Retaining Wall Design Etabs

[Jule Kue]

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???

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.

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.

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.

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.

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.

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.

One of the useful tools in ETABS program is the Shear Wall Design capability. It is not only limited in the analysis of walls but it can also perform the design and check. The designer can be opted to either use the Reinforcement to Designed or Reinforcement to be checked option. When you require the program to do the design, choose the Designed option and Checked when you let yourself assigned the wall reinforcement to be verified by the program.

When the pier section is specified to check for the rebar details specified by the designer, the program creates an analysis to determine the flexural demand/capacity ratio, the required area for shear rebar for the selected pier and other output results to determine whether the section is passed or failed.

Referred to as Simplified Compression and Tension this option can only design planar walls and has no option for a checked design. In this option, the pier geometry is defined by the length, thickness, and sizes of each member at each end of the pier. The pier section flexural design will be performed to edge strip of the wall section and it ignores the resistance from the middle strip.

The program will report the required width of the edge strip to resist the axial and overturning moment required reinforcing in the governing compression and tension load combination. When dealing with shear design the full length of the pier section and reports the required shear reinforcing per unit length. This design option is best to use to determine and design the boundary zones or the boundary elements of shear walls, especially in high seismic areas.

The uniform Reinforcing option is applicable to either design or check both planar walls and coupled shear walls (spandrels). For flexural design or check, the previously defined pier section is automatically picked up by the program and do the design or check. For boundary zones checks and shear design, it analyses the pier sections up into the planar legs and then performs the design and analysis on each leg separately. In this option, the sections assigned as pier will be designed or checked for the uniform reinforcing that the designer assigns.

To do this, the designer should select the pier section or the wall to design and go to Design>Shear Wall Design>Uniform Reinforcing and the figure below will appear. In the following figure, the designer has the option to select which bar size and spacing to provide. You can either tick the Reinforcement to be Checked and Reinforcement to be designed. The reinforcement provided is then verified if passed or failed by going to Display>Show Table>Shear Wall Summary. The interpretation of these results will be further explained in the upcoming articles to be published soon.

This section can be defined using the section designer that is built in ETABS wherein the designer can create any geometrical shapes. The designer can also play with the rebar diameters and spacing. The concept of this option is almost similar to the Uniform Reinforcing because the designer has the option to define rebar sizes and spacing. Shear design can be performed similarly with Uniform Reinforcing.

When using the General Reinforcing section go to Design>Shear Wall Design>General Reinforcing and the below figure will appear. The designer can assign which section to assign at the bottom and at the top of the pier with the option to either checked or designed.

Of the three designed/checked options available in ETABS that are mentioned above, the author personally uses the Uniform Reinforcing Options as the most efficient design uses or utilizes all the wall sections not just only end zones. You can always check the end zones of the wall though calculating the boundary zones or boundary elements manually if your engineering judgment told you so. Refer to the video below on how the Uniform Reinforcement option can be done in ETABS. You can subscribe to our channel for more videos like this.

How about you? Which method above did you usually use and why? Tell us your thoughts! Leave a comment on the section below. Subscribe to our newsletter to be updated with the latest posts or follow us on our social media pages on the below icons.

In ETABS design of shear walls or retaining walls, the programs calculate and report the required area of steel for flexure and shear based on the applied and defined load combinations. During the modeling process, the walls should be modeled as shell elements. And before the program does its tasks, the designer should define the wall labels and assign them as pier or spandrel. The pier/spandrel labeling should perform to get the design forces and reinforcement requirements. The right labeling should be imposed to ensure realistic output results. Further details have been explained in succeeding paragraphs.

A wall pier and a wall spandrel can consist of a combination of shell elements and frame elements. However, modeling it as a shell element provides more flexibility and accuracy in the design analysis and results. To extract the wall forces and to design and check the wall, the designer should define the pier labels and spandrel labels at first. You can control the pier and spandrel labels assignment accordingly. For example, if you want to get design forces of one particular part of the wall, then you can assign a pier or spandrel label on that portion only. Nevertheless, Figure 01 and Figure 02 below show the recommended pier and spandrel labeling respectively.

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