Considerations When Designing and Building A Tilt-Up Project
By: Jim MacKinnon, Saunders Construction Inc.
Despite my years of experience working as preconstruction manager on numerous Tilt-Up projects ranging from office buildings to warehouses and schools, I am still amazed how often Tilt-Up is mislabeled an “option” over another product or skin type. To compare building construction to automotive manufacturing, I view Tilt-Up as the chassis – strong, yet flexible – serving as a foundation upon which the entire car is constructed. While vehicles may have numerous options from which an owner can choose, the chassis is not one of those choices. Such is the case when selecting Tilt-Up as the building method for a project. Tilt-Up should be considered the construction system and not merely an option.
With a mindset that Tilt-Up is not an option but a standard, a design team must develop ways of thinking that include planning for Tilt-Up in the earliest stages of the design process. Architects need to be deliberate in their design approach using Tilt-Up, because the decision to use Tilt-Up significantly affects many other decisions that follow.
For example, window size and configuration in a steel-framed structure will differ greatly from that in a Tilt-Up building. The larger the window in a Tilt-Up building, the more engineering the panel will require. Thus, different dimensioning and detailing will be a natural result. As such, designers need to design for Tilt-Up intentionally, not as an afterthought. Too often I am faced with offering Tilt-Up as an option to another system when the building is 100 percent designed and within a couple of months of starting construction. At this stage, it’s far too late to change construction methods.
To avoid desires for mid-stream changes in building methods, the design team must feel completely comfortable selecting Tilt-Up. When a design team is comfortable with Tilt-Up, all members of the team understand the important considerations for using Tilt-Up and the applicability of the Tilt-Up method. When evaluating whether a project is a fit for Tilt-Up, the team should evaluate two primary considerations: design considerations and construction considerations.
WHAT DO YOU NEED TO BUILD?
First and foremost, understand almost any building can be constructed with Tilt-Up in some form or fashion. However, an honest evaluation will reveal whether Tilt-Up is a sensible, economical approach. For example, a 10-story office building in downtown Dallas is probably not a prime Tilt-Up candidate, nor would a 200-foot-tall air traffic control tower in Tulsa. How about a six- story office building or an 80-foot-tall control tower? Here, Tilt-Up should not be excluded from consideration. Today, Tilt-Up is a common building solution for schools, office buildings, churches, retail centers, municipal centers and manufacturing facilities. On rare occasions it has even been used for lighthouses, hangars and performing arts venues. With its wide range of applications, Tilt-Up provides the most economical and time-efficient building method for many projects.
In spite of its vast appeal, sometimes Tilt-Up may not be the best building solution – often because of less obvious reasons. For example, if you need to build an auto reconditioning facility that requires substantial sloped floors for drainage, the cost to install casting beds may cause the Tilt-Up system to be more expensive than a concrete block building. Another example is if the soils report for a particular project reveals the recommended floor type is a structural floor system over a crawlspace while your budget assumed slab on grade. This particular scenario can have significant cost impacts if constructed in Tilt-Up. The bottom line is to be sure to consult with a respected Tilt-Up contractor and have him or her evaluate the desired building for its application of Tilt-Up. An experienced contractor may develop a way to use Tilt-Up that you didn’t think of.
WHAT DOES THE CLIENT WANT IT TO LOOK LIKE?
Most often, a misconception of Tilt-Up’s appearance causes the most hesitation about its use. First, obtain a thorough understanding of all the various finishes applied to modern Tilt-Up so you can accurately convey these elements to clients. Do not discount certain appearances until you have researched them first through consultation with a contractor or an organization such as the Tilt-Up Concrete Association (TCA). These are the best sources for determining applicable finishes. I recall an owner who once demanded a brick building, and we offered Tilt-Up as the system with cast-in brick. We achieved cost and schedule savings over masonry, and we now have a very pleased owner.
Furthermore, know your client’s aesthetic expectations and determine if the available budget will produce these desires. Let’s face it. Some jobs are just not meant to be Tilt-Up, even with the many fine architectural enhancements we have at our disposal. If a client wants an Ivy League look and has the funds to afford it, maybe some other system will more accurately represent these sophisticated tastes. Outside of these rare occasions, Tilt-Up most often can provide a reasonable solution for almost every economic and aesthetic desire. If your client wants brick, exposed aggregate, textured surfacing, lots of glass, extensive relief and even stone cladding, Tilt-Up can offer the solution.
STRUCTURAL CONSIDERATIONS
One very important step in the Tilt-Up selection process is getting your structural engineer on board early. His or her input on foundation design, wall engineering and other related load analysis will help you understand what areas of design need attention. For example, panel openings and how they define panel thickness and reinforcing design can have significant effects on costs and other aspects of design. Having the structural engineer to evaluate the soils report early in the design process can help the team understand possible floor and foundation design. Depending on the results, Tilt-Up could possibly be eliminated as a system due to cost and/or constructability issues, or measures may need to be taken with Tilt-Up that may result in scheduling impacts.
Having both the structural engineer and a contractor at the table will go far with evaluating Tilt-Up and its merits for your project. Regarding the structural side of Tilt-Up design, following a few steps can help the process go smoothly. First hire an engineer with proven Tilt-Up experience. Next, allow the contractor to inform the engineer on structural details. These two items will save the architect and owner time and money.
