Von Mises Safety Factors from AISC, ASME, AWS, ABS and DNV.
Top 10 Unity Checks for von Mises Stress Values | Finite Element Analysis
Published on July 14, 2017
Eric Kuusisto, PE FollowEric Kuusisto, PE
Pushing the envelope in design for architecture & engineering
Introduction Most theories for developing an acceptance criterion in Finite Element Analysis (FEA) is based on von Mises stress values (distortional energy values), but a few depend on strain values . DNV states that the von Mises yield function is suitable for most capacity analysis of steel structures [1]. A non-linear FEA may analyze the structure beyond the
yield limit and try to observe a structure’s ultimate capacity by trying to model strain-hardening effects and evaluating the acceptance the results based on strain results. This is a more advanced analysis and only utilized in certain situations.
Note that a von Mises stress does not consider instability (lateraltorsional buckling, local buckling, etc.) of the structure the way the AISC allowable stresses do. It is the theoretical limit to the strain
energy density of the material. In theory, the UC for a FEA model would be the von Mises stress over the yield strength (UC = σvm/Fy); representing the maximum distortion energy of that structural model. However, that does not consider any factor of safety which considers the degree of uncertainty. For all von Mises results, a factor of safety should be considered with a unity check (UC) [2]. This is standard engineering practice to take design and risk uncertainties into account. Below is a list of allowable von Mises strengths (Fvm) that should be utilized for a UC value (UC = σvm/Fvm). Note that this methodology is tracking the various safety factors for different industries and utilizes the Allowable Stress Design (ASD) methodology. These allowable von Mises strengths (Fvm) should be compared from FEA results utilizing service loads, not factored ultimate loads. These references are also intended for an elastic FEA, as brittle
materials are more subject to fatigue concerns.
1. General Factors of Safety - Machine Designer's Reference The first resource on von Mises acceptance criteria is a list of common general safety factors intended for the design stage [3]. The following equations are commonly used when designing machinery components and could come into use if check general industrial applications. This
reference is also quoted as a resource in obtaining von Mises stress limits using FEA [2].
2. General Steel Structures - AISC AISC Steel Construction Manual contains allowable stresses based for manual methods of analysis. It is not intended that highly localized peak stresses from FEA (and which may be blunted by confined yielding) must be less than the stipulated allowable stresses [4]. For
members in shear and tension, AISC suggests the safety factor of 1.67 for a von Mises stress limit [5].
For stress concentrations and hot spots, AISC mentions highly localized peak stresses in the section Design Basis – Allowable
Stresses. According to AISC 335-89 on pg. 5-127, highly localized peak stresses determined from FEA, which may be blunted by confined yielding, are not intended to less than the stipulated allowable stresses [4].
3. Welded Connections - AWS For most FEA results, the welding material is not modeled but the welded connection is simplified by modeling the two parts with bonded contact surfaces, per DNV standard [1]. If the weld itself is of
primary concern, AWS suggested a safety factor of 1.5 as a strength design factor with von Mises stress limits on a weld [6].
4. Flat Plate Structures - API API has its own guidelines for FEA von Mises stress limits. For simple structures within the yield limit, it has a good reference for mesh sizing guidelines. For flat plate structures, the following equation is for the serviceability strength limit (yield) [7]:
API also allows for the von Mises safety factor allowable stress to be increased by a factor of 1/3 for load cases with additional design environmental conditions other than dead, live and seismic using the flat plate structures code and offshore structures code [7] [8] [4]. The required section properties should not be less than required for design dead, live loads and seismic without the 1/3 increase.
Note that most current editions of design standards (ASCE, AISC, ACI, IBC) no longer allow the 1/3 stress increase [9], as it allows engineers to potentially “double-dip”, violating the code in a non-conservative way. In lieu of using the 1/3 allowable increase, the use of updated ASD load combinations are preferred. However, the 1/3 allowable stress increase is still permitted with API and may be used with the von Mises unity check.
5. Hoisting Structures - API API lists different von Mises safety factors for structures that are used with hoisting equipment suitable for use in drilling and production operations [10]. This increases the safety factor utilized because obviously the risk of hoisting equipment failing carries a much higher degree of risk.
The load shall not exceed the maximum allowable von Mises strength (Fvm) as listed below.
