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Over-strength factor

  • 1.  Over-strength factor

    Posted 09-28-2022 10:46 AM
    Dear sir ,

    I would like to ask about the case shown in the attached picture for a building with intermediate frame system as a LFRS .

    Which elements must be designed for over strength factor as per ASCE provisions ?


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    Anas Dawas Aff.M.ASCE
    Engineer
    RAK
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  • 2.  RE: Over-strength factor

    Posted 09-28-2022 01:51 PM
    As per my understanding (someone can correct me) , Transfer beam (TR) and column B should only be designed for over strength factor ,

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    Anas Dawas Aff.M.ASCE
    Engineer
    RAK
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  • 3.  RE: Over-strength factor

    Posted 09-30-2022 07:56 AM

    Hi,  .. . . Eng. Anas Dawas. I'm glad to try to help you on this.

    I understand your question and I had to expand my DATABASE to answer with full experience.
    In Brazil we do not have Earthquakes so I had a limited experience in this design.  Because of this, I was thinking if the transfer beam with D-REGIONS and I was associate to STM approaches initially to solve localized structural themes like beam openings and variable depth, and so specific optimizations like that:  

    So Let's try to solve this!!

    I do some personal research and here some of my thoughts:

    *Unsymmetrical forms of building can have low performance than those with symmetric configurations.

    I supposed that I am seeing a structural concrete seismic resistant frame "MRF" and assuming you are giving preference to the selection of a "Structural Concrete Frame" to advance with your answer.

    Essential items in the ASCE 7-16:

    CHAPTER C2 COMBINATIONS OF LOADS

    C2.3.6 Basic Combinations with Seismic Load Effects

    "The seismic load effect including overstrength factor, Em, is combined with other loads. The purpose for load combinations with overstrength factor is to approximate the maximum seismic load combination for the design of critical elements, including discontinuous systems, transfer beams and columns supporting discontinuous systems, and collectors. The allowable stress increase for load combinations with overstrength is to provide compatibility with past practice".

    C12.3 DIAPHRAGM FLEXIBILITY, CONFIGURATION IRREGULARITIES, AND REDUNDANCY

    C12.3.2 Irregular and Regular Classification

    . . . The configuration of a structure can significantly affect its performance during a strong earthquake that produces the ground motion contemplated in the standard. Structural configuration can be divided into two aspects: horizontal and vertical. Most seismic design provisions were derived for buildings that have regular configurations, but earthquakes have shown repeatedly that buildings that have irregular configurations suffer greater damage. "This situation prevails even with good design and construction."…

    C12.3.2.2 Vertical Irregularity

    Vertical lateral force-resisting elements at adjoining stories that are offset from each other in the vertical plane of the elements and impose overturning demands on supporting structural elements, such as beams, columns, trusses, walls, or slabs, are classified as in-plane discontinuity irregularities (Type 4).

    C12.3.3.3 Elements Supporting Discontinuous Walls or Frames.

    . . . The purpose of requiring elements (e.g., beams, columns, trusses, slabs, and walls) that support discontinuous walls or frames to be designed to resist seismic load effects including overstrength is to protect the gravity load-carrying system against possible overloads caused by overstrength of the seismic force-resisting system. Either columns or beams may be subject to such failure; therefore, both should include this design requirement. Beams may be subject to failure caused by overloads in either the downward or upward directions of force. Examples include reinforced concrete beams . . . 

    "The connection between the discontinuous element and the supporting member must be adequate to transmit the forces required for the design of the discontinuous element".

    *First of all, we might recognize THAT D-REGION IS ALWAYS IMPORTANT WHEN SHEAR FORCES EXISTS!!

    *So I performed an additional Structural Analysis as well. . . for this simply imaginary simulation: 

    *In my view – It Would be important give concern where the ABRUPT CHANGES in GEOMETRY can cause the UNCONVENTIONAL FLOW on the SHEAR forces. Designing the TRANSFER BEAM and the 2 COLUMNS BELLOW would be interesting to increase RESILIENCE: (TB, C and B)  or +A in the future if you plan to remove column "B" in order to CREATE some additional Aesthetics SPACE between the C- - -A.

    I think it was my first time on this topic (SEISMIC), I hope it would be right on the majority of the explanations, if not, things not all the time have to be perfect, sometimes correct me as well if needed.
    Thanks, Andre.

    . . . and you can find an additional material over this question linking the JOINT SEI, CASE and NCSEA and the STRUCTURE MAGAZINE Initiative, by accessing our colleague:  Emily Guglielmo, P.E., C.E., F.SEI:

    STRCTUREMAGAZINE-OVERSTRENGTH



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    Andre Newinski S.E., A.M.ASCE
    Engenheiro Estrutural
    AN
    Santo Angelo
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  • 4.  RE: Over-strength factor

    Posted 09-30-2022 12:03 PM
    you are perfect and great
    I will comment my opinion as soon as possible

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    Anas Dawas Aff.M.ASCE
    Engineer
    RAK
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  • 5.  RE: Over-strength factor

    Posted 10-03-2022 05:44 PM

    Anas, your concern on the SEISMIC RESISTANT FRAMING SYSTEM made me reflect upon SHEAR. . . it is not only a SEISMIC concern, as it opened my mind to the Dangerous into general Structures. . .

