2 Story Steel Frame Reno

[mijowe]

I have a 2 story steel frame building that we are renovating. No existing building drawings are available. The floors and roof are designed for relatively heavy gravity loads, but like a lot of steel buildings of its era there does not appear to be any defined lateral system. There are some solid exterior walls of 4" brick + 4" CMU, unreinforced, with no attachment to the steel frame, they are being removed. While they may add stiffness to the building they were not intended to be a lateral system and there is not true load path.

We are removing some structural bays and all of the perimeter masonry so we are reducing the seismic load on the building. We are not in a high seismic zone and wind would govern the design.

There is a small addition to the wind load with some short parapets at the entry.

My problem is with the intent of either IEBC or IBC chapter 34. While I am not increasing the load by their threshold limits ie 10%. The language is "lateral loading to existing structural elements is not increased beyond its capacity or more than 10%" If there are not "existing structural elements" receiving load in the first place is there anything to compare?

 

[wannabeSE]

1. If the masonry attaches to the diaphragm, it could be the lateral force resisting system.
2. If it isn't, have you ruled out moment frames
3. Most people would consider Reno, NV to be high seismic
4. Just because the building code doesn't prohibit something doesn't mean it is safe

 

[JAE]

I think a lot of older engineers from that era assumed that the presence of masonry between columns was enough to "brace" the building. This, of course, was only when the masonry was between columns such that a lean in the overall framing would bear on the rectangular shaped masonry panels. Not a true, connected, load path but a load path of some sorts.

Removing masonry infill for conditions like that, and then assuming that the original frame didn't take any load to begin with, and then assuming that the code language doesn't get technically violated, would not be a good engineering practice. I would look for new means of introducing a lateral load path into the system.



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[KootK]

Flexible connection moment frames maybe? Link. What era are we talking about anyhow?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[mijowe]

"reno" was me abbreviating renovation, the project is on the east cost.

The 4" masonry is built tight to the columns and beams but no positive connection, no dowels at the bottom, no brace at the top. The exterior brick is connected to the 4" cmu backup with ladder ties, there is a gap between wythes.

Steel connections are standard gravity connections, no attempt to create a moment connection of any type. The floor girders are connected to the columns with double angles, the roof is predominantly a cantilever beam system, so beams run over the columns.

This is an early 70's suburban office building. Not uncommon in this area to not address wind and ignore earthquake at that time.

 

[clarke1973]

It sounds like you are limited for options if all of your frame connections are pinned. Are you able to introduce bracing to make it a fully braced frame, provided the floor construction can act as a diaphragm? Alternatively you could beef up some of the existing connections to make them moment resisting.
How are you going to justify the foundations if there are no as-builts available?