Leaderboard

Dear Mr. Waqas, Your study about Non Linear Analysis is quite impressive. I just want to add some more details. There are three types of design. 1- Service Level Earthquake (SLE) 2- Design Basis Earthquake (DBE) 3- Maximum Considered Earthquake (MCE) SLE is at service level. Which means that in EQ there should be no problem for non structural members. DBE: In code based design, we use DBE level earthquake. This earthquake is frwquent earthquake with return period of 475 years( I think). During this code uses actualy 100% forces and reduce these forces by reduction factor R. Means we design our structures for reduced forces as explained before. Actually in this case code allows the structure to yield at certain locations for certainm members. But code do not specify these locations and members as R is the overall division factor. So during an earthquake it is not necessary that the members will yield which we assumed to be yield. For example if beam at any location yielded then it will not transfer forces to the column. But we are not sure that column will yield first or beam will yield first. Code tried to take this by incorporating the stiffness modifiers but still it is not 100% reliable. So thats why nobody can gurrantee the building to perform 100% in earthquake when designed by Code. MCE: This is the third and extreme level of earthquake.Return period is 2475 (I think). This can only be incorporated by using Non Linear Time History Analysis. In this we can check the reinforcement designed on DBE level for capicity check by considering the actual yielding and energy disspation. We can set a limit for structure for example Life Safety, Immediate occupancy or collapse prevention. We allow the memebrs to yield upto acertain level and beyond that level we ll not allow the structure to yield. We can actually see that which member is yielding and the after yeidling what is the force and design for that force. The difference is that in code based design we aare not sure how much energy is dissipated in the structure. Code just provide 2 or 3 types or R factors depending upon the building type. So ist ios not 100 % accurate. Secondly code reduces the forces for all modes of structure but actually it can not happen. We can not reduce all modes because we can only reduce the modes which are yielding. So thats why 21.1.1 says that integrity should be maintained. But code do not give any gide line how to maintain that integrity. By using that R factor??? how can we do that????? So code just wrote that thing. Actually that integrity can only be assured by using the Non Linear Time Histoiry Analysis and stops the stucture to deform beyond which we do not require to deform. Sorry if I write something out of topic. Thanks Muneeb

Since the length of basement wall would likely to be much longer when compared to a conventional shear wall, lateral shear resisted / unit foot of basement wall would not be significant. Critical case for basement wall (generally speaking) should be lateral load due to earth + any increase in that lateral load due to earthquake. This load would be perpendicular to wall. Also, have a look at the following topic (there are 5 threads) as you may find them interesting: http://www.sepakistan.com/tags/forums/Highrise%2Bwith%2Bbasements/ Thanks.

Hello Goher, You are right most of time wind loads govern but in case of Mezzanines, Seismic forces may govern also. You are absolutely right. You will have to multiply horizontal forces with the depth of footing to get moment which you will have to apply at centroid of footing. If you want to check bearing pressure and strength design of footing, you can use SAFE but according to my knowledge, SAFE does not offer SLIDING, OVERTURNING check and for uplift, you may check pressure and pressure will tell you whether there is contact between your footing or your footing has been lifted. To apply loads in safe, you can either model a footing slab and manually assign loads at centroid (by drawing a point or by meshing footing) or there is a better way also. You can import the whole base with reactions from SAP/ETABS to SAFE. Moreover, I m attaching excel sheets which I have made for simple calculations of isolated Footings. You can use it. If you find any error, kindly report me back. I have tested them many times. These are a little rough but i think usable. footing reinforcement design sheet.xlsx footing sizing sheet.xlsx

But in reality, you r going to construct this beam. What if after making another shear wall, still it gets fail? I have reservations regarding this approach. Kindly explain.

As Muneeb pointed out, the lateral load transfer depends on stiffness of vertical elements. A concrete shell element of the same size of a line frame element has about 4% higher stiffness because of posions ratio. Thats not a big difference but a shell element when meshed has more supports at base adds up to the stiffness as compared to single support frame element.