    STRUCTUREMAG 

    I do think 3 Major Areas can be enhanced and really Motivate STRUCTURAL ENGINEERS for the Future: "Earthquake, Impact and Blast". And the reason for that is they are governed by special envelopes, I can imagine why Shear effects sometimes are not well covered along the years (neglected).  Because the codes of practice might have been test for conventional structures, so with different and newer structures it still remains uncertainty, specially when reinforcement are not present.  Elements without proper reinforcement are critical! Therefore, We should pay more attention on the design. Over repair is costly as well, and in some cases the structure has already collapsed and it is difficult to save human lives. 

    In STRUCTURAL ENGINEERING, future designs with SHEAR can be critical. Some reduction in member sizes not always compensate structures for being light, with drastic changes in geometries to a more intense energy flux the element goes heavier, thus the subject is on a higher stress, the elegant design must be complex and simply at the same time to attend several different perform objectives like thermal or deflection, or at least his own self load (dead load).  And it bring us to the case where every single point (D-region) of the structure matters. Unreinforced SHEAR is a common practice for Structural Elements such as: retaining walls, footings, silos, and earth-covered structures, the cases of "simple slabs" sometimes might not require by codes to induce Stirrups (transverse reinforcement). We also should give more attention to the likely elements while SE Profession advances, trying to reduce the size and dimension that longer spans or high altitudes requires for optimization models that can be applied to general structures in a small and large scale.  We might have to invent something flexible, high adaptable that are far away from our current technique of design, but we also have to think in present time. Maybe structures would not be made of concrete, but they can modify themselves or incorporated in something new. Maybe we can reach this new level in the next years far from those questions. 

     

    With all this in mind, we must remember how advancements will not neglect something to deal with the new and unknow (we can call this uncertainty), and it can be climate change for now, misery for the masses, or an undiscovered phenomena or another type of risk to manage. We need imagination, but must validate somehow by testing,  research and use our intuition to solve those big challenges to different possibilities and potential scenarios for favorable conditions.

    The best way is to validated that, always before put those theories in practice.

    thank you for this post! 

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    Andre Newinski S.E., A.M.ASCE
    Engenheiro Estrutural
    AN
    Santo Angelo
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  • 6.  RE: Over-strength factor

    Posted 09-30-2022 02:51 PM
    Thank you Eng. Andre Newinski ,

    This is perfect discussion with you especially that you gave me enough time to do some analysis to define clearly the effect of  abrupt change ,
    I will add my opinion , If we look to the attached picture (not picture in main post ) , we can find that :
    1) In picture (a) the over strength factor was applied for columns  C & D
    2) In picture (b) the over strength factor was applied for C only .
    the author of ASCE Guide book mentioned that in the second case (b) , the column (D) would carry the overturning forces even though the offset did not exist .

    * So we can find that the provision in ASCE need to be understood very well to select the elements they should be designed for over strength factor .
    * Most of examples in guide books are related to discontinuity of concrete walls but what about transfer beams ?

    I discussed before with one of CRSI member and he believe that transfer beam & all columns connected to transfer beam should be designed for over strength factor  !

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    Anas Dawas Aff.M.ASCE
    Engineer
    RAK
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  • 7.  RE: Over-strength factor

    Posted 10-01-2022 11:54 AM
      |   view attached
     Eng. Andre Newinski ,

    This picture is related to the previous comment

    Regards ,

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    Anas Dawas Aff.M.ASCE
    Engineer
    RAK
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  • 8.  RE: Over-strength factor

    Posted 10-03-2022 08:10 AM
    Thank you for your considerations Anas, and when you share more of your experience as well - I will respond too. Even thought the code sections are maybe simple as Chad said, there is a lot of room for interpretations - so I could agree with you  - since those type of Structures have a complexity of loads and effects to be analyzed, we cannot wait for that solution that would be simply as it can. There are ways to solve it. I would like to help you more with this - maybe in future I might have more insights on this area that is currently unconventional for me, but might play a important role ,as other areas of study as well (impact & blast). In my view, provide SAFETY is Vital to a Building located where this Type of Risk (Earthquakes), do Exists. And we should use this space as opportunities to try to help each other on these Structural Engineering subjects.

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    Andre Newinski S.E., A.M.ASCE
    Engenheiro Estrutural
    AN
    Santo Angelo
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  • 9.  RE: Over-strength factor

    Posted 10-04-2022 10:49 AM
    I am happy to share my experience with you
    I appreciate your help ,

    Regards ,

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    Anas Dawas Aff.M.ASCE
    Engineer
    RAK
    ------------------------------



  • 10.  RE: Over-strength factor

    Posted 09-30-2022 07:57 AM
    The answer is likely contained within the many provisions of ASCE 7.  While the sections are written to be simple and clear, it is important not to forget review of the accompanying Commentary volume.  In my cursory review, I see overstrength values listed for a number of components dependent on their function.  I think you are on the right track in thought of this simple example.  When applied to a real world application, definition of the member components and 3D consideration of the structure becomes important.

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    Chad Morrison P.E., F.ASCE
    Professional Engineer
    Greenville RI
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  • 11.  RE: Over-strength factor

    Posted 09-30-2022 02:51 PM
    Thank you ,

    I believe that engineering judgment always is very important  in such cases but we are designing much projects
    and much mega projects and these points still not understood well for most of our engineers due to simplicity of the code provisions in comparing with complexity of real life projects .

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    Anas Dawas Aff.M.ASCE
    Engineer
    RAK
    ------------------------------