Straps used for Caisson Construction Method

[oengineer]

I am seeking information (i.e., a detail) on the use of Caisson Straps to be used to install/construction precast riser sections for a wet well. Would anyone be able to point me in the right direction?

Suggestions/comments are appreciated.

 

[oengineer]

The image below discuss the type of Caissons Construction I am seeking info (a construction detail) about, using precast riser sections:

 

[JoelTXCive]

The city of Houston allows precast sections up to 6ft in diameter to be used as wet wells for lift stations. These wet wells are most often constructed with the caisson method in Houston since we have so much clay.

They require 6 straps to be places on the external side of the sections.

They provide loading values for these straps, but no clues on how the loading values were generated.

My assumption is that these straps serve to prevent separation of the segments during the sinking process.

Attached is the City of Houston Standard detail.

 

[oengineer]

The city of Houston allows precast sections up to 6ft in diameter to be used as wet wells for lift stations. These wet wells are most often constructed with the caisson method in Houston since we have so much clay.

They require 6 straps to be places on the external side of the sections.

They provide loading values for these straps, but no clues on how the loading values were generated.

My assumption is that these straps serve to prevent separation of the segments during the sinking process.

Attached is the City of Houston Standard detail.

Thank you so much for this information!

 

[oengineer]

Does anyone have any technical examples on how to design the straps capacity for these Caisson Straps?

 

[JoelTXCive]

Strap Capacity = .90*A_s*fy
Embed Plate Capacity = Concrete Breakout Capacity Of the Studs

I have always assumed that you need a tension capacity only to keep the segment joint closed up.

In compression, you will have concrete on concrete if the steel yields; so you do not have to look at that.

To generator any out of plane loading that would put the plates into flexure; you would have to shear the shear key off between the male and female pipe ends. If that happens; you've got bigger problems; and I doubt the plates are going to help you much.

I've googled this before and not found any answers; so it will be interesting to hear what other people think.

 

[oengineer]

Strap Capacity = .90*A_s*fy
Embed Plate Capacity = Concrete Breakout Capacity Of the Studs

I have always assumed that you need a tension capacity only to keep the segment joint closed up.

In compression, you will have concrete on concrete if the steel yields; so you do not have to look at that.

To generator any out of plane loading that would put the plates into flexure; you would have to shear the shear key off between the male and female pipe ends. If that happens; you've got bigger problems; and I doubt the plates are going to help you much.

I've googled this before and not found any answers; so it will be interesting to hear what other people think.

Thank you very much for this info!

 

[oengineer]

Strap Capacity = .90*A_s*fy
Embed Plate Capacity = Concrete Breakout Capacity Of the Studs

I have always assumed that you need a tension capacity only to keep the segment joint closed up.

In compression, you will have concrete on concrete if the steel yields; so you do not have to look at that.

To generator any out of plane loading that would put the plates into flexure; you would have to shear the shear key off between the male and female pipe ends. If that happens; you've got bigger problems; and I doubt the plates are going to help you much.

I've googled this before and not found any answers; so it will be interesting to hear what other people think.

Would it be acceptable to treat the Caisson Strap like an embed plate into a concrete wall & design it that way?

 

[oengineer]

Any other comments regarding the design of this strap is appreciated.

 

[oengineer]

I am STILL seeking information on the use of Caisson Straps to be used to install/construction precast riser sections for a wet well.

I am particularly seeking info on calculating the the applied force acting on the Caisson Straps?

Comments/suggestion are appreciated.

 

[GC_Hopi]

As JoelTXCive stated the applied force on the straps would be based on the skin friction as the caisson sinks. Worse case would be the full caisson weight and skin friction only on the upper riser section. You then would put a large factor of safety on this since a separating riser section is the last thing you want. I am sure a lot of this is based on past experience but the numbers on the 6 ft diameter caisson work out to 4.5 kip/ft around the perimeter

 

[oengineer]

I am sure a lot of this is based on past experience but the numbers on the 6 ft diameter caisson work out to 4.5 kip/ft around the perimeter

Do you happen to have a calculation example showing how you determined the 4.5 kip/ft around the perimeter of a 6 ft diameter caisson?

 

[phamENG]

The detail says 6 connections per section with a capacity of 14kips per connection. That's 84kips per section. If the section has a diameter of 6ft, the perimeter is pi*D=18.85ft. 84k/18.85ft=4.5k/ft. Pretty simple, really.

For what it's worth, I've never designed these and can't really help with the original question.

 

[oengineer]

The detail says 6 connections per section with a capacity of 14kips per connection. That's 84kips per section. If the section has a diameter of 6ft, the perimeter is pi*D=18.85ft. 84k/18.85ft=4.5k/ft. Pretty simple, really.

I actually reached out to the City of Houston to determine how they came up with their number/design and they said that they do not use precast wet wells, so they do not have structural calculations for a precast situation. The attached document is for cast in place concrete.

For this reason I am seeking info on this type of analysis.

 

[GC_Hopi]

oengineer: Do you have a geotechnical engineer on board to give you recommendations on sinking the caisson? They could give you some values (skin friction... shear, etc) to design around. Otherwise the conservative thing as mentioned is to design the connection to support the full caisson weight. Imaging it hanging just off the bottom of your excavation with all the dead load applying tension on the connections.

 

[JoelTXCive]

Got to love city permitting department employees...

Do they not realize their detail is named "PRECAST UNITS Connection Details"

I work for a large engineering firm in Houston. We submit and get approvals on at least 20+ pre-cast wet wells a year for construction in the City of Houston proper, along with annexed suburbs.

 

[JoelTXCive]

Attached is Houston's current standard detail sheet showing the precast connections and a cutting tip for precast or Cast In Place.