No Horizontal Reactions allowed 2

[dylansdad]

Folks,

I am providing shop drawings for a lightweight fabric structural system.
What I have existing is 4 HSS columns in a 30'x30' grid pattern.
Connecting these columns are 4 HSS10x6 beams. My structure (thinking trusses, etc.)
rests on top of the beams (or it appears to based on the structural drawings).
There is a note on the structural drawings stating "no horizontal reactions to the HSS
beams from DL and LL are allowed". Seismic design category = D. I have modeled
as reactions at discrete points (zoom to see) along the beams.I would appreciate some
thoughts from the group as to how this "no horizontal reactions" could be accomplished. Thanks in advance.

 

[BAretired]

Under gravity load, the reactions indicated would be vertical if properly detailed. Under lateral load, horizontal reactions will necessarily result.

BA

 

[paddingtongreen]

It looks like there will be the moment of the overhang at the eaves ends will put equal and opposite horizontal kicks into the beam, but that should add to zero for the columns.

If the HSS structure is rigid, you would be able to land only on the columns with pins and rollers to avoid lateral forces at the foundation. Landing on the beams would cause bending moment at the top of the column.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[msquared48]

If they passed a law that wind and seismic forces are not allowed in the area, maybe the restriction you refer to could work, but not in the reality of this system of things.

BA is correct. There WILL be lateral forces to the HSS sections, just not from Dead or live if detailed properly.

Mike McCann, PE, SE

 

[wannabeSE]

Dylansdad,

Is the particular problem that prevents the fabric structure from meeting these requirements? The screen capture looks like the supports of this structure are pinned. You may want to change all but a couple to rollers.

 

[Ron]

Regardless of our "desires", physics still applies. If the wind blows or the ground shakes, lateral load will be there as Mike and BAretired noted. You have to anchor the roof structure, so anchorage will result in a reaction.

 

[ztengguy]

I would label all the lateral loads as WL or EQ, that way they are not DL and LL, thus you meet the criteria.

 

[Bagman2524]

Agree with msquared. Don't see any way to avoid lateral loads unless a whole other frame is constructed to take the lateral loads.

Faith is taking the first step even when you can't see the whole staircase. -MLK

 

[BAretired]

Perhaps the OP should ask for clarification from the EOR as to the intent of the requirement "no horizontal reactions allowed".

BA

 

[dylansdad]

Thanks for all of the replies.
BA, I have asked to EOR just that and am awaiting a response.

DD

 

[BAretired]

dylansdad,
Good. How's Dylan doin'?

BA

 

[dylansdad]

BA, thanks for asking

Dylan is about like every other teenage male knucklehead.

 

[Bagman2524]

The females in that age bracket aren't any better

Faith is taking the first step even when you can't see the whole staircase. -MLK

 

[msquared48]

BA:

Sorry to be cynical here, ... well, not really, but the only thing clear from our building departments is the amount of the permit fee(s, s, s).

Mike McCann, PE, SE (WA)

 

[ChipB]

It sounds like to me, they are asking the framing system to place no thrust on the beams under DL & LL, which is possible.
But, yes, there isn't any way horizontal forces can't occur during a lateral loading event.

Wonder what kind of weasel words we can place on our drawings.... LOL!

 

[Lion06]

I think detailing the truss to impart no lateral reaction from DL or LL does more harm than good. The trusses are what connect the HSS to the diaphragm which is ultimately taking the wind load from the facade below the HSS, so releasing that connection on one side now causes the HSS on that side to span 30' in weak axis bending from wind load. The lateral reaction from the truss should be smaller than the wind load reaction. A 30' span on the weak axis of a HSS 10x6 will be so soft that 1/2" of lateral displacement will likely result in a relatively small force.

 

[haynewp]

The connections for lateral would have to be made after dead and construction live loads are put on the trusses to avoid gravity thrust. But at some point in the future, I expect live loads on the roof and possibly new superimposed dead loads to be applied through the final truss connections causing thrust.

 

[MikeHalloran]

I could read that requirement to say that every connection to the HSS except at the four corners of the grid must be made via a long-ish vertical column with ball joints at both ends, or some sort of plate/balls/plate bearing, or even no connection at all.

That last would be possible if the roof had four gables, not two, or four bowstring trusses with a fabric membrane stretched over them and the ends of radial compression members stretching from corner to corner, with cables sewn into hems forming catenary edges.

... which is sort of what I thought of When I read "... lightweight fabric structural system ...", not a trussed Cape roof with a full veranda, framed in a normal way but covered with fabric skin.


A structure vaguely similar to what I've described, with fewer compression members and no arches and no planar surfaces, got a lot of publicity when installed over the inter-terminal sidewalks at Fort Lauderdale International Airport. It got less publicity when Hurricane Wilma blew away the skin. The posts, crossbeams, and cables comprising the skeleton are still there. I guess there was no money or no plan for repairs.



Mike Halloran
Pembroke Pines, FL, USA

 

[paddingtongreen]

I took this literally, no horizontal effects from vertical load cases. I presume that the structural designer didn't know in advance, what the superstructure would look like, but was mindful that pitched roofs and portal frames both could cause equal and opposite forces on the supporting structure, ie a portal frame could kick the eastmost beam in an easterly direction and westmost beam in a westerly direction. This is easily avoided.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin