Modifying interior X bracing to accomodate doorways 1

[ErichGunther]

I have partial ownership in a three story office buildiing that is 12,000 sq. ft per floor. On each floor there are three sets of box beam type X bracing to add structural rigidty and manage wind loading. The owners/tenants of the second and first floor are doing a rennovation and want to alter the X bracing in the middle of the building to allow for doorways on the first and second floors. The proposal that came back is illustrated in the attached drawing. Basically the proposal is to remove one of the lower legs of the X on the first and second floors which makes room for the new doorways. I am an electrical engineer with only enough structural engineering background in school to be dangerous but it sure does seem to me that this change eliminates much of the capacity of the X bracing as originally designed. I am wondering if anyone on this forum can tell me what questions I should ask the engineer on this job (once I determine that there is an engineer on this job) about the impact of this change. Has anyone seen this done before in other buildings? Is the proposed change totally nuts on its face, or not uncommon as long as revised loading calculations are done?

Erich - erich<at>enernex.com

 

[msquared48]

The proposed change not totally nuts on its face, just mostly.

This will TOTALLY change the lateral capacity of the braced frame, load distibution and deflections of the structure necessitating a FULL redesign of the whole lateral system for the structure. It may also impace the foundation, which is already in place, difficult and expensife to change at this point.

DO NOT do this with a structural engineer! Preferably the one of record.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[TXStructural]

The seal on the drawing is from an experienced structural engineer. Either you trust it or not, and the engineer will have/should have done his due diligence before sealing the sheet. It is perfectly acceptable to ask the engineer for more information, like what calculations and software were used to determine the adequacy of changes, but in the end, you're relying on his seal and engineering judgment.

You will want to be sure the lateral load path and foundation were evaluated since the change effectively moves some forces from one column base and foundation element to another. He should have checked that the stiffness is still adequate, since braced frames are very stiff elements. If your area has to consider seismic, then be sure the latest requirements are being met, since modifying the lateral system requires updating the system to current requirements.

 

[ErichGunther]

I totally agree on the trust aspects. I am a P.E. and I definately take my responsibility very seriously before I stamp a document. This one is just bothering me. The responses so far are helpful in how I respond to the submitted plan. The problem so far is that all they sent me is the drawing - no commentary or analysis to back it up.

I neglected to memtion that we are in Knoxville, TN so seismic issues are not in any building codes although we are 400 miles away from the New Madrid fault near Memphis where the biggest earthquakes in history have ocurred!

Erich

 

[TXStructural]

You actually do have a seismic component requirements (it is about 5 times that of Dallas), although it might not control over wind. The quake hazard maps were updated recently (2008) so the requirements for your building could have changed.

The usual deliverable from a structural engineer is the plan sheets. They seldom deliver calculations unless the contract specifies otherwise. It would be appropriate to ask for design parameters, since these usually are provided on a plan sheet in the form of general notes.

 

[JLNJ]

I would feel better about cutting the non-continuous member and leaving the continuous member in place. These details depict cutting the continuous member, making the load capacity in the right-to-left direction nearly nil. Yikes.

On the other hand, who knows? Maybe there is a ton of redundancy in the system.

 

[msquared48]

I would not "feel better" about cutting anything. That's dangerous.

I would have a structural engineer run the numbers to see what components are affected.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[beton1]

It is stamped, and in fact the company name/logo are there. The drawing is small, but from what I can see, it appears there is an 'enlarged' detail where they address some concerns, ie. the x-brace intersection, with added plates for continuity. Whether the whole system has been checked, well that is beyond our scope here.It appears the the lines of bracing are in the same plane. Bottom level of line B x-bracing is beefed up. I would have to say if you are concerned, then to request further info or get a second opinion.

 

[msquared48]

"Whether the whole system has been checked, well that is beyond our scope here."

I don't agree here beton1. Eliminating one leg of an "X" brace will change the lateral load path affecting other members, frames, and connections. The load will have to shift elsewhere. It will require a complete re-analysis. No question here.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

Note 1.1 on the sealed drawing states that the revisions have been designed in accordance with the 2006 IBC. I am in no position to suggest otherwise, but getting a second opinion might be prudent.

BA

 

[a2mfk]

I feel I am with Mike here, but since none of us are privy to all of the details, we have to make assumptions, which can be dangerous. I may ask your SE some more questions or ask if he'd submit his drawings and calcs for a peer review.

The X brace at B, it may be more worth consideration economically to remove the existing braces and replace with more robust members. That is a lot of fabricating and field welding. Good question for a steel fab and erector. Only the first story has been strengthened, but the original design may have had enough additional capacity at the second floor bay level.

The X brace at E as modified now a single diagonal comp/tension brace. Since the other brace in tension will not be there to provide lateral buckling stability, the center connection has been modified to transfer axial force and provide stability. Essentially a column splice in the center which is the worse place to put it. BUT, that does not mean it was not checked and found to work, though it does not give me "warm and fuzzies"...

