Can you modify structrual cross bracing?
Can you modify structrual cross bracing?
(OP)
Can you modify the structural cross-bracing in a building in order to add an exit door?
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Can you modify structrual cross bracing?
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RE: Can you modify structrual cross bracing?
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.
RE: Can you modify structrual cross bracing?
You can modify it if you ensure the structure is not compromised.
RE: Can you modify structrual cross bracing?
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.
RE: Can you modify structrual cross bracing?
RE: Can you modify structrual cross bracing?
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'm sure folks do this, but I'd argue against it.
RE: Can you modify structrual cross bracing?
RE: Can you modify structrual cross bracing?
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.
RE: Can you modify structrual cross bracing?
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.
RE: Can you modify structrual cross bracing?
DaveAtkins
RE: Can you modify structrual cross bracing?
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.
RE: Can you modify structrual cross bracing?
RE: Can you modify structrual cross bracing?
RE: Can you modify structrual cross bracing?
hokie66, what are you basing your statement on? Don't you trust the ASCE 7 Code?
DaveAtkins
RE: Can you modify structrual cross bracing?
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.
csd
RE: Can you modify structrual cross bracing?
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.
RE: Can you modify structrual cross bracing?
Are you referring to the wind pressure determined using the simplified procedure per section 6.4?
RE: Can you modify structrual cross bracing?
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.
DaveAtkins
RE: Can you modify structrual cross bracing?
RE: Can you modify structrual cross bracing?
DaveAtkins