applied load factor of Plaxis result

[killi_82]

hi,

currently, we are designing diaphragm wall with help from Plaxis analysis.
However, the load factor used for reinforcement design is 1.6 (we multiply Plaxis forces result with 1.6), this one follows ASCE load factor for particular earth pressure.
1.6 seems too high, as the geotechnical engineers also have their sort of safety factors.
For your information, Diaphragm wall will be used both temporary and permanent, but Plaxis analysis only cover for temporary stage only.

Is there a requirement I can use?, that only requires this 1.6 load factor to be less, eg: 1.4

thanks

 

[r13]

The higher load factor reflects uncertainty of the load source. I recall at old ACI318, the load factor for fluid was 1.7 (treated as live load), but if it is in the container with fixed maximum depth, then we were allowed to use 1.4 (considered as dead load). So, I think it is up to your situation.

 

[IDS]

If you are deigning to ASCE, you should use the ASCE load factor.

I'm not sure what you mean by the geotechnical engineers having "their sort of safety factors", but if you are applying the ASCE factor for the structural design your base loads should come from an unfactored analysis.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[killi_82]

@IDS,

geotech team derived the soil strength parameters using statistic approach, while in my opinion, it has an inherent safety factor,
especially when they use a 'mean' value.
I believe this is a way to capture uncertainty.

however, it seems too big if I have to use 1.6 factor for concrete design (the fact that Plaxis only do temporary case analysis).
as an example, the late BS standard used 1.4 for earth pressure and not 1.6.

@retired13,

this for basement underground structure not specifically in a container.
hope I can find a way to use smaller load factors.

thanks

 

[r13]

Understand. My example is only a food for thought.

 

[IDS]

killi_82: You really can't compare isolated factors from different codes without looking at everything else. For instance the Australian Bridge code has a load factor of 1.5 for soil supporting structures, but it also has a bigger reduction on reinforced concrete strengths, so the overall factor would be much the same.

Regardless of your opinion, if the applicable code says use 1.6, you should use 1.6. If you use a lower factor and there is a significant problem with the structure you will be in serious trouble, whether the problem was associated with the lower factor or not.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[r13]

What type of problem that led you to think about reducing the load factor? Sometimes the problem can be resolved by refine/change analysis model rather than risk code violation.

 

[killi_82]

@retired13,


there are changes within projects I'm handling now,
which is now using different codes with the previous one (but similar type of construction).
using American code seems a bit higher/costly compared to previous projects using BS.

I need to seek a method (a good approach) to tackle this issue, one of them is the design of the diaphragm wall which is used only for temporary works, but it still remains long like 5 years (typical underground construction).

thanks

 

[r13]

I meant, for out of plane forces, if previously it was analyzed/designed as one way structure, but in essence it can resist loads through two way action. I hope you be able to find conservatism in the past, and change/modify it to fit your needs now.

 

[killi_82]

sounds like taking into account the 3D effect into consideration, right?
seems I cannot adopt that, as the structure is behaving almost like a continuous long wall.

 

[KootK]

I agree that, when the load source is geotechnical, there often seems to be a doubling up of the safety factors. That, especially in markets like many North American ones where geotechnical engineers are kind of late to the LRFD party and sometimes not really understanding the difference between LRFD and allowable stress geotechnical design well.

I feel that your only rational avenue of attack in this situation is to seek a relaxation of the geotechnical design values from the geotechnical engineer. I've worked on several projects where a discussion with the geotechnical engineer has netted me load reductions to the tune of 50%.

 

[r13]

So far I don't know what is the function of the wall, and what problem you are running into. It does not seem a basement wall, since said it will only be semi-permanent, then it shouldn't be retaining wall either, especially you call it a diaphragm. Can you up load a sketch, so we can understand better?

My previous response assumes you have the out of plane problem caused by the soil pressure rather than in plane shear problem. Once the configuration is made known, maybe we can find some way to attack the problem.

 

[killi_82]

@KootK,

at this point, I cant push/negotiate with geo as they seem to have reasons why they are doing like this, eg: bad soil.
however, applying 1.6 for the forces given by them is under my impression, as you know the Plaxis model contains not only earth pressure but also some live and dead loads.
nevertheless, the lateral soil is the biggest contribution in this case.
I just need a reference where it says that there is relaxation in using this 1.6 to be 1.4 or even 1.5, it really helps.

@retired13,

thanks for the reply,

it is an underground railway station case where both temporary and permanent, used same diaphragm wall.

 

[r13]

If this wall's problem is lack of complete support mechanism during construction, but the support will be there upon completion, then you might design the wall for temporary condition with reduced safety factor, but ultimately you still have to design the wall to resist the load with normal safety factor for the complete condition. I couldn't find code source to support this practice. However, the linked blog may give you some idea. A few words of caution - be prudent in dealing with underground structures. Link

 

[r13]

I wonder if this standard would help your cause. Link

 

[IDS]

killi_82 - Certainly it is worth checking that the information the Geotech people are giving you complies with the applicable code, but it sounds like you have done that already, and they have told you it does. If that is the case, just follow the procedure specified in the code you are using.

Picking factors from other codes to minimise your design actions is just wrong.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[killi_82]

@retired13,

thanks for your references, I will have a look at that further.

@IDS,

thanks for your suggestions.

@KootK,
Plaxis has the ability to use LRFD method in its analysis. Link
Would you think this will help to reduce the result compared to what they are doing now (using WSD and multiplied with a factor when you design the reinforcement)?

 

[KootK]

Would you think this will help to reduce the result compared to what they are doing now (using WSD and multiplied with a factor when you design the reinforcement)?

I couldn't say as my familiarity with Plaxis is limited. I would, however, consider an LRFD analysis to be the most consistent with modern, limit state concrete design practices. As IDS intimated though, you really have to understand the nature of the output that you're working with and ensure that your design is compatible with your analysis.