Note that API lists provisions for an ultimate strength (plastic) analysis to be performed under either of the following conditions: a)
For contact areas
b)
For areas of highly localized stress concentrations caused by
part geometry and other areas of high stress gradients where the average stress in the section is less than the yield strength limit. This is permitted by Saint Venant’s Principle, allowance of localized yielding [4], and because FEA struggles to accurately capture stress results at the point of load application. See my article on how to analyze acceptability of FEA models with stress concentrations and hot spots.
A good methodology to use this API reference to build von Mises acceptability for hoisted structures would be to use the serviceability von Mises limit (Fy) for the nominal cross section area and the plastic von Mises limit (Fu) for stress peaks or contact areas. It may be overly-conservative to compare localized stress peaks to the yield von Mises limit. Always verify with hand checks.
6. Pressure Vessels - ASME ASME provides a solution to pressure vessels that may be evaluated using a limit load analysis [11]. Limit load analysis is based on the theory of limit analysis that defines a lower bound to the limit load of a structure as the solution of a numerical model with the following properties: a)
The material is elastic-perfectly plastic with specified yield
strength
b)
The strain-displacement relations are those of small displacement
theory c)
Equilibrium is satisfied in the un-deformed configuration
The limit load is obtained by using FEA by incorporating the model and small displacement theory to obtain a solution. The limit load is the load that causes overall structural instability. The point is indicated by the inability to achieve an equilibrium solution for a small increase in load (i.e. the solution will not converge). After defining all relevant load case combinations, each load case should be evaluated. The von Mises limit is the minimum of the following two equations.
Note that the yield von Mises stress limit is modified with a temperature ratio for pressure vessels above room temperature. The temperature ratio (Ry) does not affect the tensile strength limit (Fu).
8. Ships & Steel Vessels - ABS ABS has its own criteria for von Mises stress limits. For steel vessels, the hull is the watertight body of a ship. Above the hull is the superstructure or deckhouse. The pod is a reference to the podded propulsions that are installed in an increasing number of modern ships
due to the advantages of propulsion efficiency and maneuverability of the ship. The von Mises stress limits for the pod and hull below [12].
8. Offshore Structures & Mobile Drilling Units ABS ABS has another standard for offshore steel structure and the buckling and ultimate strength behavior of its fundamental structural components [13]. ABS refers to API RP 2A-WSD [8] codes where departures from ABS formulations are recommended. For offshore steel structures, the von Mises stress limits are the following:
ABS has yet another standard for mobile offshore drilling units (MODUs), which are what the barge rigs would fall into [14]. The following is the von Mises equivalent stress criterion:
9. Shipping Containers & Portable Units - DNV DNV is a good resource for design and analysis with FEA and for interpreting von Mises results [1]. This standard applies to portable
offshore unit’s main structure, supports and features importing for the
functionality during the transport phase. The loads used to evaluate the FEA results must be the factored design loads [15]. Using these factor ed des i g n loa ds per DNV standard, the allowable von Mises stress limit is given. Note this is the only allowable von Mises limit I have listed that factors the loads.
10. Offshore Structures - DNV DNV is also good references for von Mises peak or hot spot limits [16] [17]. This specification is concerned primarily with offshore structures
and ships but some of the same concepts may be translated over when justifying peak stresses in FEA results. The following is the von Mises stress limits for general FEA results.
Note that the modified limit for plates and stiffeners allows for a lower safety factor and cannot be applied to girders, stingers, global strength elements, load-bearing elements or buckling stability elements. The von Mises stress limit under accidental loads (explosions, fire, dropped objects, crashes, etc.) for plates and stiffeners is not applicable because those members are not seen as critical for global structural stability during extreme load cases.
DNV also lists permissible von Mises stress limits for hot spots. Hot spots or peak stresses are defined as local peak stresses by fine FEA meshes in areas with pronounced geometrical changes (i.e. corners) [14]. These peak stresses may exceed the stress limits established previously for general loads provided plastic mechanisms are not developed in the adjacent structural parts. This is addressed in a creation of a peak factor, η peak, which changes based on the mesh sized utilized in the FEA model.
For ship-shaped drilling units, DNV also has special criteria for local peak stresses with pronounced geometrical changes (corners, etc.) that they may exceed the previous usage factor. Based on a guidance note on local peak stresses on FEA areas where plastic mechanisms cannot be developed in the adjacent structural parts, the von Mises peak stress limit is provided below. This source provides no guidance on the hotspot criteria with regard to mesh density [17].
