Hi all, I am working on the design of a RC water tank, and I need your help...!!

Size and Structural Form

RC Rectangular Water Tank
- Designed as a Rigid Box Structure, as the Roof, Base & Walls are all monolithically connected
- Size of the Tank = 68m (Length) x 30m (Width) x 9m (Height)
- Thickness of Roof, Base, Walls = 0.4m



Baffle Walls
- 3 Baffle Walls inside the tank to guide the flow of water
- Designed to be Full-height Structural Wall transfering load from the Roof to the Base
- Not Full-length in order to allow water flow



Foundation
- Shallow Foundation sitting directly on Soil
- Base Slab acts as Foundation

Questions

1. What is the structural behaviour of the tank under seismic loading?
- The analysis shows that the roof has large in-plane deformation, resulting in high tension at far side, and compression at near side.
- The behaviour is similar to a beam sagging under UDL (compression at top; tension at bottom).
- The Roof does not act as a rigid diaphragm, but a flexible one.
- I reckon it is the actual behaviour, as the roof and the walls have same thickness, yet the roof has a much longer span (68m) while the wall is just 9m tall. The in-plane stiffness of the Roof should be smaller than the Walls. Given the smaller relative stiffness, the roof is flexible.
- I designed the tank with Slab Foundation, using Area Spring (z-axis)& Soil Spring (x & y-axis) at Base Slab. Does it affect the results??

2. Seismic Loading of the Tank
- Could anyone advise what is the best way to model the seismic loading?
- I tried equivalent lateral load method and response spectrum modal analysis method.
- However, the eq. lateral load method greatly amplify the effect in Q1 (giving very large tension at far side at roof, and compression at near side).
- To be clear, I used EC8, and apply the equivalent lateral load (base shear) as area load distributed to roof and walls according to their mass.