seismic lateral load distribution for 3 hangers

[johndeng]

3 vertical hangers support a 1000 lbs member, the middle hanger takes 500 lbs vertical dead load, side hangers take 250 lbs each. Total lateral seismic load is 100 lbs, what's the lateral load at middle hanger, 50 lbs or 33.3 lbs?

Thanks

 

[kissymoose]

Lol, this reads a lot like a school question.
I assume the member is axially stiff enough to just distribute the load equally to each support, so 33lbs. But the difference is minute, so does it matter?

 

[johndeng]

Why it's not 50 lbs.

If we turn it upside down like a port frame, 3 columns supporting 1 beam, the middle column lateral load should be 50 lbs

 

[rb1957]

50 lbs, based on the distribution of vertical loads (which feels simplistic, but is "given")

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

 

[CDLD]

Since the middle hanger takes 1/2 the vertical load, we can assume there are 2 simply supported beams connected to the hanger. This means connections at the top of hangers must be fixed to the supports.
Assuming each hanger is the same member size, each hanger takes 1/3 of horizontal load.

 

[JoshPlumSE]

I don't generally view "hangers" as part of the lateral force resisting system. Certainly in the hospital work that I've done, we're required to put in a series of diagonal cables to take ALL of the lateral force. I can't imagine designing something that weighs 1000 lbs on hangers without some bracing or cables to take the lateral load.

 

[johndeng]

Yes assume connections at the top of hangers fixed to the supports.
Each hanger is the same member size. Please advise.

I feel it's 33 lbs due to same rigidity. But for a port frame, edge columns are always assumed to take half lateral load of the interior, also lateral is percentage of dead load supported.

33 lbs supporter: kissymoose CDLD
50 lbs supporter: rb1957
Unkown: JoshPlumSE

 

[CDLD]

It's 1/3 lateral load for sure.
Josh, I feel its more of a theoretical exercise, but I agree.

 

[WesternJeb]

Johndeng, are you referring to lateral loads from a flexible diaphragm induced be seismic forces? That seems to be what you are trying to lead to in your previous post. If that is the case, then this is much more complicated than how you describe it with simple hangers connected by some beams.

 

[johndeng]

@WesternJeb
Let say the 3 hangers fixed on roof. Lateral load is due to the 1000 lbs weight they are hanging. Middle hanger supporting 500 lbs weight, side hangers 250 lbs each.
If you think of the rigidity of the 3 hangers, you think each takes 33 lbs lateral load.
If you separate each hanger, the middle hanger takes 500 lbs weight, lateral load is 0.1*500 lbs = 50 lbs.

 

[CDLD]

Here you go John.
Lateral load is at 1000 lbs but you get the idea.

 

[johndeng]

@CDLD
great, expert of model!
Can you also model it upside down, with 2 beams on top, 3 columns fix on bottom? I bet you get 0.33 kips each, but as I know, usually for portal frame, middle column takes 0.5 kips.

 

[SWComposites]

Why care? just size each hanger for 50 lbs lateral load and be done with it.

 

[johndeng]

@SWComposites
I know, but it's just a simplified version. Actual condition makes a difference if the middle hanger takes half of the total lateral load.

 

[HTURKAK]

Your statement 'the middle hanger takes 500 lbs vertical dead load' implies the member supported with equal spans . The upper and lower limits would be ;

- If we assume the member flexible , and hangers axially rigid, the middle hanger takes (5/8)*1000= 625 lbs vertical dead load, while side hangers take (3/8)*500= lbs
- If we assume the member rigid relative to the axial rigidity of hangers , all hangers will take 1000/3 =333 lbs ,
- If we assume the member axially rigid , and the total lateral seismic load is 100 lbs, the lateral load at hangers, 33.3 lbs.

I assume ASCE 7-16 Section 13.3 ( SEISMIC DEMANDS ON NONSTRUCTURAL COMPONENTS ) is applicable .

If the max . force is more than 5 times of minimum, and the calculation is based on a lot of assumptions , i would go with max . force.

But don't forget to provide bracings ( transversal and longitudinal ) since the seismic force will be resisted by bracings not hangers.

Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM

 

[WesternJeb]

If you think of the rigidity of the 3 hangers, you think each takes 33 lbs lateral load.

Disagree with this. Elongation (gravity load) is based on PL/AE, which is the area of the section.

Bending deflection (lateral load) is based on the moment of inertia in the direction of the force.

You could have some wide flanges that are much stronger in the axial direction but relatively the same moment of inertia in the weak direction.