Structural modeling - Base Level joint

[abdallah hamdan]

As stated in section 12.7.1 in ASCE 7-16, "Foundation Modeling. For purposes of determining
seismic loads, it is permitted to consider the structure to be
fixed at the base. Alternatively, where foundation flexibility is
considered, it shall be in accordance with Section 12.13.3 or
Chapter 19."​

I'm training to design a 13 story building Using Etabs 2019, based on the Statement, Should I always model the base joints as a fixed joint to design the building and should I design the Footing (isolated or mat), to have fixed support with moment reactions?

if this isn't always the case, what are the cases that I could model the base joint as pin support?

 

[271828]

I don't think that's what the statement means. It is saying you can avoid including part of the earth in the model or avoid using springs or similar to model flexibility of the soil or rock, not that there is rotational fixity.

 

[abdallah hamdan]

thank you for the explanation
what can we consider this joint constraint, fixed or pinned?

 

[JLNJ]

You need to know the engineer's intent.

What is your connection of the bottom of the column to the foundation? Model the column end to reasonable and properly reflect this condition.

 

[KootK]

1) The ASCE provision is just saying that, for the determination of seismic loads, you can consider your building lateral system to be a vertical cantilever with a fixed base rather than a vertical cantilever on a spring that represents the soil-structure interaction flexibility. This is generally conservative for estimating seismic loads because the base flexibility usually serves to lengthen your structure period.

2) Like JLNJ said, for the base fixity of your common vertical framing elements, it really comes down to the engineers intent. For me, it usually goes something like this:

a) Wall strong axis = fixed.

b) Wall weak axis = pinned.

c) column on pad footing = pinned.

d) column on thick raft foundation = fixed.

e) column on deep grade beam or pile cap = fixed.

That list is just a gross generalization however. There are all kinds of exceptions.

 

[JLNJ]

Hey Koot

Do you ever try to model the stiffness/partial fixity of a shallow spread footing by approximating the spring stiffness of the soil?

I'm not advocating it for the OP's 13 story building but I often wonder about large single-story buildings which have a forest of columns contributing (either by design or by accident) to the LFRS. Big footings which are a product of large bay spacings can inherently offer some moment resistance but tend to give it up as the footing and column base connections rotate. Neither fixed nor pinned seems quite appropriate. And if you are looking at frame action, the stiffness of the base plays a large role in the moment distribution throughout the frame.

 

[KootK]

@JLNJ: the PCI handbook has a greatly simplified method for looking at pad footing rotational spring stiffness. I've employed that a few times but never gotten any fancier than that. I don't disagree, there's certainly some degree of incidental lateral capacity in the gravity elements of the kinds of structures that you're thinking of.