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Limit on Normalised Design Axial Force (Vd or v_d)

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1 year 7 months ago #81 by Andrew Mole
I have a couple of questions related to the “normalised axial force” v_d. It is defined as the "axial force from the analysis for the seismic design situation" divided by the product of the "area of section of concrete member" and the "design value of concrete compressive strength".

V_d = N_Ed / (A_c * f_cd)


5.2.3.7 (3)P c) "An appropriate limit of the normalised design axial force shall be respected (see 5.4.3.2.1(3)P, 5.4.3.4.1(2), 5.5.3.2.1(3)P and 5.5.3.4.1(2)) to reduce the consequences of cover spalling and to avoid the large uncertainties in the available ductility at high levels of applied axial force." (all references are reproduced below).

Q1 What is the research basis behind these limits? If the building will not go inelastic, is it possible to argue that these limits are not required? Can strain checks be used to demonstrate this?

Q2 For cores of tall buildings the individual walls will exceed the v_d limits, but the overall core will not. Is there any reason why the whole core should not be considered when assessing the v_d limit? It is essentially a “composite wall” in the language of section 5.4.3.4.1 (4) Ductile Walls : Bending and shear resistance.

5.4.3.2.1 Design for DCM : ULS verifications and detailing : Column : Resistances (3)P states: "In primary seismic columns the value of the normalised axial force v_d shall not exceed 0,65."

5.4.3.4.1 Design for DCM : ULS verifications and detailing : Ductile Walls : Bending and shear resistance (2) states “In primary seismic walls the value of the normalised axial load v_d should not exceed 0,4.”

5.5.3.2.1 Design for DCH : ULS verifications and detailing : Column : Resistances (3)P states “In primary seismic columns the value of the normalised axial force v_d shall not exceed 0,55.”

5.5.3.4.1 Design for DCH : ULS verifications and detailing : Ductile Walls : Bending resistance (2) states “In primary seismic walls the value of the normalised axial force v_d should not exceed 0,35.”

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1 year 7 months ago #84 by Edmund Booth
The most recent authoritative source of guidance and background material on the clauses of EC8 Part 1 on concrete is:

Fardis M, Carvalho E, Fajfar P and Pecker A. Seismic Design of Concrete Buildings to Eurocode 8. CRC Press 2015.

As a comment on Q1, the limit on axial force ratio in cl 5.2.3.7 (3)P applies only to primary columns in DCM and DCH structures – i.e. those designed to respond in a ductile, post-elastic manner to the design ground motions. Where the structure is designed as an ‘essentially elastic’ DCL structure, this clause would not apply, and the design for axial capacity would be based on EC2.

As a comment on Q2, EC8 Part 1 currently deals explicitly only with rectangular or flanged walls. For these configurations, the intention of EC8 is that the limitation on axial force should apply to the entire wall, not just one of its branches, with the peak compressive stress of the overall section justified by supplying a suitable amount of confinement steel. Explicit rules are not given for other geometrical forms of concrete shear cores, although it would usually be reasonable to make a similar assumption. However, explicit rules for other common geometrical forms are expected to be included in the next revision of EC8 Part 1, drafting of which is expected to start in spring 2016, but the revision will not be finally published before 2020.
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