Good HC Design Keeps Science In Mind

Posted on June 20, 2012


A perspective from another field of study can provide just the right amount of rational support for design, which can be so subjective at times.  I like numbers because they are a real illustration.  Maybe you caught my post on the  business case numbers behind why the RFP process is so fruitless for hospitals.

Some design decisions are easy when you pan back and put some simple logic and science behind it. For instance, I was doing an in-depth study of a hospital done by one of our competitors.  Haskell’s architects and engineers met with the hospital’s engineers, maintenance and facilities crew.  We spent an entire afternoon poking around, asking questions, and trying to understand the stories behind why things were designed and built the way they were.  Reverse engineering a design is impossible.  What we were after were assumptions and decisions that are “standard” with our work, but our competitor cut corners on.

One of the obvious things noticed were a lack of crash rails on the main patient corridors. These are easy to detail and specify, but they cost money, and if a client is in a rush or cash pinch they might easily overlook crash rails. Crash rails can be omitted if the walls are protected in other ways:  a more substantial wall composition or impact-resistant material like sheet vinyl, for example. These walls did not have that. Not a big deal?  Minor oversight?  Let’s look at the physics behind why hallway impact protection is needed.

An impact on a hallway is a simple physics equation:  Force = mass times acceleration, or F=ma.  Based on industry standards and information from product rep’s, I determined the average, durable hospital bed is between 700 and 850 pounds, empty. For our calculations, let’s say it is in the middle at 775 lbs.  With a 225 lb. patient with meds and equipment, the mass of a hospital bed heading down the hall is 1000 lbs.  Hospital beds can move pretty fast in an emergency, but in this case, three feet-per-second squared is a fair speed—not leisurely, but not rushed. 

In our equation, that gives us F= (1000 lbs)(3 ft/s/s).  Thus, the force hitting the corner of a corridor or unprotected hallway is 3000 lb/ft—a ton and a half of force! And that is for a mistaken collision between a bed and wall.  I walked down a materials management hallway once and saw a second-shifter repeatedly ramming a trash cart against a wall for no apparent reason—more mass, more acceleration, way more force over and over into the same spot. So a best case scenario of 3000 lbs of force against a wall makes crash rails a given. Any designer that signs off on that omission is suspect.

Using what we’ve learned, let’s look at the bed itself, or anything else the nursing staff needs to push around. A nurse needs to maneuver a 1000 lb. bed with patient down hospital corridors. Getting 1000 lbs. moving is not easy. In fact, it takes a certain amount of directional effort to get it going, M=fd, where M is the moment (directional force on the bed causing wheel rotation), f is the force, and d is the distance from the point of rotation, or moment-arm. We’ve already calculated the force, 3000 ft/lbs, so if the hospital bed has 8″ wheels (8″ = .67 ft), that gives us M= (3000 lb/ft)(.67 ft).  The rotational moment generated on a bed with 8″ wheels is 2010 ft/lbs.

If Purchasing buys a bed design with only 4″ casters, the calculation yields 990 ft/lbs, about half of 2010 ft/lbs, so wheels half the size require twice as much effort to move the object. And, if the bed had 12″ wheels, the moment would grow to 3000 ft/lbs, or 50% greater than the 8″ design.  In other words, it would take half as much effort to move the bed with 12″ wheels than 8″.

Equipment planners, procurement directors, and maintenance personnel and architects should be aware of this because this affects the building as well. The effort required for a bed to move and navigate halls should affect design decisions like…corridor widths, elevator sizes or whether to curve a hallway or not. I spoke with two VPs of Facility Planning who each had a hospital tower on his campus with a radiused floor plan.  Both hated the buildings:  too many impractical leftover spaces, too hard to wayfind, too hard to navigate with equipment. The one VP even admitted his nurses sustain far more injuries trying to push beds around the curves.  According to a Canadian Union of Public Employees 2008 newsletter, it costs $140,000 to replace a nurse injured on the job—not insignificant.

These calculations are ballpark (i.e. they don’t consider friction) and are just two instances where a little quick study can help make a design decision an apparent ‘no-brainer’, or at least provide vivid evidence for a spirited cost-benefit analysis and discussion. Think of how physics can help decide whether overhead lifts are a smart investment, or the material and strength of your waiting room furniture. The art of design and science of medicine and building for healthcare are not mutually exclusive; they should be cross-consulted for symbiosis. Consider how a perspective from another field might help you solve a design or business problem in your hospital.

Posted in: Interior Design