Steel structure with wood shearwalls 1

[Scoper]

I am looking for input on the use of wood panel shearwalls as a lateral force resisting system in conjunction with structural steel gravity framing. Can anybody tell me whether they have used this combination, or whether there are any restrictions on it in the IBC? Thank you!

 

[SteveMort]

Wood shear walls should be fine as long as they can handle the loads. The thing to be careful about is the connections. Be sure that the lateral loads can transfer from the diaphragms and other shear walls and carry the loads to the foundation. Simpson Strong-Tie manufactures wood shear walls and even stronger walls made with corrugated metal wall panels; "Strongwall" that may be helpful in your case. (

http://www.strongtie.com)

 

[abusementpark]

Why don't you want to just put in some bracing to take care of the lateral load?

 

[haynewp]

I have used this system before and I don't know of any specific IBC limitation on using steel gravity carrying members combined with wood shearwall lateral. As long as you are keeping the drift limitations by the code, I don't how the steel gravity carrying members would know they are being supported laterally by wood shearwalls versus steel bracing or some other system.

 

[msquared48]

Except in certain conditions, wood shear walls cannot support concrete laterally. As long as you do not have concrete infill on metal deck in your framing system, as mentioned above, I see no reason why the wood shear walls cannot be used.

Mike McCann
McCann Engineering

 

[haynewp]

Mike,

I have heard that several times before that you can't use wood to support concrete for gravity loads, but where is that located in IBC regarding not being able to support concrete laterally with wood shearwalls? (I am not saying it is not there, but asking where it is.)

I don't design very frequently with wood, but isn't using a 2" concrete floor topping applied over a wood sheathed floor and shearwall system pretty common?

 

[haynewp]

I found this in IBC 2006:

"2305.1.6 Wood members resisting seismic forces from
nonstructural concrete or masonry.

Wood members shall be permitted to resist horizontal seismic forces from nonstructural concrete, masonry veneer or concrete floors."

And also this:

http://www.structuremag.org/Archives/2006-4/D-CUPD-IBC-Wood-Framing.pdf

 

[msquared48]

Correct, and this has been done for years in apartments here. The key word here is NON-structural concrete. Wood can not laterally support structural concrete or masonry.

The one exception I can think of to this rule though is masonry and tiltup walls where the building has a plywood diaphragm. By code the wall has to be laterally supported and tied to the diaphragm at the top. Hence the lateral forces of the wall are thrown into the plywood diaphragm. This has been allowed for years too, and is a seeming violation, but it works. There is probably a code exception for this case, but I cannot put my finger on it.

Mike McCann
McCann Engineering

 

[hokie66]

Mike,

Suggest you read the link haynewp gave. Seems the provision has been changed at the behest of the wood people.

 

[Scoper]

I very much appreciate all the good input on this topic. In response to abusementpark, the subject combination was not my idea, and the lack of information on it made me wonder if it was compliant. I suppose a follow up question would be whether the R of 6.5 (light frame with shear walls, Table 1617.6.2) is valid for this case. Building would have a second story with steel joists, deck, and topping.

 

[haynewp]

What IBC are you looking at?

 

[Scoper]

2003. Using the 6.5, the forces are manageable. However, the 6.5 is for 'light frame' construction, which makes me wonder if it is inappropriate for this type of construction. Site is west coast with moderate seismicity.

 

[haynewp]

Be careful, see 2305.1.5 of IBC 2003. There are special conditions to be met to laterally support concrete with wood seismic shear walls it doesn't look as lenient as IBC 2006.

It appears that in IBC 2006 that 2305.1.6 (I noted above) has been added, which adds "or concrete floors" even though it is under the "nonstructural concrete or masonry" section.

Also, section 2305.1.5 of IBC 2006 has been re-titled as "Wood members resisting horizontal seismic forces contributed by masonry and concrete WALLS" instead of the IBC 2003 title "Wood members resisting horizontal seismic forces contributed by masonry and concrete".

So it looks like in IBC 2006 the restrictions are placed on taking seismic with wood shearwalls from concrete or masonry walls and not concrete floors. Anybody else interpret this differently?

 

[BPA827]

I agree with abusementpark, steel bracing would be the best choice for the lateral load resisting system.

 

[msquared48]

haynewp:

Thanks... Had not noticed this although I did have the section yellowed in my copy of the code. I guess I need new glasses...

