Considering lateral earth pressure to resist lateral loads 1

[Eng.Abdulla95]

Hi, I am new structural engineer

I have been wondering about lateral stability of structures for few months now. In the foundations we tend to make provision of tie beams to support the bottom of columns laterally, but I think it is redundant in some cases.

consider a case of column with continues reinforced strip footing and ground beam in one direction so the column is effectively tied in one direction, but not on the perpendicular direction. The building is a 2 to 3 stories villa.

Now, can we consider the lateral earth pressure of about 0.6 m deep foundation to support the column in the other direction?

Please find the attached drawing to illustrate the Idea.

 

[MIStructE_IRE]

Well unless the foundations are sitting on a bed of ball bearings then passive pressure + friction are exactly what ultimately provides the lateral resistance.

 

[Eng.Abdulla95]

@MIStructE_IRE

I dont think friction is helpful as it is normal practice here to put polythene sheets under the footing.

My concern is if the soil is loose around the footing, how will this pressure develop? what if they did not compact the soil around the footing and just do compaction below the footing?

 

[MIStructE_IRE]

Interesting, we’ve never had sheets under the footing - that would in fact replicate ball bearings!

We also pour most foundations against tye earth face rather than shuttering, so the passive capacity of the undisturbed soil remains.

You would need to ensure that they’re compacting the soil around the sides of the footing. Without that, you have very little passive resistance.

 

[oldestguy]

Those footings act as a horizontally reinforced rectangle. All parts of that rectangle work together, in addition to local soil resistance near that footing.

 

[Phil1934]

Someone may decide to put topsoil and plants near the walls so you should discount the top foot. Also cost to increase footing size and reinforcement to spread load very far from column may be excessive.

 

[Eng.Abdulla95]

thank you all I appreciate your replies.

 

[Ron]

OG...exactly!


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[WARose]

Now, can we consider the lateral earth pressure of about 0.6 m deep foundation to support the column in the other direction?

I'm not 100% sure I am following the question/situation......but be careful relying on passive pressure to resist lateral loads (i.e. wind or seismic). For one thing: most people ignore it as far as overturning resistance goes.

In any case, if it does work out that you can use it in your case......make sure your foundation system/structure can accommodate the (sometimes) large movements involved to engage passive pressure.

 

[Eng.Abdulla95]

@WARose exactly, especially the soil is loose, we are left to the rigidity of the foundation to act as horizontal beam (similar to vertical drop beams), but even this may not give enough restrains to lateral loads.

Even though we do not have earthquakes on my region and wind forces are minimal for 2 to 3 stories villas, we consider small amount of horizontal loads, so that the columns are effectively pined on the ground.

We cant relay on friction as we are using polythene sheets to protect the foundation from high salt (we are on an island), we cant relay on passive pressure as you and others mentioned soil is lose and it require large unwanted displacement to develop effective pressure. We have but one option to provide tie beams.

 

[Ron247]

Not all lateral forces come from seismic or wind. A rigid frame construction can have high thrust loads from gravity only loads. The wider the clearspan of the building, the higher the thrust load. Also, for a given clear span, the shorter building tends to have much higher thrust than the taller ones. They can easily be higher than wind and are almost always directed outwards from the building.

It does sounds like your options are limited to some sort of tie-system but it may not have to be a beam only. You could use a deadman type system.

Your protection from high salt, is that to protect the integrity of the concrete, the steel or both?

 

[AaronMcD]

I'd think it depends on the column load. You probably aren't putting tie beams perpendicular to every header post and holdown on a wood framed building. Do you want lateral resistance only to brace the bottom? If so, the footing may provide plenty of resistance. If it isn't part of the MWFRS it likely doesn't get much wind load either. But it is a pretty long footing. Here in California I would provide a tie beam if the footing span was that long.

http://www.hellodwell.design

 

[Eng.Abdulla95]

@Ron247 mainly to protect the steel, we provide 7.5 cm cover + Sulphate Resistance Concrete Mix. I do understand the high thrust, and my biggest worry is not the rigid frame itself rather the eccentricities on it.

@AaronMcD yes we do that but I am looking for other solutions, but it seems no other practical solutions (at least no body will accept other solutions).

Sometimes we try to remove some of these and we try to consider all factors to justify removing one tie beam, in most cases it is redundancy that make us remove it.

 

[Ron247]

Would placing 4" of crushed stone down before pouring concrete provide enough protection of the steel but still give you friction?

 

[Eng.Abdulla95]

@Ron247 yes we do replace 1.5 m of soil with crush fines, but this is to enhance the soil properties, and GWT can be found even 0.6 m below the soil in some sites, again the polythene sheet is like industry standard even if I dont specify it client will require, contractor will automatically do it. The rule that dose not respect the mind of its users will not be respected!

 

[CarlB]

I would not expect a thin poly layer, placed on a slightly irregular subgrade, with concrete cast on top, with a high normal force, to lower the coefficient of friction materially.
In a quick glance at these studies, friction factor was measured at 0.55 to over 0.8.

https://library.ctr.utexas.edu/digitized/texasarchive/phase2/459-1.pdf

 

[Eng.Abdulla95]

@CarlB interesting, thank you for help.