kh vs increment kae

[Thoughful]

Hello Everyone,

I have a question regarding KAE and Kh

It is silly question but I have navigated all books and materials to wrap my head about it.

In AASHTO section 11.6.5,

PIR = kh (Ww +Ws) in which PIR is the horizontal inertial force due to seismic loading. My understanding that this equation stem from F=ma

However, AASHTO also defines Pae = 0.5*gamma*h^2*Kae in which Pae is the seismic active force and Kae is the seismic active earth pressure coefficient. I understand that Kae is combining active and increment seismic earth pressure.

My confusion is the increment seismic earth pressure coefficient is determined from kh which is comes from acceleration and hence inertia. That is there is no difference between kh and increment Kae (seismic portion). Am I understanding it incorrectly?

Can someone please clarify?

Thank you!

 

[MotorCity]

I believe kh is the used to calculate static lateral earth pressure (not from inertia) and kae is used to calculate the equivalent static earth pressure from seismic forces.

 

[EireChch]

https://www.youtube.com/watch?v=KOrOnFqw2wQ&list=PL_ZYN7hwTiZJKHOSC3xiXrSnv4skQk7tp&index=9&t=1605s

this lecture from professor Kitch explains it very well.

I think he is one of those few people who are blessed with an engineering brain but also the ability to communicate brilliantly with it. Something most of us cant do.

 

[PEinc]

At the beginning of each AASHTO section, there is a definitions section. Have you checked it for k[sub]h[/sub] and K[sub]ae[/sub]?

http://www.PeirceEngineering.com

 

[Thoughful]

Thank you! I think my confusion is that I was looking at the fill on top of the heel and thinking kh and kae are applied to it. when only kh is applied to this fill while the lateral earth pressue due to seismic is applied to the wedge behind the boundaries of the retaining wall.

 

[EireChch]

hmmm, can you post a sketch as to what you mean exactly.

The weight of the fill should also be considered as part of the weight that is applied to the wall. Seismic or not....

 

[Thoughful]

I meant something like that. The weight of the fill only has kh applied to it, but Kae is applied to the active wedge behind the fill

 

[EireChch]

Kh is the horizontal seismic acceleration.

This is applied to both the soil and the wall. The red hatch you have shown is soil above the heel, I would consider this as part of the "wall".
kh is applied to the active soil wedge also, this is your blue hatch.

Kae is the seismic earth pressure and the static earth pressure.
ΔKae is the seismic earth pressure only.
ΔKae is approx 3/4kh for typical walls and soils.

Did you look at that lecture i sent?

ps - for geotechnical equilibrium we dont even cosider the seismic loading on the wall its self. We only care if the wall has sufficient mass to resist the loading. Kh applied to the wall is used by the structural lads and ladies? Structies, please confirm.

 

[Thoughful]

Thank you EireChch

I did look at the lecture, seems I didn't understand, so what you are saying is that the kh is integral of Kae? meaning it is still inertia force but only for soil which we happen to take 3/4 of it, but if we are looking the soil above heel, we are fully considering the kh, correct?

 

[EireChch]

kh is nothing more than the horizontal acceleration. This is in units of gravity. Some people, conservatively, take it as Peak Ground Acceleration (PGA). Some take it as half to 2/3rd of PGA. There is no universal guidance on this. Many codes have different values to take.

Kae is calculated based on soil friction angle, surface back slope angle, slope of back of wall, slope of failure wedge, interface friction angle etc.

What you have worked out from the above is your total seismic and static earth pressure. You can work all of these out for your wall. ΔKae is the seismic portion of earth pressure.

Your wall (if similar to the sketch you provided) is typical in terms of backslopes, friction angles etc. When you solve the above equation for Kae, and then work out ΔKae, you will see that ΔKae should be some where close ΔKae.

I would ignore the soil above the heel. Instead work out you seismic earth pressure (based on you soil failure wedge), and then seismic force and then apply it as if it acts on the back of the stem.

The structural guys need to help me here. When doing structural design of the wall, there are two seismic force that need to be considered. The lateral soil force (seismic and static) acting on the wall and the seismic force acting on the wall from its own inertia.

You need to consider the soil weight above the wedge as part of the seismic force acting on the wall from its own inertia. This is just mass (from soil and concrete weight) times kh.

 

[ATSE]

I mostly agree with EireChch, except for the very last part.
For the AASHTO method (most common, but different from the many other standards), the soil over the heel of a cantilever wall also has a pseudo-static lateral component. See 11.6.5.1 (8th Ed, 2017).
Not defending or promoting this approach over others, providing clarity re: AASHTO.