Hello everyone,
According to the existing structural analysis, I plan to carry out the following and have a few questions regarding the practical execution:
The existing two-layer reinforced Q188 foundation slab (C20/25), 20cm (8 inches) thick, is to be cut open over an area of 1.5 x 0.5 m (5 x 1.6 ft) and supplemented by a new single-layer reinforced isolated footing (C25/30), reinforcement Q335 (bottom layer, closely spaced), also 20cm (8 inches) thick. A steel column is to be installed on this footing.
During demolition, a considerable amount of the existing reinforcement remained in place.
-> Would you use this as so-called connecting reinforcement for the new footing? The structural analysis does not explicitly address this.
-> Would you "decouple" the new isolated footing from the existing foundation slab, or pour it directly against the existing concrete?
Thank you and best regards
According to the existing structural analysis, I plan to carry out the following and have a few questions regarding the practical execution:
The existing two-layer reinforced Q188 foundation slab (C20/25), 20cm (8 inches) thick, is to be cut open over an area of 1.5 x 0.5 m (5 x 1.6 ft) and supplemented by a new single-layer reinforced isolated footing (C25/30), reinforcement Q335 (bottom layer, closely spaced), also 20cm (8 inches) thick. A steel column is to be installed on this footing.
During demolition, a considerable amount of the existing reinforcement remained in place.
-> Would you use this as so-called connecting reinforcement for the new footing? The structural analysis does not explicitly address this.
-> Would you "decouple" the new isolated footing from the existing foundation slab, or pour it directly against the existing concrete?
Thank you and best regards
Hello,
It depends on how it was designed. If the new footing is supposed to transfer its load to the existing slab, meaning it acts monolithically, then you need to connect them.
If it was designed as an isolated footing that does not transfer loads to the slab, then you must decouple it (for example, to avoid differential settlement).
If you decouple it, you must consistently separate the reinforcement as well.
Almost the same answer as above, but with an addition:
If the slab is designed monolithically, you have to ensure that forces can be transferred via the reinforcement from the old slab to the new one. This requires maintaining proper overlap lengths (lap splices). In your case, with 1.05 m x 0.50 m (3.4 ft x 1.6 ft), it depends on the direction whether that is sufficient. The structural engineer must confirm this.
Further, the structural engineer must specify whether additional reinforcing needs to be installed, for example anchored into the old slab (bonded connection).
To put it briefly:
All these questions must be answered by the structural engineer who did the design.
Best regards,
Jann
daytona schrieb:
-> Would you decouple the new isolated footing from the existing slab or cast it directly against it?
It depends on how it was designed. If the new footing is supposed to transfer its load to the existing slab, meaning it acts monolithically, then you need to connect them.
If it was designed as an isolated footing that does not transfer loads to the slab, then you must decouple it (for example, to avoid differential settlement).
If you decouple it, you must consistently separate the reinforcement as well.
daytona schrieb:
During demolition, quite a bit of existing reinforcement remained in place.
-> Would you use this as so-called reinforcing for the connection to the new footing? The structural engineering documents do not explicitly mention this.
Almost the same answer as above, but with an addition:
If the slab is designed monolithically, you have to ensure that forces can be transferred via the reinforcement from the old slab to the new one. This requires maintaining proper overlap lengths (lap splices). In your case, with 1.05 m x 0.50 m (3.4 ft x 1.6 ft), it depends on the direction whether that is sufficient. The structural engineer must confirm this.
Further, the structural engineer must specify whether additional reinforcing needs to be installed, for example anchored into the old slab (bonded connection).
To put it briefly:
All these questions must be answered by the structural engineer who did the design.
Best regards,
Jann
Hello Jann,
Thank you for your answers, although honestly, I was hoping for more specific information ;-).
But just for understanding: What if I can no longer contact the author or the structural engineer? Is it not possible to derive the assumptions or the resulting construction measures (answers to my questions) from the existing structural calculation? For example, from the referenced standards, terms, numbers, or even the sketch and symbols (standardized?).
Since I am an engineer myself, although not a civil engineer, this should be possible in the structural calculation—as an officially recognized document, right?
And if not, what would be the lesser evil: to decouple or not?
Thanks and regards
Thank you for your answers, although honestly, I was hoping for more specific information ;-).
But just for understanding: What if I can no longer contact the author or the structural engineer? Is it not possible to derive the assumptions or the resulting construction measures (answers to my questions) from the existing structural calculation? For example, from the referenced standards, terms, numbers, or even the sketch and symbols (standardized?).
Since I am an engineer myself, although not a civil engineer, this should be possible in the structural calculation—as an officially recognized document, right?
And if not, what would be the lesser evil: to decouple or not?
Thanks and regards
Hi,
if you can no longer contact him, you could ask another structural engineer to review the structural analysis. They can derive the answer from the calculations provided. I understand that you were hoping for a clear answer, but it’s not that simple in this case.
It’s difficult to say which option is better.
Here is a brief explanation of why I won’t make a definitive statement:
Assuming the column was designed to transfer the loads to the adjacent slab, thus transferring the shear force into the slab. This transfer also increases the area over which the load is distributed into the ground, meaning a lower soil pressure would be sufficient.
Now, if you decouple the foundation from the slab—
the consequence is that the shear force is no longer transferred to the slab, which could lead to punching failure, where the column pushes through the foundation and the load above follows downward.
Another possible failure mode is excessive soil pressure, causing the column and foundation to sink further into the ground, leading to settlement.
All these scenarios are possible but not guaranteed. Since no one wants to take responsibility for this, no one can simply give you a definitive answer.
Therefore, my advice for your case:
Take the documents and hire a structural engineer officially and pay them accordingly.
if you can no longer contact him, you could ask another structural engineer to review the structural analysis. They can derive the answer from the calculations provided. I understand that you were hoping for a clear answer, but it’s not that simple in this case.
It’s difficult to say which option is better.
Here is a brief explanation of why I won’t make a definitive statement:
Assuming the column was designed to transfer the loads to the adjacent slab, thus transferring the shear force into the slab. This transfer also increases the area over which the load is distributed into the ground, meaning a lower soil pressure would be sufficient.
Now, if you decouple the foundation from the slab—
the consequence is that the shear force is no longer transferred to the slab, which could lead to punching failure, where the column pushes through the foundation and the load above follows downward.
Another possible failure mode is excessive soil pressure, causing the column and foundation to sink further into the ground, leading to settlement.
All these scenarios are possible but not guaranteed. Since no one wants to take responsibility for this, no one can simply give you a definitive answer.
Therefore, my advice for your case:
Take the documents and hire a structural engineer officially and pay them accordingly.
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