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I have observed few issues while getting the design results of reinforced concrete members from ETABS (v 9.7.4). I wanted to share this issues with you and get your input and observation on it. ETABS always considers the minimum eccentricity for selecting the design moment of columns irrespective of the probable behavior of the column, whether short or long column. See section 10.10.6.5 and its commentary of ACI 318-08 which deals with minimum eccentricity of long columns. You should always check the design moments that ETABS uses for columns if you want to bring down the cost of construction. If your model has lateral loads, ETABS will give you design moments in column irrespective of its status as braced or un-braced as per ACI 318 criteria. You should investigate if the storey under consideration is braced, or un-braced (10.10.5.2), and decide appropriate design moments of columns. ETABS has a tendency to select a time period of the building that is considerably less than the value obtained by the approximate method, Method A, of the section 1630.2.2 of UBC 97. To quote the FEMA 451 document: ''Because this formula is based on lower bound regression analysis of measured building response in California, it will generally result in periods that are lower (hence, more conservative for use in predicting base shear) than those computed from a more rigorous mathematical model". So, there is no need to use the value of time period that is lot less than Ta. One should always check the time period used by the software; ETABS can overestimate the seismic force by more than 2 times. Method A gives lower T and higher V, so FEMA 451 has advised not to use the value of time period less than this value even if rigorous analysis gives a lower value. I have seen the results where Etabs have use the value of time period less than Ta; in-fact as low as 0.5Ta, which can increase the base shear two times.

There may be several reasons for this. For example, - provided columns are of a larger size, or - there are a number of shear walls, or - you might have erroneously assigned support property to the column joints at all storey levels, instead of those at base level, etc. You should recheck your model carefully to determine, what might be the real reason for the results obtained. Assigning of diaphram is necessary for proper accounting of stiffness of the structure, to resist lateral forces (earthquake etc). Meshing is essential for the transfer of slab load to the edge beams more uniformly. HTH

Aoa dear S.Es Wat is the criteria for selection of a particular type of a foundation especially with reference to an RCC building? e.g A RAFT or SINGLE FOOTING?

1.The structural load are so high or soil conditions so poor that spread footings would be exceptionally large. As a general rule of thumb, if spread footing would cover more than 50 % of the building footprint area, a mat or some type of deep foundation will usually be more economical. 2.If the soil is very erratic and prone to excessive differential settlements (heaves). The structural continuity and flexural strength of a raft will bridge over these irregularities. 3.The uplift loads are larger than spread footings can accommodate. The greater weight and continuity of a mat may provide sufficient resistance. 4.The bottom of structure is located below the GWT, so water proofing is an important concern. Because mats are monolithic, they are much easier to waterproof. The weight of mat also helps resist hydrostatic uplift forces from the groundwater.

1. P/A + My/I,xy If area req for isolated footing is so large that it combines with other footings, provide raft. 2. Soil report recommendations including uplift and water table, soil type, liquification etc.

salaam.. hope so all will be fine i m the civil structure engineer using etabs or sap for designing but i would like to prefer designing by hand calculations.. actually i need the hand calculation for mat or raft design if any one have the calculations regarding these please email or upload it.. i will be very thankful to you..thanks

I have worked with some engineers that like to assign high stiffness modifiers to rafts to get conservative flexural and shear design of foundation. Assigning modifiers would increase amount of rebar in your raft. I personally think that this is good practice as nothing is perfectly rigid and cracking in inevitable, which would result in loss of inertia and high flexural stresses. The general procedure is to create two models. One with no stiffness modifiers and one with modifiers for foundation/ raft. You should use the first model to calculate piles reactions and the second to do flexural and shear design. Thanks.