TX_Eng
Structural
- May 14, 2021
- 1
Hi all -
I'm working on a mat foundation for a small retail building. The one-story building is lightly-loaded with wood-framed walls and trusses, with one intermediate bearing line in the long direction. Building dimensions are roughly 50' x 75'.
The geotechnical engineer has identified a liquefaction risk and has recommended a mat foundation to support structure. Following the liquefaction analysis, the geotechnical engineers have indicated a total seismic settlement of 1-3/4 inch and total settlement (static + seismic) of 2-3/4 inch, with differential settlements being 1/2 of total (7/8 inch and 1-3/8, respectively). I don't have a ton of seismic experience so this is a nice project to learn from. Settlements occur over a distance of 40' per the geotech.
I have a number of questions so I'll try to ask them in an order that makes sense:
1) ASCE 7-16 has a new section on shallow foundation design. We don't meet the exception in 12.13.9 so liquefaction must be considered. Moving on, shallow foundations appear to be acceptable per 12.13.9.2 so long as both Part A and Part B are adhered to. Note: the geotechnical report doesn't go into detail re: lateral spreading to check the box for Part A but I'm assuming that this is not much of an issue or a failure mechanism for a mat foundation. However, then in Section 12.13.9.2.1.2 written specifically for mat foundations, "the flexural demands caused by liquefaction need not be considered if the mat is detailed in accordance with the requirements of 18.6.3.1 of ACI 318 (basically reinforcing the heck out of the mat)." Does that mean that differential settlements are not required to be analyzed for or accounted for at all if you simply reinforce the mat per ACI?
2) Assuming we do need to account for differential settlements, is there a threshold below which you don't really worry about the magnitude and "ignore" settlement effects? 1-3/8" seems pretty small to me, but maybe it isn't. There's an NEHRP design guide for mat foundations that loosely indicates mat foundations can accommodate up to 2 inches of differential settlements, but that statement doesn't indicate how stiff the mat needs to be, how it's reinforced, etc.
3) If settlements occur over a distance of 40' but my building dimensions exceed that in both directions (50' and 75') do the settlements provided need to be extrapolated over the length of the building? For example, if total settlements are 1-3/8" over 40' are they then (75/40) x 1-3/8" = 2.6" over 75'?
4) Reviewing design examples, most address how to deal with differential settlements with larger concrete/steel buildings where they "drop" an interior column or an exterior column in order to produce some worst-case conditions to see what happens with stresses and bearing pressures. However, this is less clear to me with a smaller mat with a perimeter line load. In RAM Concept I can modify springs to simulate a differential deflection, but how do I determine exactly where this differential deflection occurs to design my mat foundation? Do I assume the building tilts in a few different directions (i.e. 0" on the right side of the building and then the full differential settlement on the left side of the building)? More or less, I'm struggling to understand how to approximate where liquefaction actually occurs within the footprint of my foundation for analysis.
I've been toying around with analyzing a slab similar to what I'm describing in Note 4 (tilting the building) and then checking to see if deflections and soil pressures across the building in the direction of the tilt are fairly linear. That way I'd assume the slab is behaving as a stiff/rigid element and impacts to superstructure will be minimal. Per ASCE 12.13.9.2b, if I'm accounting for liquefaction I need to detail my superstructure to be able to accommodate the differential settlements but that's not such an easy task with a light-framed, wood structure.
Hopefully my thoughts and questions are coherent. Realize this is a lot to unpack so I appreciate time/responses.
I'm working on a mat foundation for a small retail building. The one-story building is lightly-loaded with wood-framed walls and trusses, with one intermediate bearing line in the long direction. Building dimensions are roughly 50' x 75'.
The geotechnical engineer has identified a liquefaction risk and has recommended a mat foundation to support structure. Following the liquefaction analysis, the geotechnical engineers have indicated a total seismic settlement of 1-3/4 inch and total settlement (static + seismic) of 2-3/4 inch, with differential settlements being 1/2 of total (7/8 inch and 1-3/8, respectively). I don't have a ton of seismic experience so this is a nice project to learn from. Settlements occur over a distance of 40' per the geotech.
I have a number of questions so I'll try to ask them in an order that makes sense:
1) ASCE 7-16 has a new section on shallow foundation design. We don't meet the exception in 12.13.9 so liquefaction must be considered. Moving on, shallow foundations appear to be acceptable per 12.13.9.2 so long as both Part A and Part B are adhered to. Note: the geotechnical report doesn't go into detail re: lateral spreading to check the box for Part A but I'm assuming that this is not much of an issue or a failure mechanism for a mat foundation. However, then in Section 12.13.9.2.1.2 written specifically for mat foundations, "the flexural demands caused by liquefaction need not be considered if the mat is detailed in accordance with the requirements of 18.6.3.1 of ACI 318 (basically reinforcing the heck out of the mat)." Does that mean that differential settlements are not required to be analyzed for or accounted for at all if you simply reinforce the mat per ACI?
2) Assuming we do need to account for differential settlements, is there a threshold below which you don't really worry about the magnitude and "ignore" settlement effects? 1-3/8" seems pretty small to me, but maybe it isn't. There's an NEHRP design guide for mat foundations that loosely indicates mat foundations can accommodate up to 2 inches of differential settlements, but that statement doesn't indicate how stiff the mat needs to be, how it's reinforced, etc.
3) If settlements occur over a distance of 40' but my building dimensions exceed that in both directions (50' and 75') do the settlements provided need to be extrapolated over the length of the building? For example, if total settlements are 1-3/8" over 40' are they then (75/40) x 1-3/8" = 2.6" over 75'?
4) Reviewing design examples, most address how to deal with differential settlements with larger concrete/steel buildings where they "drop" an interior column or an exterior column in order to produce some worst-case conditions to see what happens with stresses and bearing pressures. However, this is less clear to me with a smaller mat with a perimeter line load. In RAM Concept I can modify springs to simulate a differential deflection, but how do I determine exactly where this differential deflection occurs to design my mat foundation? Do I assume the building tilts in a few different directions (i.e. 0" on the right side of the building and then the full differential settlement on the left side of the building)? More or less, I'm struggling to understand how to approximate where liquefaction actually occurs within the footprint of my foundation for analysis.
I've been toying around with analyzing a slab similar to what I'm describing in Note 4 (tilting the building) and then checking to see if deflections and soil pressures across the building in the direction of the tilt are fairly linear. That way I'd assume the slab is behaving as a stiff/rigid element and impacts to superstructure will be minimal. Per ASCE 12.13.9.2b, if I'm accounting for liquefaction I need to detail my superstructure to be able to accommodate the differential settlements but that's not such an easy task with a light-framed, wood structure.
Hopefully my thoughts and questions are coherent. Realize this is a lot to unpack so I appreciate time/responses.