This pertains to gravity loads, but it is still a change. Deflection would be a major consideration driving larger members or more supports, shorter spans, otherwise the slab will crack and become unserviceable with time.



Mike McCann
McCann Engineering

 

[haynewp]

Mike,

It seems at the end of that article that it may be implying wood shear walls being used with Hambro composite steel joist and concrete system, but it is vaguely implied if at all. I may be reading that into it.

But back to the IBC 2006, I don't see where it disallows the use of wood shear walls in combination with concrete floors. It looked more obvious in the IBC 2003 where it gave specific ways around it, but 2006 IBC has revised these sections. I had previously interpreted that you could always laterally support structural concrete floors with wood shear walls under IBC 2006, but now I am not 100% confident just based on the previous restrictions I saw in IBC 2003.

 

[msquared48]

haynewp:

I think here you have to look as to what is not said, but really implied Section 2305.1.6 was an addition to the 2006 code after 2003 and addresses only non-structural concrete, non-structural masonry veneer, and non-structural concrete floors. The insertion of the words "non-structural" here are implied by the title of the section.

What is not addressed in this section is structural concrete, structural masonry, structural brick, or structural concrete slabs. The implication that I percieve is that the resistance by wood shear walls of the lateral forces induced by these masses is not allowed.

Also, I perceive that the same is implied in Section 2305.1.5 where it speaks only of walls. If the intent was to say OK for structural slabs, I feel that it would have been only logical that structural slabs be addressed and included here too. Since it was not, I can only assume that structural slabs are not OK for wood shear walls.

Probably could use some code clarification here - seen any commentaries?

And I could see how you could read the use of wood shear walls into the Hambro system. Again, clarification would be helpful here.

Mike McCann
McCann Engineering

 

[haynewp]

On this review of IBC 2006, it states:

"Wood shear walls can resist seismic forces generated by
horizontal concrete but not from concrete or masonry walls per IBC 2305.1.5."

http://www.seaonc.org/pdfs/SEAOC Seismology Wood Overview.pdf

And it doesn't say that the horizontal concrete has to be non-structural. Or, I think they would have called it "concrete floors" if they meant it to be non-structural in nature (engineers usually call horizontal structural concrete as "slabs" and not "floors").

I guess as a side note, if I wanted to use the Hambro composite system supported on wood walls, how would I make the connection so that no seismic force is induced in the sheathed walls? And also, when I looked at IBC 2003 the other day, it appeared you COULD use wood shear walls to laterally support STRUCTURAL concrete (slabs and walls) as long as you met certain conditions. So now in the IBC 2006 they changed it to where you can't laterally support horizontal structural concrete at all??

Right now, I am thinking the following:

1. Section 2305.1.6 pertains to NON-STRUCTURAL concrete exclusively by title, so I will say this has nothing to do with the discussion and I shouldn't have mentioned it previously. It seems poorly worded in that it states "non-structural concrete" and then mentions "concrete floors" separately as if concrete floor topping is different from other types of non-structural concrete.

2. If I remember correcty, Section 2305.1.5 previously pertained to laterally supporting all STRUCTURAL concrete in IBC 2003 with certain stipulations. But now, in IBC 2006 it has been completely re-written to only put stipulations on laterally supporting concrete walls. So should I infer that by them not specifically mentioning horizontal structural concrete in section 2305.1.5 that it is forbidden to be laterally supported by wood altogether now??

So it looks like we are understanding each other completely as this reiterates what you said, but I am seeing it as by them not mentioning structural slabs in 2305.1.5 that they are NOT excluded, where you are seeing that it means they are excluded. BTW, I appreciate the conversation on this.

 

[msquared48]

Haynwep:

Thanks for your insight too...

Let's agree to agree that a clarification of the intent needs to be made here by the powers to be...

Mike McCann
McCann Engineering

 

[haynewp]

I emailed the APA and their reply (in summary) was that it is not the intention to exclude structural slabs from being laterally supported by wood shear walls due to loads from seismic. So essentially 'wood shear walls are allowed to support seismic loads from structural slabs' was their response to IBC 2006 section 2305.1.5, for what it is worth...

I would like to take this opportunity to acknowledge the help I have gotten over the years from the APA help desk. Even though I do not design a lot of wood buildings, they have always seemed to be very fast and thorough in answering any question I have asked.