4 story wood building with steel beams and columns at the first story

[Engr08]

I am designing a large 4-story wood residential building with wood stud walls and wood truss floors. The half of the building at the first story is parking garage, framed with steel columns and beams which support the wood structure above. My question: is the steel frame needed to be braced by itself, such as using moment frame? Since the floor truss will sit on the top of the steel columns and beams, I am concerned that if the steel frame can be braced by the plywood floor diaphragm above. Any comments appreciated, thanks in advance!

 

[Ron]

If you are asking about span bracing, the wood trusses and flooring will suffice. If you are asking about overall lateral load resistance, you'll have to either use a moment frame or provide some type of shear wall/ cross bracing.

 

[msquared48]

I agree with Ron here, but I am surprised that the first floor is not PT. Depending on the footprint, and the larger the better, PT should be more economical.

Mike McCann
MMC Engineering

 

[Engr08]

Thanks for your reply, Ron. My question is about the stability of the whole steel frame. The overall lateral load resisting system of the building will be the wood stud shear walls in other parts of the building. My concern is that if the lateral loads acting on the steel frame can be transferred to the wood shear walls by the floor diaphragm since the diaphragm and the steel frame are separated by the floor trusses. If I use moment frame or cross bracing for the steel frame, the building will have two different lateral load resisting systems and I have to use the smaller R value, I don't know if I really need to do that.

 

[Engr08]

Hi, msquared48, What is PT? You mean pressure treated wood? thanks

 

[hokie66]

PT is post-tensioned concrete.

 

[rowingengineer]

It would have to be a huge residence for PT to be economical, I would suggest a normal reo slab would be more functional for the builder, he probably get the same guys to do the 1’st floor that he will get to do the SOG.

When in doubt, just take the next small step.

 

[Engr08]

The floor is 3/4" plywood sitting on the wood trusses or I joists. Concrete is not an option.

 

[MiketheEngineer]

Fire resistance??

 

[msquared48]

If this is a residence, unless it is Bill Gates' house, PT will not be more economical.

However, if it is a decent-sized multi-family "residence", you should consider replacing the first floor framing with PT.

Whatever works though...

Mike McCann
MMC Engineering

 

[neffers]

Have seen a lot of this type of construction using precast hollowcore concrete plank at first floor above grade...then wood construction up from there.

neffers

 

[Engr08]

Thanks for all the comments above. It is too late to change materials or structural systems now. My question is that Should the steel frame in the part of the first floor be designed as a moment frame or braced frame, or can it be designed only as a gravity system? My opinion is that it can be designed only as a gravity system since lateral loads will be resisted by the wood stud shear walls in other parts of the building. Is there anybody has done a similar project to this one? I really want to get my thought confirmed.

 

[mfstructural]

I'm not familiar with this, but why would you use hollowcore planks at the 1st floor above grade and wood above? Why not use HC planks all the way up? Reducing the foundation/wall loads? But I guess if you did that would need either reinforced masonry or precast walls for the loads and then that becomes uneconomical for residences plus it's not as aesthetically pleasing for the architect and potential buyers.

 

[apsix]

Engr08
If you have a viable load path from the steel framing to the timber shear walls I can't see that 'extra' bracing is required just for the steel frame.
Your local design codes may dictate otherwise, but I'm not familiar with those.

 

[sandman21]

As long as the column and beam connection can not take moment, i.e. they are simply supported. I don’t see why you would be required to use the steel columns as your lateral system. Where is the project located?

 

[msquared48]

mfstructural:

Depending on where you are, some jurisdictions limit the number of wood stories to anywhere from three to five stories.

If you have an eight story building then, it is common to have the parking levels in concrete (PT), and as many of the living floors as necessary to meet the maximum number of wood stories.

In some jurisdictions, the top of the PT or concrete level, commonly called the Podium Slab Level, is used as the seismic base, with none of the lateral forces in the PT having to be transmitted to the wood stories. This helps immensely where allowed, and it does work where the lateral stiffness of the concrete floor system is much greater than the wood floors. Generally, this difference in stiffness must be proven via calculations to be accepted and allowed. It is tedious, but doable.

Mike McCann
MMC Engineering

 

[vandede427]

yes, use the wood structure to laterally brace the beams top flanges for flexure.

however, i'm surprised that the first elevated level is not concrete. see if you can explore this option with your architect.

 

[engr567]

In east-coast, we see many buildings of this nature, and we design it all the time. If you can justify the load path (I am talking about the lateral forces), and the wood shear walls at the first floor is sufficient to transfer the cumulative shear force, you can design the steel frames as gravity frames only. I do not see the reason why you can not.

 

[Engr08]

Thanks you guys for the comments. Seismicpe: This project is also in east-coast and I am glad to know that you designed this type of structure before and have your comments. I finally decided to design most of frames as a gravity system except for an outmost one since the floor diaphragm cantilevers more than 25'.