Existing Beam Help

[KOTOR1]

I have a situation where I have to remove two columns at an existing beams(3 span beam) and place a new column to make it a two span beam. Looking at the existing conditions, there were originally two beams, one was continous across end bearing to the futhermost column. While the other beam spanned from the futhermost colummn to the other end bearing conditions.

For designing the new beams, I assumed one beam to be continous with a cantilever while the other beam be simply supported. Is that a safe assumption. The existing splice on the futhermost column also seems to work. Do I need to replace the splice there.

My original spans were 21'-10";25'-6" and 27'-5". Now my spans become 33'-10" w/ a 12'-4" cant and 27'5". Is that assumption correct.

 

[msquared48]

Sounds like you are trying to convert to a link-span condition, and it sounds to me like the 12.33' cantilever is a little large to optimize the moments - usually no more than 15% of the main span which would be 5 feet, not 12.33. More than likely, the shear at the cantilever will control the design.

Mike McCann
MMC Engineering

 

[KOTOR1]

The existing beam is a W36x135 which I think is pretty huge for the original spans. If a look at the design of the beam w/ the cantilver it actually works and also meets the defelction criteria. Although, I have to replace one of the end columns due to the new loads.

 

[kslee1000]

Original

--------------- -------------------------------
| | | |
| | | |

New
Cant
---------------?-------------------------------
| Link | |
| | |

Correct? Where was the existing splice?

 

[kslee1000]

Please note the 3 columns under the original continuous beam have shifted to the left, as well as the 2 columns under the beam with cantilever.

 

[KOTOR1]

Well, the original condition was as follows

I--------II--------V--------I

I- Column End Bearing Condition
II-Beam Cont. Across Column
V - Column w/ beam splice exactly above it.

The new condition

I------------A----III---------I

III-Splice At Same Location as that of the 3rd Column from the left. (at V as per existing condition)
A - new column location
End Bearing Condition remain the same.

 

[haynewp]

That is the correct analytical model from what you have stated and it is a stable configuration as long as you keep the beam continuous over the new column.

 

[KOTOR1]

The beam is going to remain continous at the existing column. Should the existing splice (now at the cantilvered end) be redesigned? It works with new loads.

 

[haynewp]

I don't know if the design is adequate or not. If it works for the new loads, then I am confused with the redesign question.

 

[KOTOR1]

I misquoted abut the existing splice. The splice does work w/ the new loads. I was wondering if I need to provide any additional reinforcing at the splice.

 

[kslee1000]

THe splice is a link in the new system as opposed to the continuity connection in the old system. You may consider to make it a moment connection, or leave it as is (simple link taking shear only). For the latter, the splice will experience higher load than before (1/2 of load from the beam to its right), also you shall have noticed the large deflection at this location. If you have check the bolts, connection plates (from both faces), and the deflection is acceptable, then there is nothing much you need to do about. Maybe just replace the bolts and plates to ensure the quality. However, pay attention to details on the right-most beam column connection, the end rotation tends to create stability concerns. Also, your cantilever is rather longer than ideal, make sure the leftmost beam is adequate for reversed stresses (beam deflect upward, is the floor still level at this instance?), and far left end connection is adequate to resist uplift force if present. This is an interesting project. Good luck.

 

[BAretired]

Make sure the compression flange on the leftmost beam and cantilever are adequately braced for unbalanced load (a) at the new column (b) in the span and (c) in the cantilever.

Best regards,

BA