Greenalleycat
Structural
- Jul 12, 2021
- 580
Howdy all
A trend here in NZ is for almost all fixings (at least in residential) to go to epoxy post-fixed anchors rather than cast-in.
As engineers we much prefer to spec cast-in bolts for capacity, but contractors push for post-fixed as they give significant tolerance
This is particularly useful for portal baseplates (or for epoxying in wall starters because they inevitably did not put enough care into getting them into the right place to start with).
However, a topic of hot discussion in our office is whether post-fixed anchors really are an equitable substitute at all and whether we, as an industry, should be backing off our use of these in primary structural elements.
Our concerns are that the actual load path of an epoxy anchor is substantially different to a cast-in fixing
Cast-in fixings generally rely on a washer etc at the bottom as the origin of the cone, meaning that the failure cone of the anchor will spread out and engage bars around it
The reliability of the load path can be checked by assuming a failure cone angle and checking that steel is developed within this cone
Epoxy anchors, by contrast.... we calculate say a shear and tension force on the portal baseplates, apply these to our Hilti Profis model (or similar) and get a tick saying that our proposed 2/4 anchor arrangement is satisfactory - job done, design out the door, black box software happy
However, epoxy anchors generally fail by pulling out a relatively shallow cone that starts at an unknown point above the base of the fixing
This means that there is a reduced ability to develop reinforcement bars within the depth of the cone
The strength of the anchor therefore ultimately relies on concrete in tension - something you aren't allowed to do in our codes
This problem becomes even worse when considering the tendency for contractors to stuff up their starter bars and try to epoxy in new starters to substitute
The original hooked starter has well-understood performance in terms of strength of connection, formation of concrete strut on the inside of the hook etc
The epoxy bar...does anyone know how these fare when you have a mixture of epoxied and cast-in starters?
What is the strut and tie model for an epoxy bar? When the capacity of the epoxy is exceeded, what happens (presumably the whole thing fails very brittly)?
There are also practical issues such as the fact that the QA on these is generally non-existent - you're reliant on the contractor following the spec, not anchoring into cover concrete, etc
I am curious to know what other engineers' views are on this topio as we are starting to view it as one of those unspoken issues being pushed by the manufacturers and contractors and not resisted by engineers
Cheers!
A trend here in NZ is for almost all fixings (at least in residential) to go to epoxy post-fixed anchors rather than cast-in.
As engineers we much prefer to spec cast-in bolts for capacity, but contractors push for post-fixed as they give significant tolerance
This is particularly useful for portal baseplates (or for epoxying in wall starters because they inevitably did not put enough care into getting them into the right place to start with).
However, a topic of hot discussion in our office is whether post-fixed anchors really are an equitable substitute at all and whether we, as an industry, should be backing off our use of these in primary structural elements.
Our concerns are that the actual load path of an epoxy anchor is substantially different to a cast-in fixing
Cast-in fixings generally rely on a washer etc at the bottom as the origin of the cone, meaning that the failure cone of the anchor will spread out and engage bars around it
The reliability of the load path can be checked by assuming a failure cone angle and checking that steel is developed within this cone
Epoxy anchors, by contrast.... we calculate say a shear and tension force on the portal baseplates, apply these to our Hilti Profis model (or similar) and get a tick saying that our proposed 2/4 anchor arrangement is satisfactory - job done, design out the door, black box software happy
However, epoxy anchors generally fail by pulling out a relatively shallow cone that starts at an unknown point above the base of the fixing
This means that there is a reduced ability to develop reinforcement bars within the depth of the cone
The strength of the anchor therefore ultimately relies on concrete in tension - something you aren't allowed to do in our codes
This problem becomes even worse when considering the tendency for contractors to stuff up their starter bars and try to epoxy in new starters to substitute
The original hooked starter has well-understood performance in terms of strength of connection, formation of concrete strut on the inside of the hook etc
The epoxy bar...does anyone know how these fare when you have a mixture of epoxied and cast-in starters?
What is the strut and tie model for an epoxy bar? When the capacity of the epoxy is exceeded, what happens (presumably the whole thing fails very brittly)?
There are also practical issues such as the fact that the QA on these is generally non-existent - you're reliant on the contractor following the spec, not anchoring into cover concrete, etc
I am curious to know what other engineers' views are on this topio as we are starting to view it as one of those unspoken issues being pushed by the manufacturers and contractors and not resisted by engineers
Cheers!
