Comparing moment capacity of concrete connection with metal insert

[hitch22]

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.

 

[Qshake]

There is no attachment.

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.

 

[hitch22]

Thanks Qshake for the quick reply and for pointing out the attachment was missing.

What do you say about there being more steel at the top than the bottom?

 

[BAretired]

How are the struts loaded? If by gravity, the end moments are negative, i.e. tension on the top. If by sway, one is positive and the other is negative. If in combination, gravity and sway, the negative moments would be additive so that the maximum moment would be negative. It could be at either end, depending on the direction of sway. The positive moment would be primarily due to gravity and not significantly influenced by sway.

The following is not clear to me:

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).

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

 

[hitch22]

Hi BAretired,

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?

 

[BAretired]

I agree that if the end connections are unable to take the negative moment, then moment redistribution will occur and positive moment will increase. But you calculated a factored moment capacity of the connection to be 147kN-m or about 1.69 times the 87kN-m value cited. Perhaps the original engineer was talking about a WSD (Working Stress Design) value instead of a USD (Ultimate Strength Design) value.

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

 

[hitch22]

OK thanks a lot BAretired. I wasn't sure if what I was thinking made sense. Thanks for your opinion.

 

[ishvaaag]

I see there more than anything the intent of building a simply supported end of strut, guided by the slotted bolts, and supported by the corbel. Differences of magnitude might be explained by the original numbers being service level. Furthermore, the insert seems quit short at first sight (even if today we see proprietary connectors for shear also quite short) so it may proceed from some reliable calculations or quite proven practice.

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.

 

[hitch22]

ishvaag:

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?

 

[dhengr]

Hitch22:
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?

 

[ishvaaag]

You are entirely right, hitch22 I was misdirected in my thoughts by the apparently slotted connections, that then must be just be a buildability device. Too quick a look. For the problem dhengr refers to, a FEM model would be great; but one may try some interpretation with just strut and tie for the capability to transfer some moment and shear at the frontal face of the beam.