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Free Resources to Downlaod: (Updated every week)

• Beam design procedures to BSI
• Load bearing wall design procedures to BSI
• Isolated footing design procedures to BSI
• Thrust Block design procedures
• ACI-fundamentals powerpoint lecture

In focus(Updated every month)

Earthquake  Design  procedure  for  concrete  building structures  in  accordance  with  the  UBC 97

 Select the Lateral Force Procedure to be adopted in the design ( UBC 1629.8)

 – Static

 – Dynamic

Equivalent Static Force Procedure

 Derive the total design base shear  (UBC 1630.2.1)                      

 Calculate the general parameter to be fixed for a specific project.

 -  Seismic Zone Factor ( UBC   Tbl 16 – I)

- Soil profile type  ( UBC Tbl 16 – J)

- Seismic Dynamic Response spectrum coefficients Ca & Cv (  UBC   Tbl 16 – R & 16 –Q)

- Importance Factor I (UBC   Tbl 16 – K)

- Measure of Over Strength R (UBC  Tbl 16- N) 

Derive the period TA from Method A

                  TA = C_t (h_n ) ^0.75

Limiting that period by 1.4TA for zone 2,3 and 1.3 for zone 4 (Method B - UBC)    

The Design Base Shear, V = (C_vI / RT ) W

  But check min. and max : 0.11 C_a I W < V < (2.5 C_a I /R) W

  Where C_a is determined from Tbl 16 – Q

 Derive the ultimate earthquake force E from clause 1630.1.1

 E = ? E_h + E_v

E_h = V determined above applied  

 E_v = ±  0.5 C_a I D applied in the    V ,where D is dead load

 For static procedure, distribute the shear to each story in accordance with  1630.5

Design the structure for torsional moments arising from the accidental torsion, applied by assuming the mass of any storey is displaced by 5 % of the plan dimensions (1630.6)

 If a torsional irregularity exists, the accidental torsion has to be amplified as required by                    1630.7) using mode

Calculate storey drifts in accordance with 1630.9. Check this value against 1630.10.

 The drift should be calculated based on both translational and torsional deflections and should adopt inertia values based on cracked sections (1630.1.2).

Moreover, for torsion accidental force application, check mode 1  and see if the building is twisted or not and check the ratio (max. displacement/ average displacement < 1.2) other wise you need to amplify your worst combination (check 1630)

 Check the following irregularities where applies

Weak Storey ( Tbl  16 – L)

Torsional Irregularity ( Tbl 16-M)

 If there is any Torsional Irregularity exists , Orthogonal effects shall be considered  (Ref. 1633.1)

Dynamic Procedure

 Carry out a Natural frequency analysis to estimate the natural frequency of building and the mass participation.

 Check period T = 1 / f –    (f - frequency)

 Check   1.4 TA > TB , use TB = 1/f

 Carry out a spectral response analysis in accordance with 1631.5 using  specified Zone  spectrum

Compare the Base Shear results against dynamic base shear determined from clause 1631.5.4

i.e. V_des  = (90% regular ) or (100%  irregular)   x statics

        Minimum V_des = V _dynamic from Spectrum/R

Use the greatest value to determine the scale factor

 Scale Factor  =  V_des / V_dynamic

General Notes:

• It is preferred to have the same stiffness for you building in both directions in order to resist the earthquake forces...

• UBC-97 load combination is very close to ACI-318M-08 combinations

• Earthquake force in UBC-97 is factored by 1.4, so in service load combination you will notice that earthquake force is divided by 1.4

• Mass participation should be at least 90% in both direction X, Y

• For Z direction you need to perform vibration analysis

• P-delta analysis needs to be performed as geometrical nonlinearity

• Scale factor based on NEWMARK table should be between (1 - 4) max. otherwise, your structural system needs to be changed.

• orthogonal load combinations (100%Ex + 30%Ey....ect) can be used if you do not use SRSS (square root of sum of squares) and CQC (completing Quadratic combinations)

• Ductility of you structure is very important and you need to check it with acceptance percentage  specified 

• ETABS can calculate for you the spectrum function directly, STAADPro you need to imported, and Robobat gives you multiple options and multiple methods to calculate the spectrum and modal seismic analysis...

In the next article, asdd-engineering.com will discuss the spectrum functions in details. At the end, asdd-engineering.com thanks Eng.V. and Eng.MD for their help ...