The reinforcement in basement wall is similar to reinforcement in slabs. So design basement wall as a slab. Model an area element resembling your basement wall with suitable edge conditions... apply back fill load and gravity UDL loads. Mess/Divide the wall. Run the model and read moment at critical locations. and Provide reinforcement for these moments. You can also read shear force. Check thickness of wall against this shear force. But if your wall is very long and acts as one way cantilever slab, then you can design it as beam or slab. Basement walls are basically designed as three edge supported slabs. But in high seismic regions, i think your walls will also behave as shear walls so proper consideration should be given to seismic induced forces.

Dear Goher, If you are using prestress or pre-engineered columns, then in that case how you are going to connect the columns with foundation. If you provide some cast inplace pedestals then you can assign horizontal loads on them in addition to vertical loads. This will solve your problem. Thanks Muneeb

yes, you have to calculate the net tensile strain in extreme layer. Approx it will be somewhere near to 0.02

A maximum reinforcement ratio for beams and slabs is not directly given in ACI 318-05. Instead, Section 10.3.5 requires that non-prestressed flexural members must be designed such that the net tensile strain in the extreme layer of longitudinal tension steel at nominal strength t is greater than or equal to 0.004. In essence, this requirement limits the amount of flexural reinforcement that can be provided at a section. Using a strain compatibility analysis for 4 ksi concrete and Grade 60 reinforcement, the maximum reinforcement ratio is 0.0206. Thanks.

No dear, the maximum area of steel aims to limit the area of steel put in the section so as to control the strain in steel to be equal to or larger than 0.005. This will not differ between slabs and beams. Regards

W salam, These concepts need some fundamental theory of development of seismic analysis procedures (static procedures) in codes as briefed below, When engineers decided to go for an earth quake resistant design,then they initially proposed to assign a horizontal load of "0.1 x Weight of structure" to cater for seismic forces.With the passage of time several geo-technical and site specific response characteristics were included in analysis for evaluation of seismic forces and structural members were designed to resist these forces in their elastic range i.e to not yield under these forces. Structures designed accordingly surprised engineers, as they were observed to show little tolerable non structural damages in seismic events considerably greater then those considered in evaluation of seismic forces.This leads to the development of concept of energy dissipation and over strength factor i.e under cyclic seismic loading structures have the ability of resistance beyond the elastic range of stresses in members (after yield), in proportion to their ductility.Since then started consideration of this structural over strength characteristics that consists in reduction of design seismic forces in accordance with their energy dissipation characteristics or mathematically reduction of base shear by division with over strength factor. For ex in accordance with UBC97, if on a structure the actual seismic force i.e Cv.I/W=550 & over strength or ductility factor is R=5.5 then Seismic shear will be 550/5.5 = 100, then structural members will be designed to remain elastic or not yield under the lateral force of 100, whereas they will dissipate the remaining 450 in inelastic range or in terms of energy it can be said that this structure is able to dissipate 450/550x100 = 81% seismic forces through its ductility and is required to design elastic only for 19% of actual seismic forces. In the lights of above these clauses could be defined as follows, ") 21.1.1 says, ........................................For which, design forces , related to earth quack forces, have been determined on the bases of ENERGY DISSIPATION IN NONLINEAR RANGE OF RESPONSE". For every structure,seismic forces are evaluated in accordance with corresponding over strength factor that indicates the extent of probable energy dissipation. 2) Commentary of R 21.1.1 says, The integrity of the structure in the inelastic range of response should be maintained because the design earth quack forces, defined in documents such as ASCE/SEI 7, the IBC, the UBC and NEHRP provisions are considered less than those corresponding to linear response at the anticipated earthquack intensity. As defined above, structures are designed for seismic forces that are reduced by over strength factor however actual fores are times greater than that, therefore code requires that when structure is subjected to actual seismic forces(plastic state),then although structural damages in members are tolerable but integrity of structural members should necessarily be maintained so that structure will not collapse.This condition is another form of philosophy of safety in code under seismic events that says "Under major earth quake,structure should be designed to have structural & non structural damages but should not collapse".