beam force resulting from end beam relative displacement 3

[TTTKAO]

Hello All,

I am designing a 6.6m steel beam with 20mm displacement difference at the ends. this beam is supported by beams, one end is moment connection and the other end is shear connection. Is there anybody have suggestion regarding to how to consider the load resulting in the beam end relative displacement or know any document discussing about this topic?

Currently, i doubled the beam length to 13.2m and add 20mm displacement at the center of the beam to get the additional load, is there any risk or concern to use this method?

Thank you for your advice and time in advance.

Regards!

 

[JoshPlumSE]

As I understand it, the excessive deflection is coming at the end with the moment connection. Right? It looks to me like the deflection is caused by whatever is loading the "huge beam" (built up 1000x350x60x40) that supports the 350WC280.

It almost looks like this built up beam is a transfer girder supporting a column from above. If so, then nothing you do with the 350WC is going to affect that deflection. You have to look at that transfer girder if you want to reduce the deflection.

 

[BAretired]

Josh, I don't think anyone is suggesting that the deflection is excessive. We don't know the span of the supporting beam D-E-F, but judging by the isometric drawing, assuming it is to scale, it appears to be about 15m. An unfactored deflection of 20mm is only L/750 which does not seem excessive.

There is a cantilever platform on the opposite side of the beam. We don't know much about it...the size, the load or just how it is supported, but I'm assuming that it adds a moment to the end of beam B-G-E which will change each reaction by M/6.6, an increase at point E and decrease at point B.

 

[TTTKAO]

Hi Josh,

the deflection is mainly caused by the 6.6m beam which is a transfer beam need to support two columns above. i can do nothing to the supporting girder. the cantilever platform load is very small. the torsion load from the cantilever platform can be resist by the supporting girder directly.

regards

 

[TTTKAO]

Good morning All,

Thank you so much for all your opinions, ideas and pervious experiences. I appreciate a lot.

I think i get confidence to ignore the beam end relative displacement for this situation. let me use factored deflection shown in the sketch to get quick calculation conservatively.

as the supporting beam is very huge comparing to the 6.6m 35WC280 beam. i ignore torsion deformation on supporting beam.

1. 6.6m beam rotation value due to self-deflection, (37mm-(34mm+5mm)/2)/(6.6m/2)= 0.005 rad
2. 6.6m beam rotation value due to end relative displacement. (34mm-5mm)/6.6m= 0.005 rad
3. the total rotation value is 0.01 rad which is less than 0.02( this is connection experiment failure value)

base on the calculation above, we may say the shear connection is still work well and we can assume this is pin connection in our analysis model.

my original thought for this post is to check if we need add extra force due to the relative end displacement, with the discussion here, i think we can control deflection ratio/rotation value to make sure the structure is safe.

Thank you again all.

 

[rb1957]

don't quite understand 1) ? the rotation of the end of a simply supported beam under a uniformly distributed load is not that (what is given) !?

I think it's in Roark, but don't have a copy to hand (and don't really care) ... I'm sure others will have it to hand.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[TTTKAO]

Hi rb1957,

you are right, for item one, the actual deflection should be parabolic one, but i assumed triangle one to simplify the calculation.

 

[BAretired]

the deflection is mainly caused by the 6.6m beam which is a transfer beam need to support two columns above. i can do nothing to the supporting girder. the cantilever platform load is very small. the torsion load from the cantilever platform can be resist by the supporting girder directly.

It appears from the isometric sketch that deflection in the supporting beam is mainly caused by the column load at point E, which appears to be almost centered (and probably should be centered) on the supporting beam. A rigid end plate connection bolted to the web would be preferable to the detail shown in the top left corner of your sketch.

I suspect the 6.6m transfer beam will resist the eccentric moment from the platform, as its bending stiffness is likely much greater than the torsional stiffness of the supporting beam.

 

[TTTKAO]

Hi BAretired,

Thank you for the suggestion, the moment connection is better than pin connection at E point

Regards!

 

[BAretired]

Another point which I should have mentioned earlier:

i used factored deflection in the sketch, the factored deflection 29mm, the unfractured deflection is around 20mm.Sorry for the confusing.

Deflection should always be based on service loads, not factored loads, because the expression for deflection assumes stresses below yield. Using the expression for factored loads would permit some or all of the beam to be above yield strain, so the results would be incorrect.

 

[TTTKAO]

Hi BAretired,

well noted, thank you.