Roof Lifting Structural Integrity: Typical LIFTEX Practices for Addressing Lifted Roof Loads
Introduction.
Roof lifting is not only a structural challenge but also a test of safety and precision.
In this post, we’ll explore the built-in redundancies, independent lifting points, and impressive load capacities that make LIFTEX the trusted partner for structural engineers.
Vertical & Gravity Considerations.
The calculated dead, snow, and roof live loads often remain the same from pre-lift to post lift on the raised portion of roof. Items such as the addition or replacing of roof insulation, new RTUs, or solar panels are considered as they are often desired in conjunction with the roof lifting scope.
For our proprietary lifting columns, there are two approaches. One is a sleeve outer member (A992 Gr 50 Channels with A36 connector plate built up members that form a tube) installed around the original wide flange column. The other is a total replacement, a square HSS within a 1” larger square HSS. Sizes are calculated using a stepped column calculation based on AISC Design Guide 7, Appendix B. This determines new effective lengths for each of the inner and outer members, in both the strong and weak axis, and those lengths are then analyzed for sizing accordingly. The lifting column remains as the final column on completion. LIFTEX has additionally analyzed & defined a set of standard inner member to outer members connection details, typically using three feet of overlap.
Other than rare exceptions, interior column foundations typically do not need to be enhanced or even exposed, as they are treated with transition plates as described in the series two post.
Lateral Load Considerations.
Since LIFTEX literally “pops the top” of the cube, the challenges are in checking the existing roof structure and then connecting it back to the foundations. The new height of the roof generates an increase in the overall lateral and uplift loads. LIFTEX begins by checking the roof diaphragm and structure to see if items such as additional uplift bridging or diaphragm bracing may be required. Typically enhancement is considered in scenarios when the project calls for roof ballast removal or when the existing deck has a diaphragm that cannot be calculated – such as gypsum or tectum type decking.
After validating the roof, getting the roof diaphragm loads back down to at least the original shear walls, if not down the foundations, is the next item addressed. Means and methods often depend on the size & integrity of the existing shear walls, footings and general connections. For in-plane loading, cross bracing (normally angle) or chevron bracing (normally HSS) is the typical treatment, depending on the calculated loads. Often when the perimeter walls are used in the approach, no foundation enhancement is required. When they are not, foundation enhancement is more likely, all depending on the existing conditions. Other approaches have been used before, such as moment connections or strapping of vertical stud enclosure walls. The LIFTEX engineering team considers all these factors in designing a comprehensive lateral force resisting system.
So how about perpendicular to the wall (also known as out of plane)? What happens there, how do you support the top of the old wall and how do you account for the dreaded “hinge” point? LIFTEX addresses all these concerns. For wall support, typically there is a horizontal wind girt installed either on top of or inside of the top of the wall, spanning from column to column and anchored periodically to the concrete or grouted CMU wall.
Rather than replacing perimeter columns, the approach is to install a telescopic HSS lift column adjacent to the perimeter column, connecting them together at designated intervals. Once the roof is lifted, the lifting column remains as a “strongback”, and the original column is extended. The extended column is now braced by the lift column so that it can handle gravity loads, and the combination of the columns is checked to ensure it is capable of supporting the out of plane lateral loads. The remaining lift column, referred to as a “strongback”, increases the depth of the column perpendicular to the wall. All welded “tie back” connections, including spacing & sizes, are detailed so that the existing extended column and new “strongback” work together to eliminate any hinge points and resist the new lateral forces. In scenarios where this combination is still inadequate, the addition of perimeter “wind” columns is a common treatment.
Structural Modifications & Mezzanine Adjustments.
When adding or removing a mezzanine, LIFTEX carefully evaluates the conditions to determine whether footing enhancements or other reinforcing to the existing is required or ensure that the removal does not compromise the building’s structural stability, or that a new installation does not impact the raised roof structure.
Our goal is to maintain the building’s original structural integrity while enhancing vertical clearance, all without fundamentally altering the layout or support mechanisms.
Working with the Best.
By integrated means and methods that are time tested, developed by steel erectors and validated many times over by structural engineers, the LIFTEX approach is the ultimate balance of innovative engineering and efficient use of time and materials. The design-build approach has allowed us to develop systems that work safely, effectively, and efficiently in getting the roof raised and getting the building back to its intended use without unneeded costs and time.
Contact LIFTEX to learn more about their cutting-edge lifting system.
Let’s Start the Conversation.
LIFTEX specializes in providing state-of-the-art roof-lifting solutions that transform retail properties into thriving industrial and multi-use spaces. Contact us today to learn how roof lifting can repurpose your vacant property and elevate its value for the future.
