Steel frame with (sort of) masonry infill

[shepherd]

I'm investigating adding some windows to a four story office structure that was built in the 1960's (Maryland). It's very difficult to tell what the original designer intended for lateral resistance (beginning to suspect if it was even considered). The majority of the exterior walls have 5 foot windows with 1 foot brick piers between. The walls where the windows are proposed, have what appears to be full height self supporting brick veener (tied back to the structure) and 4" Concrete Masonry that is "built around" the beams full height. The Masonry does not appear to be mechanically attached to the beams or columns, but does appear to built "tight" around the beams. Does anyone have any suggestions for quantifying the lateral resistance of this type of arrangement, or should any lateral resistance be assumed? Thanks in advance.

 

[rlflower]

I am an engineer employed in California, so my point of view would naturally be quite conservative. Maryland is Zone 1 whereas California is Zone 4, according to the Uniform Building Code.

If you can get your hands on a 1960's issue of the buiding code that has been locally approved, a quick review of the lateral design requirements would tell you how this structure should have been designed. On the other hand, maybe you can obtain a set of the original construction drawings.

Construction in Maryland in that time period perhaps would not make it obvious whether or not the structure was properly designed. What I mean by that is: structural hardware might not be visibly protruding from the piers. A little destructive investigation of one of the existing piers may be necessary to identify the structural elements. Only then could you verify what you have: it may be masonry piers, steel columns within a masonry veneer, or concrete columns with a masonry veneer.

If these are masonry piers, then you may want to submit samples of the brick, mortar and reinforcement to a lab for analysis.

Keep in mind that the building code has advanced significally since the 1960s, primarily in the subject of connections. With that in mind, consider the existing connection hardware with anticipation of modifying them. The construction may look "tight" simply because the capacity of the connections has not been "put to the test" in a significant seismic event.

 

[shepherd]

rlflower,

Thanks for your response. The brick piers appear to be strictly veneer. From the existing drawings it does appear they are self supporting full height and tied back to the beams at each floor level. The entire building is steel framed, but the beam/column connections were simply a weld on either side of the beam web to the column flanges. I'm assuming this as a shear connection only (although it does provide some rotational restraint since the beam flanges appear to be tight fit against the columns). This leads me to believe that the 4" concrete masonry infill between the beams was intended for lateral restraint (if it was considered). The elevator core was constructed in the same manner (beams built into the wall each floor level). My biggest concern is, will I reduce the stability of the structure, if I remove this CMU infill to place the new windows. Thanks for your help.