External steel staircase

[nisha1980]

have a small query , if you have an idea please let me know . it is appreciated.

i am designing a external steel fire escape staircase as below. columns are pinned . all member connections are pinned . this need some bracing to not to too wobbly when is fully loaded (live load is 4kpa) and also to prevent deflection from wind . All threads and landings are checker plates .

My question is i m trying to find a way to make this without bracing

1. One i found it if we can have all columns fixed to the footings with bored piers , i found bracing not needed. but there is an existing one and after demolishing it we are going to use the existing footings if sufficient , so this option is not possible

2. If we have moment connections at top like in portal frames i think we do not need bracing( eg haunch). but i m not sure whether we can do it for small structures like stairs. can we do it for these small members , columns are 100SHS and all other members are 150PFC.

3. Any other way to make the stair case rigid and not wobbly?

any idea is appreciated .

 

[jayrod12]

There's one more option, not sure if it's applicable. At the landing levels that line up with existing building floor, provide appropriate plan bracing and tie it into the building's diaphragm. Your columns would need to be designed for the lateral load from mid-ht landings to be able to dump it into the floor diaphragm, but that shouldn't be unmanageable.

As an aside, bracing is the cheapest and most effective way to make a structure stiff. I would argue the method above would be the second stiffest load path (if applicable), the other two options (moment frame or cantilever column) will be extremely flimsy solutions without significantly increasing the size of the members.

 

[milkshakelake]

Besides what jayrod12 said, you can reinforce the existing foundation and then make a moment connection at the bottom by using a welded base plate and post-installed anchors, or encase the bottom of columns with concrete piers that tie into existing footings.

I noticed that your bracing is eccentric. You can add knee bracing with square tubes to reduce the bracing (need to check columns in axial+bending). This can go over openings and help with torsion. Remaining members can be moment connections to help a little bit. Might need to increase member sizes.

 

[JLNJ]

1. Determine all the loads (including wind and seismic).
2. Choose your lateral force resisting system.
3. Understand the load path.
3. Design your components.

 

[Ron247]

How tall is this stair? At some height, the fixed base option is still "wobbly" unless you use very heavy columns because of the poor aspect ratio. Are you in a high seismic area? I agree with other posts that X-bracing is the best option assuming it is not in your way. Attachment to the existing already mentioned is a logical 2nd choice.

I assume you are not in a high seismic area. Fire escapes can also be earthquake escapes. Wobbly would be a problem if you are in a high seismic. My choices would be different depending on wind vs seismic.

The drawing appears you have a 3-story building. Your lower stair stringers are the stiffest item present in the lower level, but they are eccentric and are stiff in one-direction only. It looks like you could X-brace the lower level where the 2 stringers meet the floor beam above. You could also x-brace the underside of the stringers themselves. When I say X-brace, I am referring to tension and compression type x-bracing not tension only bracing.

 

[nisha1980]

Hi

thanks for everyone's input and appreciated ,

Ron247- the stair is about 4.2m high in total . yes i m in very low seismic area,

1. what are the possible sections for X- brace (for both compression and tension). i assume circular tubes would be good.

2. When it come to X bracing, What if i design these bracing as tension only X-bracing is the case where cables ( or small steel rods) are used as the members. The positively can only take tension.so i assume the compression member will buckle and only the tension brace is acting to resist the lateral loads.

3. So what will be the difference of using X bracing with both Tension and compression ?

Appreciate your reply

Thanks

 

[milkshakelake]

1. Circular or square/rectangle tubes are both okay. If you're welding everything, square/rectangle tubes are easier. If you're bolting with gusset plates, both options are annoying to detail; probably better to use double angles. Since this is a fire escape, I'm assuming you're not attaching studs and drywall to it.

2. It should be fine to use cables or steel rods as tension-only if the deflections work out. Make sure to run a non-linear analysis, and try to reduce your torsion by spreading the bracing evenly in the x and y direction.

3. More stiffness. If you want to reduce the difficulty because of the connection at the middle of the X, you can try using chevron braces (V or inverted V) if your codes allow it. It reduces the number of connections from 5 to 3. Just remember to also run the analysis for gravity without the V, in case the V fails due to wind/seismic. Or you can use a single diagonal with a larger size to reduce the connections to 2; check for buckling.

 

[Mihdi Caballero]

Since it is an open structure, the wind loads wouldn't have much area to press on, and the dead load applied is small, so being in a low seismic area with low dead load is not going to be a problem either.

Thus, you resultant lateral force shouldn't be so big and you could try to use fixed connections and eliminate the bracing. On way to achieve the fixed connections is welding the elements in the joints and checking the moment capacity, without the need of haunches.

Nonetheless, as it's been said before, using tension only bracing is the cheapest solution and the one with better results in deformation of the structure.