BONUS
These allowable von Mises strengths are based on Allowable Stress Design (ASD). This is because it is easier to track safety factors for acceptability. Another factor is it translates better from civil/structural engineering to other professions like mechanical, aerospace, petroleum, etc.
Today in Civil Engineering classes in college, Load Resistance Factor Design (LRFD) is the gold standard; which purpose is to split the safety factor into the load and strength portions for more precise control of unknowns. This works great with traditional building and highway design, but make it trickier for cross-discipline applications like FEA.
For this post, which compares safety factors suggested by different industry standards, ASD methodology was used as it makes it easier to track the factor of safety. There is not much literature on the reduced strength capacity (ϕRn) compared to the ultimate loads (Qu) for von Mises stresses.
REFERENCES
1. DNV-RP-C208 – Determination of Structural Capacity by
Non-linear FE analysis Methods (2013). pg. 16 2. Finite Element Analysis Concepts via SolidWorks – J. Ed Akin, Rice University (2009) 3. Machine Designer’s Reference – J. Marris, P.E. (2012). pg. 14 4. AISC 335-89 – Specification for Structural Steel Buildings
Allowable Stress Design (1989). 5. AISC Journal – Combined Shear and Tension Stress (1986). pg. 125 6. AWS Journal – Stress Analysis and Design of Double Fillet-
Welded T-Joints(1998). pg. 94-s 7. API BULL 2V – Design of Flat Plate Structures (2004). pg. 19 8. API RP 2A-WSD – For Planning, Designing and Constructing
Fixed Offshore Platforms (2010). pg. 38 9. AISC Modern Steel Construction Journal – The One-Third Stress
Increase: Where is it now? (2003). 10.
API SPEC 8C – Drilling and Production Hoisting
Equipment (2012). pg. 6 11.
ASME Title VIII Division 2 – Boiler and Pressure Vessel
Code (2009). pg. 5-12 12.
ABS – Steel Vessels - Part 3: Hull Construction and
Equipment (2015). pg. 177 13.
ABS – Buckling and Ultimate Strength Assessment for
Offshore Structures - Commentary Guide (2005). pg. 52 14.
ABS – Mobile Offshore Drilling Units - Part 3: Hull
Construction and Equipment (2001). pg. 64 15.
DNV 2.7-4 – Portable Offshore Units (2011). pg. 22
16.
DNV-OS-C102 – Structural Design of Offshore
Ships (2012). pg. 20 17.
DNV-OS-C107 – Structural Design of Ship-Shaped
Drilling and Well Service Units (2008). pg. 13
ABOUT
Eric Kuusisto is a registered Professional Engineer (Civil-Structural). He has worked in a wide range of structural engineering projects, from skyscrapers to transmission towers to oil & gas. Currently works for HALFEN USA as an Technical Sales Representative. Please like and comment! ShareShare Top10 UnityChecks for vonMises Stress Values |Fi nite Element Analysis
LikeTop10 UnityChecks for vonMises Stress Values |Finite Element
Analysis
Comment
ShareShare Top10 UnityChecks for vonMises Stress Values |Finite
Element Analysis
FollowEric Kuusisto, PE
Eric Kuusisto, PE
Pushing the envelope in design for architecture & engineering
9 articles
3 comments Newest
Sign in to leave your comment
3mo
Minh Minh abc at 1234 Do you have PDF file ? I can't see from limit [5] Like
Reply 4mo
Krzysztof Dyk Owner at CORDEM Sp. z o.o. I have the same question. Do you have PDF version of this? Like
Reply There is 1 other comment. Show more.
Don't miss more articles by Eric Kuusisto, PE
o
o
o
Acceptance Criteria for Nonlinear FEA | Buckling, Instability, Strains, & More! Eric Kuusisto, PE on LinkedIn
Do You Know Your Known Unknowns? Top 5 Relative Uncertainties in FEA Eric Kuusisto, PE on LinkedIn
SHELLS vs. SOLIDS | Finite Element Analysis Quick Review Eric Kuusisto, PE on LinkedIn
Looking for more of the latest headlines on LinkedIn? Discover more stories
Help Center About Press Blog Developers Careers Advertising Talent Solutions Sales Solutions Small Business Mobile Language Online Learning ProFinder Search Jobs
Directories
o o o
Members Pulse Companies Universities
LinkedIn Corporation© 2018 User Agreement Privacy Policy Community Guidelines Cookie Policy Copyright Policy Unsubscribe
