Calculating Load, Deflection, and Rotation for Overhead Medical Support Grids Unistrut Service Co.
Ceiling Support Grids for Medical Applications: Why Accurately Calculating Load, Deflection, and Rotation is Important
August 27th, 2013
Thanks to its ease of installation and adjustability, Unistrut channel is often specified during the construction of overhead medical equipment support grids . The safe and effective use of catheterization c arms, radiology equipment, lights, and service booms suspended by Unistrut support grids requires proper calculations of load, deflection, and rotation. This Tech Talk post discusses how to make accurate calculations to ensure both optimal equipment performance and the safety of your installation.
One common mistake is the misuse of the “beam load tables found in the Unistrut General Engineering Catalog and on the Unistrut Service Company website. Please note that the standard “beam charts” shown below provide the MAXIMUM ALLOWABLE UNIFORM LOAD. Most actual cases will require some type of “reduction factor” to properly determine the actual load capacity of the strut.
Click to Enlarge Image
To help engineers and architects make accurate calculations, we also include notes and sample calculations as shown here:
1. Beam loads are given in total uniform load (W Lbs) not uniform load (w lbs/ft or w lbs/in).
2. Beam loads are based on a simple span and assumed to be adequately laterally braced. Unbraced spans can reduce beam load carrying capacity. Refer to the linked page for reduction factors for unbraced lengths from 2 to 20 feet .
3. For pierced channel, multiply beam loads by the following factor:
- KO Series .95%
- T Series .85%
- HS Series .90%
- SL Series ..85%
- H3 Series ..90%
- DS Series ..70%
4. Deduct channel weight from the beam loads.
5. For concentrated midspan point loads, multiply beam loads by 50% and the corresponding deflection by 80%. Follow this link for information regarding other load conditions .
6. All beam loads are for bending about Axis 1-1.
Sample calculation #1:
Here is a sample calculation to demonstrate the beam loading charts, and the load reduction factors.
Say we have a 6’ span of P1000sl, and want to see what the load capacity is for a point load.
Per the charts, P1000 is good for a maximum allowable uniform load (not load per foot) of 560 lbs. In referencing the notes below, we have the following load reductions:
- X .85 for the sl
- X .5 for a point load in the center
- X .78 load reduction per page 56
Therefore 560 lbs x .85 x .5 x .78 = 185 lbs. Sample Calculation #2.
Say we have a 5’ span of P1001, and want to see the load capacity for a point load.
Per the charts, p1001 is good for a maximum allowable uniform load (not load per foot) of 1,910 lbs. In referencing the notes below, we have the following load reductions:
- X .5 for a point load in the center
- X .97 load reduction
Therefore 1,910 lbs x .5 x. 97 = 926 lbs.
Note that these assume that the strut connection is able to handle the load. It is the responsibility of you and your Engineer to review all calculations and connections, and to verify that all items are installed per Unistrut requirements. Unistrut Service Company takes no responsibility for the misuse of information provided. Local codes may dictate more stringent requirements.
Catheterization and x-ray equipment are also highly sensitive to deflection and/or rotation. The “typical” Catheterization or Radiology support requires 1/16” deflection, and the “typical” light or service boom requires .2 degrees of rotation. Designing supports for overhead medical equipment solely on material stress without considering deflection/rotation will compromise both the performance of your equipment and the safety of your personnel and patients .
For example, an inadequate overhead support structure may cause excessive wear and eventual failure of service boom brakes and clutches. With an inadequate support grid, heavy service booms may initially perform sufficiently, but with time, the interior boom components will begin to wear and continually fail. Eventually, the dynamic load may cause failure of the support, causing the equipment to come crashing down.
Making accurate calculations of load, deflection, and rotation when designing your overhead ceiling support grid will ensure your medical equipment performs to manufacturer specifications while keeping lab technicians and patients safe. If you have any questions regarding load, deflection, and rotation calculations for medical support grid applications, contact Unistrut Service Company for assistance.