the increase in
two-way prying action forces in the bolt?
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4. Does two-way prying action allow for a greater increase in connecting member strength
that one-way prying action?
5. Does two-way prying action create internal forces that demand a reduction in the
connecting member strength?
6. Is there a ration between member size and applied load that would indicate the presence
of two-way prying action?
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References
Steel Construction Manual. (2017). American Institute of Steel Construction.