Cumulative tutorial 19
Auditing an analysis you did not run
Six checks in order, on an output that looks entirely normal.
A subconsultant has issued a modal and response-spectrum analysis of a nine-storey concrete frame. The report is neat, the mode shapes are plotted, and there are no warnings. Run the checks before reading a single member force.
What you are given
- Nine storeys, 31 m tall, plan 540 m²
- Reported first period 0.38 s
- Reported total seismic weight 47 000 kN
- Reported base shear 18 000 kN
- Reported first-mode effective mass 74%; cumulative over 9 modes 97%
Tutorial 19
Work the problem
4 questions
Question 1
What first period does the rule T ≈ 0.075H^0.75 predict, in seconds?
Question 2
What is the seismic weight per unit floor area, in kN/m²?
Question 3
What is the base shear coefficient?
Question 4
By what factor is the model's stiffness in error, taking the expected period as 0.985 s?
What this establishes
Ten minutes, four numbers, and the whole submission is established as unusable before anyone opened a drift table. Note also what passing all six checks would NOT have established: only that no fault of these kinds is present, which is a much weaker and more honest statement than 'the analysis is correct'.