Not without analyzing the building for the new brace configuration per the currently applicable bldg code.
This could be very difficult or impossible because the lateral force resisting system might've been designed under an older bldg code. Newer bldg codes sometimes require much larger forces and/or more stringent detailing requirements.
Might wanna look at other options if at all possible.
I disagree in principle with 271828 on this one. Unless your code specifically prohibits changes to an existing building without bringing the whole structure up to current standards, the general rule is that modifications should not adversely affect the existing structure. Therefore, if you replace the capacity lost by a bay of cross bracing by e.g. adding equivalent bracing in a different bay which is designed to resist the same load and take it to the ground, I think you would be acting responsibly.
I should add a caveat that modifications of this sort should only be done if you assess the building as competent with reliable load paths, whether strong enough to meet modern codes or not.
You need to check both the structural framing and foundation, especially in the areas where you are removing the brace and adding others, to ensure the bracing mods do not adversely affect the stability of the building.
hokie66, I guess we'll just have to disagree on this one, LOL!
By moving the cross-brace elsewhere, different members now act as struts. Foundations that formerly resisted only gravity loads now have uplift and sliding.
I don't disagree that the struts, columns, and footings have to be checked. I just have found that it is often possible. Many times the other structural elements are not controlled by the bracing.
Yeah, I agree with you. My original "re-analyze the whole bldg" was too broad.
When I read a question like that, I envision somebody either removing the brace and not doing anything or just moving the brace over a couple of bays and not analyzing anything.
If you live in a location like I do (Wisconsin), where the old building code required that everything had to be designed for 20 psf wind load, most of the time bracing can be completely removed, or modified, because the IBC only requires 10 psf wind load. Just make sure you justify by calculation what you are doing.
I believe that it would be wrong and unprofessional to so what you are suggesting. Don't care what your code says, 10 psf is not enough wind anywhere on the face of this earth. And the code will probably change again in the other direction at some stage.
ASCE 7-98, referenced by the Wisconsin version of the IBC, is used in Wisconsin. If the building is low rise, the Exposure is B, and the Importance Factor is 1.0, 10 psf wind load will govern for the design of MWFRS.
hokie66, what are you basing your statement on? Don't you trust the ASCE 7 Code?
Have you thought of chevron bracing in the same bay. you should be able to fit a door under.
It will be stiffer than the the cross bracing though, so it may attract more than its share of load.
The other option is to portalise the frame, i.e. add a perpendicular portal frame at the location of the cross bracing to take the bracing load.
Be careful to check ALL affected members from footing to existing adjecent members. Changes like this that have not been fully checked are one of the major causes of structural failures.
There has been discussion about wind loads as it relates to this, but it would seem that seismic loads also have to be considered.
2006 IBC, 3403.2: "Additions or alterations to an existing structure shall not increase the force in any structural element by more than 5 percent, unless the increased forces on the element are still in compliance with the code for new structures.
This has been problematic for me here in central PA in trying to explain to owners/architects that are renovating old unreinforced masonry buildings that if they affect the masonry structure too much that it will have to be brought up to meet the new seismic requirements. And unreinforced masonry performs miserably for seismic design. Under the old BOCA code, most of PA was Zone 0 or 1, and for the most part, seismic loading had not been considered. Architects and owners here (and unfortunately many engineers) generally scoff at the idea of an earthquake in PA (we have had some <4.0).
So, back to the original question; if the building was originally designed under the old 0-4 seismic "zones" and falls in Zone 0 (or Zone 1 with I<1.5), it may not have been designed for any seismic loading. If the building has any substantial mass to it, the seismic loads may now control.
And while that 5 percent number seems a little artificial to me, given the in-exactness of the seismic load modeling, it would seem even minor revisions to bracing of this nature could thoeretically change the forces elsewhere in the structure to that degree, let alone the potential that the entire system may not make it for the new seismic requirements.
If you use Method 2, wind load not near a corner will be 8.5 psf, and wind load at corners will be 12.8 psf. Most of the time the 10 psf minimum wind load governs over the 8.5 and 12.8 psf loads.
Wouldn't the 8.5 and 12.8 be for C&C, not MWFRS? I thought MWFRS only had one value and you shouldn't use the C&C loads for MWFRS. I am wrong in saying that you shouldn't be comparing the two (for use in the MWFRS)?
