This is not a SFA software-related problem. But this limit has to be set manually when we use SFA.
But I didn't find any provisions for it in US codes.
In VERIFICATIONMANUAL, 75% is used.
Zhao,
Your question:
But what made us confused is special rules for mat foundation. In previous version of SFA, weight of soil is not considered for mat foundation as it is not designed as rigid foundation.
Answer:
As far as I remember, all versions of SFA have considered the weight of soil as an additional load on mat. In older versions, the soil weight was applied by converting it to an equivalent selfweight of the mat. You can find the details here.
https://communities.bentley.com/products/ram-staad/f/ram-staad-forum/103571/selfweight-of-mat-foundations-in-staad-foundation-advanced/599428#599428
In the current version, the program applies the soil weight as a pressure load on the mat. For each load case, the pressure is calculated as
dead load factor * Unit weight of soil * height of soil on top of mat
SFA creates a STAAD.Pro file (.STD file) containing the finite element model of the mat. If you open that model in STAAD.Pro, you can see the pressure value applied as an element pressure load for each load case on all the elements that constitute the mat.
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In the following picture, it says SFA checks soil bearing capacity under load combinations of service ultimate state. But in calculation report, SFA also checks it for strength ultimate cases.Is this step intended to get the net soil reaction presussure?
We agree that we need to mention in the manual that the recent versions of the program check the soil pressure for strength load cases too. The pressure needs to be calculated for the strength load cases because the bending moments and shear force on the footing are based on the soil pressure.
When checking footing shear capacity and calculating reinforcement area, net soil reaction is used. That is the reason why SFA calculates Pmax under ultimate load combinations(Then minus the pressure induced by selfweight of soil and footing).
But should we compare Pmax with soil bearing capacity in order to conclude if dimension of footing is enough? I don't think so.
And there is also no need to calculate contact area percentage under ultimate load combinations.
But if Chinese Code is selected, this step is not included in calculation sheet.
This is a bit confusing.
When checking footing shear capacity and calculating reinforcement area, net soil reaction is used. That is the reason why SFA calculates Pmax under ultimate load combinations (Then minus the pressure induced by selfweight of soil and footing).
That is correct.
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That is a choice left to the user. We agree that in most engineers do not check the ultimate soil pressures against a bearing capacity (factored by a number between 1 and 2 typically). But some companies do. So, we have provided that option.
--------------------------------------------------------------Your question:
This too is the user's choice. Some users have told us that for ultimate load cases too, they want to ensure that the loss of contact is within a reasonable limit, especially if the structure is supporting costly equipment like turbines. If it is not a matter of concern to you, you can ignore it.---------------------------------------------------------------Your question:
But if Chinese Code is selected, this step is not included in calculation sheet. This is a bit confusing.
We will look into it.
Answer Verified By: ZHAO BIAO
Thank you very much.
I need to confirm, in SFA, if checking soil bearing capacity and contact area is optional or compulsory.
If it is compulsory, we have to create 2.sfa models.
Yes, these are compulsory checks at present. However, in order to ensure that they do not govern, specify a large number (2 or greater) as the multiplying factor, and a small value like 5% for contact area for ultimate load cases. Later this year, we will have a version of the program where these checks are optional. You do not need to create 2 separate models.
I am using version 9.5.062. I do not see how to turn off contact area check for combined footing. So I set min contact area as 5% and set overhang, thickness and calculate width. The width is increased until there is no negative pressure. How do I get it the uplift and calculate rebar and width for my minimum width of 24"
It looks like the e = M/P <= L/2 is the controlling factor
On additional investigation that e = M/P is not governing - I was looking at the resisting moment/P, and it is L/2 just a coincidence? the Demand M/P is about 90% of L.
Can someone help me with this?
In the window below the drawing area, the program displays a number of messages as it progresses through the calculations. You could either scroll through them, or do a right mouse button click in that window, save the data to a text file, and view the text file's contents in Notepad.
If you do that, do you see a statement that looks like this?
FOOTING IS NOT IN FULL CONTACT FOR CURRENT ULTIMATE LOAD CASE. SFA WILL TRY TO INCREASE THE SIZE AND KEEP THE FOUNDATION IN FULL CONTACT .....