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Saturday, December 29, 2018

Uplift Verification for Underground Structures: Solved Example

It is widely recognised that an object will float in water, if the weight is less than the upthrust. Upthrust is an upward force exerted by a fluid that opposes the weight of an immersed object. This also applies to structures that are buried under the ground, and subjected to ground water action. Structures such as basements, foundations, underground tanks, and swimming pools are at risk if the dead weight is less than the upthrust, especially when the structure is empty.


Clause 10.2 and 2.4.7.4 of EC7 (EN 1997-1-1) recommends an approach for verifying structures against uplift. Uplift failure (UPL) is verified under ultimate limit state (ULS), and in this post, we are going to show how you can verify UPL using Staad Pro software and manual analysis. To check for uplift, we must confirm that the destabilizing actions (both permanent and variable), are less than permanent stabilizing actions. We do not consider favourable variable actions when verifying uplift. Let us look at an example below;

Example
Let us verify the uplift stability of a 3 m x 2 m x 4.0 m concrete tank that is buried under the ground (see figure 3.2). The water table is 1.0 m below the natural ground surface. Thickness of all elements is 350 mm, unit weight of concrete is 25 kN/m3, and unit weight of water is 10 kN/m3. Verify the uplift stability of the tank during construction before the installation of the first floor slab.





Solution
(a) Stabilizing Forces
Weight of the base = 3.7 x 2.7 x 0.35 x 25 = 87.41 kN
Weight of the walls = 2(3.7 x 4.0 x 0.35 x 25) + 2(2.0 x 4 x 0.35 x 25) = 259 + 140 = 399 kN
Total weight (Gstb) = 486.41 kN

We are neglecting side friction and other possible stabilizing forces

At ultimate limit state, Gstb,d = 0.9 x 486.41 = 437.769 kN

(b) Destabilizing Forces
Head of ground water above the base = 4.35m – 1.0 m = 3.35 m
Vdst,d = (3.7 x 2.7 x 3.35) x 10 = 334.665 kN
At ultimate limit state Vdst,d = 1.1 x 334.665 = 368.131 KN

Vdst,d/Gstb,d = 368.131/437.769 = 0.84 < 1.0 Ok

Therefore the tank is safe against uplift at that stage of construction.

If the structure had failed uplift verification, the remedial actions that could be adopted are;

(1) Increase the thickness of the structural elements and/or projecting the base beyond the walls
(2) Employing drainage and ground water lowering techniques
(3) Introducing anchor piles

To download our publication on design of swimming pools and underground water tanks, click HERE;

To download our publication on design of swimming pools and underground water tanks, click HERE;


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