Peer Reviews:
I had a similar situation where I was hired to peer review a design as well as look at an in-field construction problem. A steel framed floor had deflection issues prior to the concrete being placed (self weight only). I had a face to face with the other SE after doing some preliminary calcs. It was a funky balcony space frame for a theater that he modeled in 3-D FEA (he showed me his model and walked me through his design). If you were to just look at it, it would appear like a more basic structure, and my prelim calcs showed several members not working.

Anyway, my original thoughts and assumptions were proven incorrect because I did not fully understand the design intent of the final built structure. The deflection was due to a lack of shoring until all of the members were up and connections were all made.

Peer review is my final opinion. It can be informal if you want, have him meet with another SE face to face to review the design. OR, you can have him submit calcs and drawings for a full PR. You may get the same result either way. You are an owner so cost is a consideration in all of this.

HTH,
Andrew Kester, PE
Structural Engineer
Florida

 

[msquared48]

Further, in looking more at the center detail of the "X", it looks like these are tension only braces. If that is true, then one direction is being structurally compromised at the lower two stories. The lateral force will have to be shifted somewhere.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[beton1]

Mike, point taken. Intuitively, I think we are all on the same page. But as BA has stated, and a2mfk elaborated, there are two facts: It is a stamped drawing, designed by another engineer and we do not know all the facts. So either you trust that the engineer of record has done his duty or you cautiously and professionally raise your concerns.

 

[TXStructural]

Look closely at the drawing. The detail shows that the other frame has been reinforced, and the remaining diagonal on the affected frame has been reinforced. Whether it does that effectively or not can't be determined from what we see here, but suggests that the engineer did evaluate that aspect.

X-bracing with gussets at the central intersection will be tension only, unless the continuous member can brace itself and the gusseted member.

 

[csd72]

Two potential issues with this one.

1. instead of going through two diagonal braces the load is now going through one and potentially doubling the load in the one remaining brace.

2. That diagonal is required to take compression as well as tension and it has a massive weakness at mid span. This is not so much of an issue when the other diagonal is in tension but definately is in this case.

Scary!

 

[ToadJones]

I agree with some here that if the bracing was designed as tension-only, then there are some problems, especially with the associated shears at the base of the columns. Ultimately the lateral loads go into the foundation and some of the the potential paths have been eliminated.
The bracing seems a little bit heavy to be tension only, but the gussets at the mid point do suggest to me that it may be tension only.
Having said that.....
An analysis of this type of modification can rather easily be done with software and as long as everything checks out (including the foundations and anchorage), I don't really see a problem with the changes.
This type of configuration happens quite frequently in industrial buildings in order to allow for penetrations.
The engineer should probably also take a look at/ model any diaphragm action from the floors (can't see if there is horizontal bracing in the floor). Counting on the floors can make the load paths more complicated, but sometimes that's all you've got.

 

[ToadJones]

Another question this brings up....
this type of change may change the torsional stiffness of the building. Even wind from one direction can cause torsion on the building with an asymmetric (for lack of a better term) bracing scheme.
This should be investigated in terms of deflections and possible effects on the interior finishes/ windows etc.

 

[MiketheEngineer]

Agree with all - get an experienced Structural...

A "K" brace may work here - but needs to be engineered

 

[CTW]

I agree with the others about the brace now taking compression. The x-brace detail, “Enlarged Detail at X-Brace Intersection”, appears to address this by adding the continuity plates and trying provide more stiffness at the remaining joint to keep the brace from buckling. This is being done at mid-length of the brace so the design has to be robust to keep the brace from buckling.

I think the key to the modification lies at the “Enlarged Detail at X-Brace Intersection”. This detail shows a continuity plate welded on each side of the tube. The detail calls for a ¼” fillet weld, 6” long on each side. Although the weld is called out to be typical, it’s not clear whether this weld needs to be provided where the plate crosses the tube that is to be cut. To me, the typical part of the weld implies that the weld is the same on the other end of the plate, not across the plate to the other tube. In order to provide the necessary stiffness, I think the plate has to be welded to the remaining tube. I’ve marked up the detail in the attachment to help visualize my explanation.

I’m not sure why the continuity plates are 24” long. The engineer is calling out a 6” fillet weld on a plate section cover the tube that is longer than 6". I would prefer to see this completely welded along each edge.

Also, the existing gusset plate shown in the detail is larger than the plate shown in the elevation view and the detail in general looks different than the configuration in the elevation view. I’ve tried to note this in the attached drawing. I’m not sure the detail provides enough stiffness if the ends of the tube are cut as shown in the elevation view.

 

[IceNine]

Looks to me like he already has an experienced structural. Experienced enough get licensed. One of the other bays has been reinforced. Looks like an analysis was done, but it wouldn't hurt to ask.