TUNNEL DESIGN ON SAP2000

[Hira Malik]

Kindly share any material on how to design a tunnel on sap2000. Attached is a pic of a model i found somewhere. this person has braced the tunnel by using "link supports" which I have no idea about since i have ever used them. Whats the purpose of these supports? Its properties have terms like coupled or uncoupled stiffness. and factors for line area and solid springs etc. That image is attached too and the values this person has entered are 1000 and 1000000. Its going above my head. Please guide about this and also how to calculate stiffness and when to define stiffness for a member?

Edited January 3, 2018 by Hira Malik

[UmarMakhzumi]

Links can be used to model any structural element. To give you an example, for the case of analysis of structural steel supporting pumps/ compressors etc, piles and soil impedance (damping + stiffness) needs to be provided in SAP2000. So, a lot of engineers use another software called DYNA to get those values and instead of support provide link with properties obtained from DYNA to create an analysis model that reflects the actual condition. This is just one use of link. For the above case, links might have been used to provide support properties that could not have been modelled directly. I assume the link represent some kind of soil structural support response but that is my guess as I am not aware of tunnel design.

On ‎2018‎-‎01‎-‎03 at 12:14 AM, Hira Malik said:

Its properties have terms like coupled or uncoupled stiffness. and factors for line area and solid springs etc

Coupled stiffness means the response of the link would consider stiffness values together in the direction specified

 

On ‎2018‎-‎01‎-‎03 at 12:14 AM, Hira Malik said:

That image is attached too and the values this person has entered are 1000 and 1000000.

If these are metric units, then the properties are defined for 1m and 1m^2.

Check this out for further details: https://wiki.csiamerica.com/display/kb/Link

Thanks.

 

[SALMAN CH]

@Hira Malik  @UmarMakhzumiFrom picture attached, he probably wants to model the behavior of soil around your structure means soil-structure interaction using a link element. You can use Line springs to your frame elements in-spite of Link elements. 

As far as property of link element is concerned, it might be easier for you to explain in line with normal bridge modeling, many bridge designers uses link elements to model neoprene bearings in-spite of hinge condition. You can specify actual stiffness of the bearing in flexure and shear.

This guy is using "mm units". 1000 and 1000000 is nothing simply factors to convert this to "m" like you define the properties of Line springs and area springs for any element frame and shell elements to model your soil interaction. Please see the image below. Factors 1m = 1000 mm, 1m2 = 1000000 mm2. If you will use units as "m", you should put 1 and 1 in both factors, like attached below.

See last picture whereby a bearing is being modelled using a link element.

Edited January 10, 2018 by SALMAN CH

[Hira Malik]

Ohk i got that. Thank you both. Ok i hAve a few more question on the topic,

1. The stiffness is calculated for radial and tangential SPRINGS  in the attached pic. That's for circular tunnel. How to calculate stiffness if springs or links are assigned to non-circular sections likes rafts or mat foundations? Whats the formula for  STIFFNESS?

2. I have attached another pic of loading. How can I apply varying load on a structure like this? Like load is more at bottom and less at top. To do this, all the members must act as one circle but when i try to join lines, nothing happens.

3. There is also horizontal rock load, and again i have no idea how to apply it on a circular section.

[Gossetto]

You can not model this problem in SAP2000 cause soil is no lineal and SAP is not able to do this. You need a no lineal software made to model soils.

[Hira Malik]

SAP2000 is used to model tunnels all the time. How is that the soil or rock forces cant be applied in SAP2000?

 

[SALMAN CH]

@Hira Malik..

Please see following response for your questions

Q1. Soil spring stiffness for normal rectangular mat/raft is taken as :

For area spring, :        from mod of subgrade reaction which is generally taken as K = 90xB.C for raft. (Kn/m2/m)

For Joint springs,       If  you are applying joint springs, you have to multiply it with equiv tributary of that joint.(Kn/m2 xm2= Kn/m/m)

For Frame springs:     multiply the value of K with width of frame element, and if the spacing of frames are different than 1m, also multiply with spacing 

2. Generally, the triangular loading is applied via joint pattern for shell elements. Would you please tell us you are modelling it as a pipe, shell or a 2d frame element. I can explain you better, if i know what are you doing. You can also apply varying load on frame element, go to distributed load you can give value of loading after each distance rather than equally distributed.

3. It is very easy to apply this circular circumferential loading e.g for frame element, first turn on the local axis of the element, go to distributed load while applying load change the axis from global to local, it may be in local axis 3 of the frame element.

 

@Gossetto.. you can also apply nonlinear soil properties by using link element, in-spite of linear link, select Gap link element.see this

https://wiki.csiamerica.com/display/kb/Pile+lateral+support+based+on+P-y+curves

[Hira Malik]

7 hours ago, SALMAN CH said:

@Hira Malik..

Please see following response for your questions

Q1. Soil spring stiffness for normal rectangular mat/raft is taken as :

For area spring, :        from mod of subgrade reaction which is generally taken as K = 90xB.C for raft. (Kn/m2/m)

For Joint springs,       If  you are applying joint springs, you have to multiply it with equiv tributary of that joint.(Kn/m2 xm2= Kn/m/m)

For Frame springs:     multiply the value of K with width of frame element, and if the spacing of frames are different than 1m, also multiply with spacing 

2. Generally, the triangular loading is applied via joint pattern for shell elements. Would you please tell us you are modelling it as a pipe, shell or a 2d frame element. I can explain you better, if i know what are you doing. You can also apply varying load on frame element, go to distributed load you can give value of loading after each distance rather than equally distributed.

