strucbells
Structural
- Mar 25, 2020
- 173
Paging JedClampett..not sure if there's a way to message users directly here.
I was taught at my last job to design the mat slab to wall joint for reinforced concrete liquid containing basins for the mat to have at least the same flexural capacity as the wall at the joint. This is for an open top, fixed base basin wall. Typically this results in a mat slab 2-3" thicker than the wall (due to increased cover and waterstop detailing) & with similar reinforcing near the joint at least.
I am reviewing rebar shop drawings for a project in construction that was designed a few years before my starting at this company that has a slightly thinner mat with much smaller bars than the wall above. Doesn't feel right, and the engineer who did the design is no longer at the company and there has been some "brain drain" prior to me arriving here. The saved project calcs are rough but it appears they used Enercalc's "Beam on Elastic Foundation" method to show that some of the moment gets directly transferred to the subgrade and therefore the mat can have much lower flexural capacity at this location.
The wall is 14" thick with #8 @ 6" OC dowels each face, and the mat slab is only 12" thick with #4 @ 8" OC T&B each way. The ldh is insufficient for #8 dowels, but appears to work when taking the As_reqd/As_provided reduction for the loads applied. The dowels are primarily sized for the earth pressure (liquid level inside the basin is much lower than grade outside), the mat is about 12' below grade and has an 8" extension beyond the wall face, so it is a bit like a retaining wall with a really short heel and really long toe (continuous mat to the opposite side).
A few questions:
1. How to determine how much of the "toe" aka mat slab inside of the basin is effective with respect to the moment applied at the base of the wall? The mat is continuous to the opposite wall about 20' away.
2. Is this a common and acceptable way to design this joint? Do others assume the mat slab does not need to resist the full wall moment due to the presence of the subgrade below?
3. Any good technical resources that cover this topic?
Trying to determine if it is worth raising the red flag and changing the design at this late phase in the project. ACI 350 and other relevant documents do not seem to provide much direction related to detailing this joint.
I was taught at my last job to design the mat slab to wall joint for reinforced concrete liquid containing basins for the mat to have at least the same flexural capacity as the wall at the joint. This is for an open top, fixed base basin wall. Typically this results in a mat slab 2-3" thicker than the wall (due to increased cover and waterstop detailing) & with similar reinforcing near the joint at least.
I am reviewing rebar shop drawings for a project in construction that was designed a few years before my starting at this company that has a slightly thinner mat with much smaller bars than the wall above. Doesn't feel right, and the engineer who did the design is no longer at the company and there has been some "brain drain" prior to me arriving here. The saved project calcs are rough but it appears they used Enercalc's "Beam on Elastic Foundation" method to show that some of the moment gets directly transferred to the subgrade and therefore the mat can have much lower flexural capacity at this location.
The wall is 14" thick with #8 @ 6" OC dowels each face, and the mat slab is only 12" thick with #4 @ 8" OC T&B each way. The ldh is insufficient for #8 dowels, but appears to work when taking the As_reqd/As_provided reduction for the loads applied. The dowels are primarily sized for the earth pressure (liquid level inside the basin is much lower than grade outside), the mat is about 12' below grade and has an 8" extension beyond the wall face, so it is a bit like a retaining wall with a really short heel and really long toe (continuous mat to the opposite side).
A few questions:
1. How to determine how much of the "toe" aka mat slab inside of the basin is effective with respect to the moment applied at the base of the wall? The mat is continuous to the opposite wall about 20' away.
2. Is this a common and acceptable way to design this joint? Do others assume the mat slab does not need to resist the full wall moment due to the presence of the subgrade below?
3. Any good technical resources that cover this topic?
Trying to determine if it is worth raising the red flag and changing the design at this late phase in the project. ACI 350 and other relevant documents do not seem to provide much direction related to detailing this joint.
