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  • 1.  Seismic Design of Diaphragm and strengthening design of existing buildings

    Posted 21 days ago

    Hi All,

    If someone needs to have some discussion around the diaphragm design or seismic component of new buildings or seismic strengthening of existing buildings. Please reach out to me. Happy to assist the way I can.

    Cheers

    Muneeb



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    Muneeb
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  • 2.  RE: Seismic Design of Diaphragm and strengthening design of existing buildings

    Posted 20 days ago

    Hi Muneeb,

    Could you please recommend a resource for designing diaphragms for post-tensioning system according to Australian standards, as well, is it possible to detail the diaphragm reinforcement bars and replace them with post tensioning strands, another question, does Ram Concept (a software for designing reinforced concrete and posttensioning elevated and slab on ground systems) design the required diaphragm reinforcements. I am looking forward for your kind assistance regarding this matter.

    Best regard



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    [Luay] [Al-loos]
    B.Sc. M.Sc. MIEAust. NER (Structures)
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  • 3.  RE: Seismic Design of Diaphragm and strengthening design of existing buildings

    Posted 20 days ago

    HI Luay, Thanks for the message.

    Gravity design: Out of plane forces

    Diaphragm Design: In-plane forces. Diaphragm is designed for collector elements, tension and compression chords and shear friction reinforcement. 

    Post tension design of slab is for the gravity loads, while diaphragm design is related to lateral loads, particularly for seismic loads. Diaphragm reinforcement can not be replaced with tendons as tendons are for gravity loads. While the reinforcement in diaphragm takes in-plane tensile forces which can not be resisted by tendons. 

    As per Australian Standards, diaphragm design needs to be completed by strut and tie 15.2.2.1 & C15.2.2.1 (or previously by finite element analysis). Diaphragm can be completed by any finite element software if you are designing it based on finite element analysis and by using section cuts. For strut and tie, the easiest software is SpaceGass. You just need to apply the in-plane forces and balance the forces to transfer these to lateral resisting elements.

    Please let me know if you have any further questions.

    Cheers

    Muneeb



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    Ahmad Muneeb Badar
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  • 4.  RE: Seismic Design of Diaphragm and strengthening design of existing buildings

    Posted 19 days ago

    Hi Muneeb,

    Thank you for your message.

    According to the "Design guide for reinforced concrete diaphragms" CRSI concrete reinforcing steel institute, which refers to ACI 12.4.2.4, in-plane design moments, shear forces, and axial forces in diaphragms must be determined using any method satisfies equilibrium and the design boundary conditions. Methods of analysis based on the following models are permitted to be used:

    • A rigid diaphragm model where diaphragm can be idealized as rigid.
    • A flexible diaphragm model where diaphragms can be idealized as flexible.
    • A boundary analysis in which the design values are the envelope of values obtained by assuming upper bound and lower bound element model considering diaphragm flexibility.
    • A finite element model considering diaphragm flexibility.
    •  A strut-and-tie model in accordance with ACI23.3.

    Is it possible to use the upper mentioned methods in the analysis rather than the ones specified by AS3600-2018 as an alternative method (some being mentioned).

     Regarding post tensioning strands, through my previous reading I came across an example of compression and tension members designed using both conventional and prestressed strands, please refer to the reference "Design of prestressed Concrete to AS3600-2009" Chapter 13.

    Thanks again for your assistance

    Best regards



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    [Luay] [Al-loos]
    B.Sc. M.Sc. MIEAust. NER (Structures)
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  • 5.  RE: Seismic Design of Diaphragm and strengthening design of existing buildings

    Posted 19 days ago

    HI Luay, Thanks for the message,

    Indeed, you mentioned correctly, the diaphragm design method should satisfy the equilibrium so thats why Australian codes (and New Zealand code, ASCE) has asked to design the diaphragm based on strut and tie method. In case of response spectrum method, the equilibrium can not be achieved as the total forces are the SRSS or CQC of different modes and these modes are in different directions. So thats the reason, diaphragm design should be by strut and tie method. 

    The first assumption of assuming the diaphragm as rigid, is not correct. Although, the in-plane capacity of diaphragm is very high but still not infinite. And if you use Rigid diaphragm option in ETABS model, the results obtained might not be correct as the force distribution will be based on diaphragm as rigid and other factor will be ignored. Hence rigid diaphragm assumption is not suitable. there is a lot of material which CSI (ETABS) has published on the rigid diaphragm assumptions. Always use Semi-Rigid diaphragm.

    As far as flexible diaphragm is concerned, this is just steel rod cross bracings which will act as flexible diaphragm and it will be designed totally based on tributary area method while Rigid / Semi Rigid diaphragm will be based on stiffness.

    AS3600 mentions Strut and Tie model. hence the safest, easiest reliable and conservative method is strut and tie.

    I have not read that book about using PT strands in diaphragm design, but I think PT strands can not be used as diaphragm reinforcement. As mentioned above, diaphragm is designed for collector elements, tension and compression chords and shear friction reinforcement. PT strands can only have some effect on tension Chord reinforcement design where the perimeter of the building diaphragm is in bending. All other elements will not have any effect from PT Stands. Secondly, there is a lot of research completed where it is mentioned not to combine gravity reinforcement/ crack control reinforcement for diaphragm design. Slab is always taking gravity load and hence the reinforcement is already stressed to a certain level. And in case of seismic event, the available yield strength is quite low and if you have used same reinforcement to act as diaphragm reinforcement then diaphragm might crack. 

    Cheers

    Muneeb



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    Ahmad Muneeb Badar
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  • 6.  RE: Seismic Design of Diaphragm and strengthening design of existing buildings

    Posted 19 days ago

    Hi Muneeb,

    Thank you for your message and for the detailed information, regarding the prestress book, the example is for column not a diaphragm, I'm happy to share with you this chapter if you like.

    Best regards,



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    [Luay] [Al-loos]
    B.Sc. M.Sc. MIEAust. NER (Structures)
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