This past winter was a tough one here in New York, thanks in part to the flu bug. I had the flu just a few months ago, and not being able to get out of bed reminded me of a topic I have been meaning to write about for some time. It's related to that sagging feeling brought on by the flu virus.
Buildings have their own version of this symptom, generally referred to as deflection. And while the flu symptoms we suffer from are not related to those of a building, they do have one important thing in common: sometimes you see it coming (anticipated deflection) and sometimes you don't (unanticipated deflection).
Anticipated deflection is what our structural engineers account for when designing buildings. Architects generally respond to this type of deflection by employing control joints in finishes, slotted connections in curtain walls, and slip tracks at partition headers, to name just a few.
Unanticipated deflection is different and not always apparent. This is the head scratching type of deflection that affects everything from doorframe and curtain wall installation to MEP, roof pitch, and ceiling heights. Oftentimes there is no clear answer as to why unanticipated deflection occurs - and being an architect, I leave it to our structural brethren to figure out.
What architects do need to understand is that unanticipated deflection has several consequences: multiple meetings, numerous phone calls, RFIs, emails, sketches, change orders, and more meetings. On construction sites, this is simply about time, money, and resolving the issue. Keep in mind that it's not just the cost of labor and material to correct the problem. Usually the far bigger issue is logistical, as it involves removing MEP already installed (and possibly tested) in order to access structure needing reinforcement. Not to overstate it, but unanticipated deflection should be avoided like the plague...I mean the flu!
For these reasons, the following actions should be taken during the design development (DD) and construction document (CD) phases as well as during construction:
Discussion with Structural Engineer
Discuss your concerns about deflection with the structural engineer during the DD and CD phases. Ask him or her about any problems they've had with unanticipated deflection in the past. Focus on cantilevered areas, loads on long spans, transfer beams, slab edges, and spandrel beams. Make sure the engineer is aware of all MEP equipment loads and beam penetrations. Also review the façade design so the structural engineer is aware of all loads along the building's edge that might impact the envelope's design or installation. Any other heavy equipment or unusual live loads should be addressed at this time.
Discussion with Cost Estimator: Allowances and Unit Prices
Should unanticipated deflection occur, and if the contract type permits, control costs up front by including allowances or unit prices in the general conditions of the contract. Trades to consider include steel, concrete, self-leveling, temporary shoring, and MEP, as well as removal and dismantling work. When developing unit price lists and allowances, think about the trades affected and what processes the contractor will need to employ to correct the problem. If your project has a cost estimator, consult with him or her to develop this schedule.
During Construction
Verify that the contractor forwards all concrete reports and steel mill reports to the structural engineer for their review. Specifically, concrete that is testing too heavy should be carefully scrutinized by the structural engineer. Concrete inspectors should closely monitor slab thicknesses during the pour and report back to the structural engineer on any deviations from the design.
Additionally, alert the structural engineer if the contractor substitutes or provides a different manufacturer for building systems such as cooling towers or air handlers as this may increase the weight and/or impact the load points for this equipment. As a rule of thumb, if the structural engineer designed for it during DD and CDs, then they will need to review if it has changed during construction.
Finally, owners usually have structural steel surveys performed before and after concrete pours. Surveys should be reviewed by the structural engineer for deflection above and beyond what was designed. Inconsistencies should be reported to the owner immediately.
Unfortunately, when unanticipated deflection occurs, you can't just tell your building to "take two of these and call me in the morning." But there are steps that can be taken to minimize the odds that something like this will happen. Think of it as a flu-shot for your building.
Buildings have their own version of this symptom, generally referred to as deflection. And while the flu symptoms we suffer from are not related to those of a building, they do have one important thing in common: sometimes you see it coming (anticipated deflection) and sometimes you don't (unanticipated deflection).
Anticipated deflection is what our structural engineers account for when designing buildings. Architects generally respond to this type of deflection by employing control joints in finishes, slotted connections in curtain walls, and slip tracks at partition headers, to name just a few.
Unanticipated deflection is different and not always apparent. This is the head scratching type of deflection that affects everything from doorframe and curtain wall installation to MEP, roof pitch, and ceiling heights. Oftentimes there is no clear answer as to why unanticipated deflection occurs - and being an architect, I leave it to our structural brethren to figure out.
What architects do need to understand is that unanticipated deflection has several consequences: multiple meetings, numerous phone calls, RFIs, emails, sketches, change orders, and more meetings. On construction sites, this is simply about time, money, and resolving the issue. Keep in mind that it's not just the cost of labor and material to correct the problem. Usually the far bigger issue is logistical, as it involves removing MEP already installed (and possibly tested) in order to access structure needing reinforcement. Not to overstate it, but unanticipated deflection should be avoided like the plague...I mean the flu!
For these reasons, the following actions should be taken during the design development (DD) and construction document (CD) phases as well as during construction:
Discussion with Structural Engineer
Discuss your concerns about deflection with the structural engineer during the DD and CD phases. Ask him or her about any problems they've had with unanticipated deflection in the past. Focus on cantilevered areas, loads on long spans, transfer beams, slab edges, and spandrel beams. Make sure the engineer is aware of all MEP equipment loads and beam penetrations. Also review the façade design so the structural engineer is aware of all loads along the building's edge that might impact the envelope's design or installation. Any other heavy equipment or unusual live loads should be addressed at this time.
Discussion with Cost Estimator: Allowances and Unit Prices
Should unanticipated deflection occur, and if the contract type permits, control costs up front by including allowances or unit prices in the general conditions of the contract. Trades to consider include steel, concrete, self-leveling, temporary shoring, and MEP, as well as removal and dismantling work. When developing unit price lists and allowances, think about the trades affected and what processes the contractor will need to employ to correct the problem. If your project has a cost estimator, consult with him or her to develop this schedule.
During Construction
Verify that the contractor forwards all concrete reports and steel mill reports to the structural engineer for their review. Specifically, concrete that is testing too heavy should be carefully scrutinized by the structural engineer. Concrete inspectors should closely monitor slab thicknesses during the pour and report back to the structural engineer on any deviations from the design.
Additionally, alert the structural engineer if the contractor substitutes or provides a different manufacturer for building systems such as cooling towers or air handlers as this may increase the weight and/or impact the load points for this equipment. As a rule of thumb, if the structural engineer designed for it during DD and CDs, then they will need to review if it has changed during construction.
Finally, owners usually have structural steel surveys performed before and after concrete pours. Surveys should be reviewed by the structural engineer for deflection above and beyond what was designed. Inconsistencies should be reported to the owner immediately.
Unfortunately, when unanticipated deflection occurs, you can't just tell your building to "take two of these and call me in the morning." But there are steps that can be taken to minimize the odds that something like this will happen. Think of it as a flu-shot for your building.