What is Floor Deflection?

A floor is a flat surface that contains air space between its layers. Air spaces are created due to friction or other forces acting on the floor.

These air spaces act like springs and when they move, it causes the layer underneath them to move too. If you have two floors with identical thicknesses but one has a small gap between it’s layers, then the air pressure will cause those gaps to close up making the second layer stronger than before. When the gap closes up, it reduces the force exerted on the layer beneath it and therefore makes it less likely that that layer will collapse.

How Do You Calculate Floor Deflection?

The first thing you need to know is how much force you want to apply against your wall. For example if you have a 10 inch thick wall and want to increase its strength by 5%, then you would use a 5% increase in force (5%) multiplied by 10 inches = 0.05 pounds per square inch (psi).

Now you just multiply that number by the thickness of your wall. So if your wall is 1 foot thick, then you would use 0.005 psi to calculate the amount of force required to increase its strength by 5%.

Now you can figure out how much force you need to apply against each side of your wall. To determine which way to go, first decide whether or not there is any room between the top and bottom edges of your walls.

You need to build up the wall if there is no space between the edges. You can do this by using plywood or metal sheets. You can use drywall alone if there is a small space between the top edge and the ceiling.

If you are going to add sheets of plywood to the top of your wall, then use this formula: Load from above (psi) = 0.005 * (wall thickness + plywood thickness).

For metal sheets, use 0.0035 * (wall thickness + metal sheet thickness). If you are going to use drywall alone to build up the top of your wall, use 0.0005 * (wall thickness) for the load from above.

Now that you have the load from above, you need to apply a force against the bottom side of your wall. You can do this by using earth or concrete.

Use this formula: Load from below (psi) = 0.0003 * (wall thickness) * 0.4 for earth and 0.0002 * (wall thickness) * 0.8 for concrete.

For example, if you have a 10 inch thick wall with 5% more strength, then use 0.0005 for the load from above and 0.4 for the load from below (0.005 * 0.1).

This means that you can use earth or concrete against the bottom of your wall. Make sure the earth is packed and leveled.

Good Luck!

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__Sources & references used in this article:__

- Deflection calculation for reinforced concrete structures—why we sometimes get it wrong (RI Gilbert – Structural Journal, 1999 – concrete.org)

https://www.concrete.org/publications/internationalconcreteabstractsportal/m/details/id/779 - Limitations and uncertainties in the long-term deflection calculation of concrete structures (B Vakhshouri, S Nejadi – Vulnerability, uncertainty, and risk …, 2014 – ascelibrary.org)

https://ascelibrary.org/doi/abs/10.1061/9780784413609.055 - Deflection calculation and control for reinforced concrete flexural members (W Zhou, T Kokai – Canadian Journal of Civil Engineering, 2010 – NRC Research Press)

https://www.nrcresearchpress.com/doi/abs/10.1139/L09-121 - Efficient Design for the Calculation of the Deflection and the Shear Force Capacity of Slim-Floor Girder (U Kuhlmann, G Hauf – … Construction in Steel and Concrete VI, 2011 – ascelibrary.org)

https://ascelibrary.org/doi/abs/10.1061/41142(396)16 - Shrinkage, cracking and deflection-the serviceability of concrete structures (BO Aalami – ADAPT Corporation, 2008)

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.493.1680&rep=rep1&type=pdf - Deflection of two-way floor systems by the equivalent frame method (RI Gilbert – Electronic Journal of Structural Engineering, 2001 – Citeseer)

https://www.concrete.org/publications/internationalconcreteabstractsportal.aspx?m=details&i=11132&m=details&i=11132 - Virtual machining considering dimensional, geometrical and tool deflection errors in three-axis CNC milling machines (AH Nilson, DB Walters – Journal Proceedings, 1975 – concrete.org)

https://www.sciencedirect.com/science/article/pii/S0278612514000442 - Characterization of the protective capacity of flooring systems using force-deflection profiling (M Soori, B Arezoo, M Habibi – Journal of Manufacturing Systems, 2014 – Elsevier)

https://www.sciencedirect.com/science/article/pii/S1350453312000926