3. It is very easy to apply this circular circumferential loading e.g for frame element, first turn on the local axis of the element, go to distributed load while applying load change the axis from global to local, it may be in local axis 3 of the frame element.

 

@Gossetto.. you can also apply nonlinear soil properties by using link element, in-spite of linear link, select Gap link element.see this

https://wiki.csiamerica.com/display/kb/Pile+lateral+support+based+on+P-y+curves

Thanx alot for your explained answers.

2. I am designing the tunnel elements as 2D frame sections. So if I apply varying load by selecting one element that will only be for that same element. What I want is that I want to model the whole tunnel as one element somehow, such that when I give value of loading after each distance by selecting all elements, the load is automatically applied as more at bottom and decreases gradually towards the top. I just have 2 values of internal water pressure, 803 at top and 837 at bottom. Want to model them as varying load.

[Hira Malik]

I have attached the screenshot of the loading and how I applied it on SAP. So what I did was simply select the top, left and right frame sections of the tunnel and applied the loading as 39.59 and 48.6 as uniformly distributed load. While applying loads I selected local axis 3. Is it right? Also it confuses me to select the frames for a specific load. Like on how many frame members shall I apply the top loading? And how can I be sure that the members below those will only be used for the lateral loading?

 

Edited January 13, 2018 by Hira Malik

[SALMAN CH]

@Hira Malik.. Actually i cant see clearly in pic, only model can tell me right, but this seems to be wrong direction, this load should be applied in 2-2 local axis of frame, what you applied is in longitudinal direction not lateral load. 

See the loading i attached,axis 1 is in red,3 is in cyan and 2 is in green, your pressure will act in 2-2.

In one element i have applied loading in 3-3 which is lonngitudinal and in other element i have applied in 2-2 which is right direction. If you will draw a tangent this circle, you will see on every point, the lateral pressure will be like at right angle so for this cross sectional 2 d frame,2-2 is the right direction.

Even if you will draw a curve on CAD and import it SAP2000 will divide it into series of linear elements, you can not just select one and assign it once. For more precise analysis you can further divide these segment.

Read this:

https://wiki.csiamerica.com/display/kb/Curved+frame+objects

 

Edited January 14, 2018 by SALMAN CH

[Hira Malik]

On 1/14/2018 at 12:32 PM, SALMAN CH said:

@Hira Malik.. Actually i cant see clearly in pic, only model can tell me right, but this seems to be wrong direction, this load should be applied in 2-2 local axis of frame, what you applied is in longitudinal direction not lateral load. 

See the loading i attached,axis 1 is in red,3 is in cyan and 2 is in green, your pressure will act in 2-2.

In one element i have applied loading in 3-3 which is lonngitudinal and in other element i have applied in 2-2 which is right direction. If you will draw a tangent this circle, you will see on every point, the lateral pressure will be like at right angle so for this cross sectional 2 d frame,2-2 is the right direction.

Even if you will draw a curve on CAD and import it SAP2000 will divide it into series of linear elements, you can not just select one and assign it once. For more precise analysis you can further divide these segment.

Read this:

https://wiki.csiamerica.com/display/kb/Curved+frame+objects

 

OK. So is this only for the lateral rock pressure or the other pressures such as internal and external water pressure, and grout pressures be also applied in the 2-2 direction? I will attach the model with the loading details. Kindly check it if you have some time.

[Hira Malik]

Check the attached model. Rock pressures are as attchaed in previous post. Other loads are as follows:

Top and bottom external pressures (kn/m) 2303.5 and 2336.5

Top and bottom internal pressures (kn/m) 863.5 and 896.5

Grout pressure is also attached.  No idea why is it only acting at top 120 degrees

[uetlhrcivil]

On 1/12/2018 at 8:53 AM, Gossetto said:

You can not model this problem in SAP2000 cause soil is no lineal and SAP is not able to do this. You need a no lineal software made to model soils.

You are right about non linear behavior of soil. However, in SAP one can model compression only behavior of soil.

[uetlhrcivil]

On 1/18/2018 at 11:34 AM, Hira Malik said:

Check the attached model. Rock pressures are as attchaed in previous post. Other loads are as follows:

Top and bottom external pressures (kn/m) 2303.5 and 2336.5

Top and bottom internal pressures (kn/m) 863.5 and 896.5

Grout pressure is also attached.  No idea why is it only acting at top 120 degrees

This grout is usually done at crown, that's why top 120 degree is chosen by the designer.  However, you may realistically reduce this pressure intensity & also change the angle from obtuse to acute. My advice, conservatively neglect internal pressure (Empty condition). 

[Hira Malik]

3 hours ago, uetlhrcivil said:

However, you may realistically reduce this pressure intensity & also change the angle from obtuse to acute

How? How can I select all circular members at once and apply variable intensity of any pressure? Such that its max at top and decreases towards the bottom.

[uetlhrcivil]

17 hours ago, Hira Malik said:

How? How can I select all circular members at once and apply variable intensity of any pressure? Such that its max at top and decreases towards the bottom.

In my personal opinion, you have two options (I am not much aware of software options, other members may correct me)

i) Use area element and generate varying load using joint pattern options in SAP.  You may not get software generated design in this case.

ii) Use dummy area elements with zero property modifiers. Then load these dummy area elements as per step 1 above. During analysis, loads will be transferred from dummy area elements (Having zero stiffness) to frame elements & you can get the software design for these frame elements. (Some engineers do it for wind load analysis on frame buildings, where wind loads applied on non-structural walls is transferred to columns by this method).

Alternately, in STAAD there is a direct option of hydrostatic loading on frame elements. Snapshot below. 

 

 

Edited February 8, 2018 by uetlhrcivil