Comparing moment capacity of concrete connection with metal insert
Comparing moment capacity of concrete connection with metal insert
(OP)
Hi all,
Currently I have a problem finding the moment resistance of a connectio in an existing concrete struture. While it may sound easy, my results don't convince me. Attached is a sketch and this is a description of the problem:
I am analyzing existing concrete frames. These frames are connected via struts and these struts have metal inserts in them (10W33=W250x49). The drawing for this connection indicates max shear is 25kip(112kN) and max moment is 64 ftkip(87 kNm). The problem arises when I use 87 kNm as the max moment (negative moment) at the connection of the frames and struts. This is because 87 kNm is so low that most of the moment is transferred to the middle of the strut and it becomes positive moment. It is strange that positive moment should be considerably larger than negative moment becaue there are 3 rebars at the top (#9) and 2 rebars at the bottom (#8). Therefore, it seems to me that this struts were designed with negative moment in mind. Addtionaly, if the structure does behave this way (connection limited to 87 kNm) then this structure should have failed at the positive moment region ULS. However, this structure is over 40 years old.
Another thing to note is that this structure was made before ULS came into use. Therefore, there is some difficulty in comparing values.
I have checked the maximum shear and the value that I obtained is approximately 142kN which is larger than 112 kN. Although this is not very close my moment capacity calculated is 147 kNm which is much larger than 87 kNm. This connection is fully welded and so I expect it to be a moment connection. Rebars run full length of strut.
Could someone please give me some pointers? Thank you very much in advance.
Currently I have a problem finding the moment resistance of a connectio in an existing concrete struture. While it may sound easy, my results don't convince me. Attached is a sketch and this is a description of the problem:
I am analyzing existing concrete frames. These frames are connected via struts and these struts have metal inserts in them (10W33=W250x49). The drawing for this connection indicates max shear is 25kip(112kN) and max moment is 64 ftkip(87 kNm). The problem arises when I use 87 kNm as the max moment (negative moment) at the connection of the frames and struts. This is because 87 kNm is so low that most of the moment is transferred to the middle of the strut and it becomes positive moment. It is strange that positive moment should be considerably larger than negative moment becaue there are 3 rebars at the top (#9) and 2 rebars at the bottom (#8). Therefore, it seems to me that this struts were designed with negative moment in mind. Addtionaly, if the structure does behave this way (connection limited to 87 kNm) then this structure should have failed at the positive moment region ULS. However, this structure is over 40 years old.
Another thing to note is that this structure was made before ULS came into use. Therefore, there is some difficulty in comparing values.
I have checked the maximum shear and the value that I obtained is approximately 142kN which is larger than 112 kN. Although this is not very close my moment capacity calculated is 147 kNm which is much larger than 87 kNm. This connection is fully welded and so I expect it to be a moment connection. Rebars run full length of strut.
Could someone please give me some pointers? Thank you very much in advance.






RE: Comparing moment capacity of concrete connection with metal insert
Also consider that as part of any yielding, the loads may have redistributed and thus the ULS is not reached in one section of the frame as your calcs indicate. Another reason may be that the actual loading on the structure has never reached the ULS and so would not fail.
Regards,

Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
RE: Comparing moment capacity of concrete connection with metal insert
What do you say about there being more steel at the top than the bottom?
RE: Comparing moment capacity of concrete connection with metal insert
The following is not clear to me:
What do these two sentences mean? A negative moment cannot become a positive moment. How did you determine that the positive moment is larger than the negative moment? It does not seem to make sense.
BA
RE: Comparing moment capacity of concrete connection with metal insert
The struts are gravity loaded. As for the negative moment becoming positive moment, I meant to say the magnitude of the positive moment changes. If the connection is limited to a moment of 87 kNm then the magnitude of the midspan (positive moment) is larger than the moment at the midspan if the connection is permitted to develop a moment larger than 87 kNm.
This would be moment redistribution since a plastic hinge develops at the connections if 87 kNm is the maximum moment at the connection. Correct?
RE: Comparing moment capacity of concrete connection with metal insert
USD was first introduced into ACI 318-63 but WSD was also permitted in that code. If the building is over forty years old, it could have been designed by either method.
BA
RE: Comparing moment capacity of concrete connection with metal insert
RE: Comparing moment capacity of concrete connection with metal insert
It wouldn't be difficult to assess the end by FEM today. As preliminary checks some strut and tie schemes on where the loads are going may help to give an idea, but the FEM model will give more interesting information.
RE: Comparing moment capacity of concrete connection with metal insert
Why do you see the intent of building a simply supported end of strut when there is a 1/4" weld connecting the insert? How would this allow rotation?
RE: Comparing moment capacity of concrete connection with metal insert
I think I have a fair handle on plastic moments in steel, rotations at the hinges and redistribution of the moments. But, the bigger question might be, given the proportions you show in your sketch, how are you going to transfer the same general moment magnitude (or range) where the steel stub beam enters the conc. beam section? How do you keep the steel stub beam from just cranking out of the conc. section?
RE: Comparing moment capacity of concrete connection with metal insert