A soil shear failure can result in excessive building distortion and even collapse. Excessive settlement can result in structural damage to a building frame nuisances such as sticking doors and windows, cracks in tile and plaster, and excessive wear or equipment failure from misalignment resulting from foundation settlement analysis.
It is necessary to investigate both base shear resistance (ultimate bearing capacity) and settlements for any structure. In many cases, settlement criteria will control the allowable bearing capacity.
Except for occasional happy coincidences, soil settlement computations are only best estimates of the deformation to expect when a load is applied.
But, as the technology is growing fast and becoming more popular day by day, for any important project, Foundation Settlement Analysis is done with much modern software. Here, in this post, I will discuss how to analysis foundation settlement with STAAD Pro.
Footing Type support under foundation support in STAAD Pro V8i
Almost all the times the STAAD Pro users preferably use a fixed or pinned type of supports for frame analysis. However, there are times when the soil below the foundation is elastic & the joint displacements due to settlement in soil have to be taken into consideration in analysis, at such times one can use the foundation support feature provided in the Supports page of STAAD Pro.
Process of Analysis
Here below I am going to brief about the creation of footing type support for a column-
First create a column member of height 3.5 m & cross-section 0.30 m x 0.30 m.
In the next step go to General page >> Load & Definition subpage & create a primary load case. In the created primary load case define a nodal load of Fy = -50 kN & assign it on the top node of the column.
Now, go to General page>> Supports sub-page & from the datasheet area, click on create a button in Supports-Whole Structure window.
By doing so the Create Support window will be prompted on-screen, now choose Foundation tab from above.
Under the foundation tab three options viz. footing support, elastic mat & plastic mat are available. In this post, will discuss how to model a footing type of support using Length (L) & Width (W) values.
Define the values for ‘L’ & ‘W’ as 1.5 m, choose Y direction for spring generation & define the value of subgrade modulus as 2000 kN/m2/m (for SBC of 100 Kn/m2 & 50 mm allowable settlement) & click ADD. After adding the support to Supports-Whole Structure window, assign the support to the bottom node of the column.
Now, in this example, we have modelled a column of 3.5 m high & 0.30 m x 0.30 m section size supported on 1.5 m x 1.5 m size footing loaded with an axial load of 50 kN.
Finally, go to Analysis & Print Page & assign No Print command to the structure, afterwards give Run Analysis (Ctrl+F5) command from Analyze menu.
Once the analysis is complete, close the STAAD Analysis engine window & go to Post Processing mode from the Mode bar.
In the Post-Processing Mode go to Node Page >> Displacement Subpage >> Node Displacements Table (on the right side of the screen). Here in the Node Displacements table check for the displacement of support node in the Y direction, the value provided generated here is in mm & this displacement value is calculated on the basis of Area of Footing (L x W), Soil Subgrade Modulus, Spring Constant ‘K’& Load on Column.
Kindly, refer to the below for the manual calculation of support displacement for verification & for any query or suggestion kindly post your comment in the comment box.
Assume, Footing section of L = 1.5 m and W = 1.5 m
Safe Bearing Capacity (Q) = 100 kN/m2
Allowable Settlement, = 50 mm = 0.05 m
Area of Footing = L x W = 1.5 x 1.5 = 2.25 m2
Subgrade Modulus = Q/ = 100/0.05 = 2000 kN/m2/m
Spring Constant (k) = Area x Subgrade Modulus = 2.25 x 2000 = 4500 kN/m
Column section due to nodal load = 50 kN
Support Displacement = Column Reaction/Spring Constant = 50/4500 = 0.01111 m = 11.11 mm
Check for this diaplacement value in Post Processing Mode of STAAD Pro.
I hope this article will help you. You may also want to see my other post from my Blog. If I have missed anything here, please let me know about that in the comment below this post.
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