Basements and Substructure Uplift Checks | Hydrostatic Uplift Check Calculations

Summary

This video explains the importance of managing hydrostatic uplift in basement construction. It highlights the need for calculating the upward water pressure and ensuring that the structure’s mass can resist it. The video details how to conduct hydrostatic uplift checks, with a focus on the groundwater table and weight of the structure. It also walks through a sample calculation, showing how to ensure the structure's mass is sufficient to counteract water pressure. The video emphasizes the necessity of these checks during the design phase to prevent structural mishaps during construction.

Takeaways

• 😀 Hydrostatic uplift is the upward pressure exerted by water on structures with basements, and it must be considered during construction to avoid mishaps.
• 😀 Although hydrostatic uplift is accounted for in design and reinforcement, the real question is whether the structure's weight is sufficient to resist this upward force from water pressure.
• 😀 Dewatering procedures help manage water around the construction site, but it is not economical to continue dewatering throughout the lifespan of the construction.
• 😀 A hydrostatic check and calculations should be performed by a structural engineer to ensure that the design can resist the uplift pressure during construction.
• 😀 ASCE 7 2010 Section 3.2.2 requires that the upward pressure of water be considered in the design of basement floors or slabs below grade, and it should be measured from the underside of construction.
• 😀 The hydrostatic uplift check should be made based on the groundwater table's height relative to the foundation, which is typically indicated in the soil investigation report.
• 😀 To perform a hydrostatic uplift check, the structure’s downward load (due to its mass) must be greater than the upward load exerted by water pressure.
• 😀 The code does not specify a safety factor, but it is recommended that the downward load be at least 25% greater than the upward load due to hydrostatic pressure.
• 😀 The uplift check equation ensures that the mass of the structure overcomes the uplift pressure, with a minimum safety factor of 1.25.
• 😀 If the structure fails to satisfy the uplift check equation, increasing the slab or raft thickness can help. Alternatively, the weight of the upper floors may be used to neutralize the uplift pressure.

Q & A

What is hydrostatic uplift and why is it important in basement construction?

Hydrostatic uplift refers to the upward pressure exerted by water on a structure, like a basement. It is important because this pressure can potentially overturn or collapse a structure during construction if not properly managed.

What role does water pressure play in the design of basement floors?

Water pressure must be accounted for in the design of basement floors, as per ASCE 7-10 code. The upward pressure from water is considered when designing basement slabs, ensuring the structure can resist this force.

How can the risk of hydrostatic uplift be managed during construction?

Hydrostatic uplift can be managed by performing calculations and ensuring that the mass of the substructure is greater than the uplift force caused by water. Additionally, controlling groundwater through dewatering is common, though it’s not sustainable long-term.

When is a hydrostatic uplift check necessary during construction?

A hydrostatic uplift check is necessary if the groundwater table is higher than the bottom of the foundation level. This check ensures that the structure's mass can resist the uplift caused by the water pressure.

What factors should be checked before performing a hydrostatic uplift calculation?

Before performing the calculation, the groundwater table should be checked. If the table is above the foundation level, a hydrostatic uplift check is essential. The soil investigation report usually provides this data.

What is the key equation used to check if the structure’s mass is sufficient to resist hydrostatic uplift?

The equation used is: the mass of the structure (WT) divided by the uplift pressure should be greater than or equal to 1.25, accounting for a 25% safety factor to resist the uplift pressure.

What happens if the structure’s mass does not meet the hydrostatic uplift requirements?

If the structure’s mass is insufficient, the thickness of the slab or raft foundation can be increased to balance the uplift pressure. Alternatively, the weight of upper floors can be considered to neutralize the uplift.

How is the water pressure calculated for hydrostatic uplift?

Water pressure is directly proportional to the depth of the water and the specific weight of water. It acts equally in all directions, and the pressure increases with depth.

What is the significance of the 25% safety factor in hydrostatic uplift calculations?

The 25% safety factor ensures that the downward load from the structure’s mass is significantly greater than the upward water pressure, providing additional stability and safety during construction.

What is the recommended procedure if increasing slab thickness is not possible during construction?

If increasing slab thickness is not feasible, the weight of upper floors can be used to help neutralize the upward pressure from water. It’s important to ensure that the total mass of the structure is sufficient to resist the uplift.