Property Modifiers For Retaining Walls

[asadishaq]

Retaining walls in ETABS absorb much of base shear if our whole basement is confined by retaining walls,what is the correct way of modelling retaining walls? can we neglect them in the model?or if we model them what are the correct modifiers for them,

I am vetting a project in which the designer put all the modifiers to 0.7 in membrane and bending and retaining walls is absorbing 54%of base shear in X direction and 46% in Y direction is this the correct way of modelling???

[Ayesha]

0.7 for walls should be acceptable as per ACI-XX 10.10.4.1. The retaining walls will be acting more like shear walls. How many basement levels do you have?

I think its time that someone writes a detailed tutorial about buildings with multiple stories below basement. This questions has been around forever. 

[UmarMakhzumi]

I would use the ground floor as base for seismic loads instead of basement. The reason is that the combination of ground floor diaphragm and perimeter basement wall offers a very rigid system. This statement would be true for buildings with 1 or 2 underground stories. If you have got more stories than a separate analysis for sub structure and superstructure should be done. 

 

The above quoted rule is based on my own personal experience. You should ensure that all limit states are satisfied and do your due diligence. I am keeping things as simple as could be kept while replying. Please have a look at the attached documents that explain some aspects about subject thread.

 

Basement Wall with Seismic Earth Pressure.pdf

Seismic Performance Of Structures with Underground Stories.pdf

 

Thanks.

[WR1]

Interesting discussion. I am in to have your more detailed views on this. So Umar you mean we dont need to apply EQ on 2 basements because of stiff diaphragm and walls?

 

Asad, yes 0.7 on all is perfect.

[UmarMakhzumi]

I think thats what the post says . The reason however makes more sense for single story basements only. All I am asking is to exclude sesimic weight of basement story. This also assumes that walls are all around perimeter and diaphragm is rigid.

Thanks.

[asadishaq]

The building is located on hilly area near murre and it has 8 basements and 10 upper stories 4 basements are totally confined from all sides,after B4 level it is confined from 3 sides.foot print of the building is such that B8 footprint is very small than B7 and step by step it is increasing till B4 and after B4 rest of the basements are same.I am attaching the sections of building also,the designer choose the B4 level to the lower story for seismic force.when i am checking my base shear at B4 level only 54% of base shear is taken by columns...since there is no special consideration been made by the designer to design the retaining walls for seismic forces,retaining walls are designed as a normal walls.the modifiers assigned to retaining walls are 0.7 for bending only the membrane modifiers are 1.the thickness of the whole retaining wall is 4 feet.I am from client side and checking the model kindly give me you valuable comments about this project.

A-301-MAIN BUILDING & BALLROOM SECTION-Model.pdf

[asadishaq]

following load combinations are used in the model.design preferences are ACI 318-02,Building is in zone three with SMRF system for lateral loads.and there is an inactive fault near the building just 500 meters away from the site,a special geological study has been conducted which shows zone 2B but the vetting consultant asked the designer to design it for zone 3.load combinations are as follows.

1.  1.4D
2. 1.2D + 1.6L
3. 1.2D +1.0L + 1.6WX
4. 1.2D +1.0L - 1.6WX
5. 1.2D +1.0L + 1.6WY
6. 1.2D +1.0L - 1.6WY
7. 1.2D + 0.8WX
8. 1.2D - 0.8WX
9. 1.2D + 0.8WY
10. 1.2D - 0.8WY
11. 0.9D + 1.6WX
12. 0.9D - 1.6WX
13. 0.9D + 1.6WY
14. 0.9D - 1.6WY
15. 1.2D + 1.0 EQX
16. 1.2D -  1.0EQX
17. 1.2D + 1.0EQY
18. 1.2D - 1.0EQY
19. 0.7D + 1.0EQX
20. 0.7D - 1.0EQX
21. 0.7D + 1.0EQY

dynamic analysis is performed using eigen vector.

 

 

 

 

 

 

[asadishaq]

inter story drift is exceeding in basement levels,also torsional irregularity greatly exists at different levels.should we consider the underground stories for seismic drift and torsion???

[UmarMakhzumi]

I would suggest that you should consider seismic loading at all levels and also check your underground stories for drift and irregularities. For load combinations please see this discussion:

http://www.sepakistan.com/topic/1307-etabs-load-combinations/?hl=%2Bload+%2Bcombination

 

Thanks.

[Syed Umair Haider]

Asad,

It is not necessarily required to extend the seismic analysis's storey range till basement levels as for buildings with several below grade levels supported by basement walls, two stage static analysis procedure is used (ASCE 7-10 Section 12 & UBC97 section 1630.4.2) that consists in distribution of building in flexible upper portion (above basement levels) and rigid lower portion (basement levels), provided the lower portion have a stiffness minimum 10 times greater than upper and time period of whole structure should not exceed 1.1 times of flexible upper portion's period while it is considered as a separate structure.

You can simply check these limitations as,

1, by computing stiffness ratio (EI/L ratios of basement walls + LFRS in rigid lower portion) to the (EI/L ratios of LFRS in flexible upper portion)

2, computing time period of whole structure (Eigen vector) and computing time period of upper portion alone modeled without basement levels.

Having satisfied these, seismic analysis is required to be performed till base of upper portion only & rigid lower portion is required to design only for seismic forces transmitted at the base of flexible upper portion modified by the factor Rupper/Rlower.

In ETABS you have to define "ground level" as bottom storey in analysis storey range and seismic shear imparted on ground level will be automatically transmitted to the levels below through diaphragm action.It will be just required to compute "R" value for lower portion considering it separate and to modify seismic load case's scale factor by Ru/Rl for the design of below grade structure.

In this way the maximum seismic shear will be acting at the ground level not at B4, that will reduce the magnitude of force and could be beneficial in mentioned below grade serviceability issues particularly drift will be considerably reduced (also compute drift using user defined time period obtained from eigen vector analysis see UBC Section 1630.10.3).

As long as below grade torsion is concerned, it is just required to satisfy that Ax (UBC97 Eqn 30-16) should not exceed 3 and required to be noted that amplification of diaphragm eccentricity is of no meaning there since seismic forces are imposed from upper portion and are not calculated & applied separately.

Secondly, load combinations should be inclusive of minimum seismic vertical effects and dynamic load combinations.

[UmarMakhzumi]

Asad,

You may find this thread useful too.

http://www.sepakistan.com/topic/1444-amplification-of-forces-in-etabs/

Thanks.