SCHEDULE/KEY DESIGN CONSIDERATIONS
Consideration must be given to schedule. By now it is a foregone conclusion that Tilt-Up as a system is typically quicker to construct than steel and masonry. But architects, engineers and owners need to realize how much time goes into the front end of planning a Tilt-Up project compared to other structures. For example, once a contractor is released to begin the project, reinforcing steel and detailed panel shop drawings need to be generated and approved before materials can be fabricated and delivered. Depending on the size and complexity of the project, production of shop drawings can take anywhere from four to 10 weeks. I have seen projects where we were released to start construction only to have the anxious superintendent standing around on his slab waiting on materials so he could start the panels.
The result of the above scenario is wasted time and money. If the contractor can be brought on-board early in the process, then most often this scenario will never happen. If you have a project that is fast-track or under pressure to get started, your critical path runs through the structural engineering. Give the structural engineer the information he or she needs, and then turn your attention to getting the contractor on board so he will begin work on shop drawings.
The bottom line is to consult with a trusted and respected contractor early or bring him or her on board as soon as possible. A team approach will most assuredly yield the Tilt-Up project everyone wants and at a price the owner is willing to pay. Architects, owners, developers or contractors should not make the mistake of discarding the Tilt-Up system too quickly. For every project you think isn’t a fit for Tilt-Up, I’ll show you one just like it that was – and one that even received a TCA Achievement Award. If you’re pursuing Tilt-Up as a design method, a good decision is the fruit of two essential practices: Do your homework and understand the design considerations used to apply the Tilt-Up system.
This addresses the considerations for designing a Tilt-Up project, but how about the needs to consider with regard to construction when evaluating a possible Tilt-Up project. When a preliminary set of plans hits my desk, I immediately start looking at the following areas, not necessarily in any specific order of importance:
- Site layout and topography
- Foundations (type, depth, design- if any)
- Slab
- Panel design
SITE LAYOUT AND TOPOGRAPHY
When looking at the site plan of a potential Tilt-Up project, it is important to know if all the panels can be cast on the proposed building’s floor slab or if casting beds will need to be constructed. If casting beds are needed, is there enough room directly around the building? Now the combination of site topography and space both become critical. A flat area around the building will be needed for casting beds along with an access path for the erecting crane. If casting beds are not required and all your panels can fit well on the slab, crane access and picking requirements still need to be evaluated. Determine how much grading is necessary to get a flat, graded path for your crane. Occasionally, this will require spending additional dollars for grading and maybe gravel roads for the crane.
Every Tilt-Up project requires a panel layout diagram to detail exactly where the panels will be cast on the slab. This is helpful in many ways, because it will assist in determining if the panels do indeed fit on the slab so that casting beds are unnecessary. Also, it forces the contractor and panel erector to get on the same page so that there is agreement on what panels get picked when and from where.
On many projects, Tilt-Up may be an “option” you’re considering over steel, precast or masonry. Oftentimes, contractors are presented with a project the design team thinks may be a terrific candidate for Tilt-Up, but the initial review of the site and building layout may prove to the contrary. For example, I received a call one time from an architect about a distribution warehouse project he thought would be perfect as a Tilt-Up project. I excitedly rushed over to his office to take a look and hopefully secure a nice opportunity. As I looked at the site plan, my optimism languished. It was a two-story building in the side of a hill with virtually no lay down area and excessive slope all around the building. To make matters worse, the walls would have to be designed to resist significant load while acting as a retaining wall. Compared to other structural systems, Tilt-Up was not a very desirable system for this particular building.
FOUNDATIONS
Foundation types vary all over North America depending on soil conditions and climate. Panels can be erected on drilled piers, pad footings, continuous footings, grade beams, stem walls, etc. But what if you’re comparing different building systems to Tilt-Up? I have often been asked to provide cost comparisons between Tilt-Up and masonry, precast, steel with studs and more, and the one area that people fail to investigate fully is the foundation system. For example, precast panels need to be transported so they are limited in width, usually between 13 feet and 15 feet. Thus, if you have a caisson foundation system you’ll have similar spacing for your piers. If looking at Tilt-Up, panels can be designed much wider, most often twice the width of its precast counterpart, thus saving lots of money in caissons. Likewise, a masonry wall composed of concrete masonry units is usually built on a grade beam, whereas a Tilt-Up wall directly founded on caissons or footings acts as a grade beam itself. Therefore, the grade beam can be a differentiating factor with regards to cost. Be sure to consult the structural engineer to determine appropriate foundation design when doing these comparisons.
THE SLAB
One commonly overlooked aspect of Tilt-Up construction is the importance of the slab.
When planning a Tilt-Up project, be sure to consult with your panel erector to determine if the panels will be erected while the crane is on the slab or outside the structure. This one important discussion can dramatically impact crane size and productivity. Erecting on the slab can have impacts on the slab thickness and reinforcing requirements, so be sure to evaluate the crane size and how this may affect slab design. Another concern for erecting on the slab will be how the slab will look after a crane has been on it. The client needs to be aware that there will be inherent wear and tear on the slab and that it can only be protected and cleaned to certain limits.
When bracing panels to the slab, be sure to understand the proper brace engineering requirements. Many brace engineering companies will evaluate the loads or forces on the braces, thus defining the structural requirements of the slab. Factors involved with defining these forces and loads include panel height, width and openings as well as anticipated wind load. I can’t tell you how many times I’ve seen designers plan for Tilt-Up on tall structures but specify a lightly reinforced 5-inch slab. If you’re bracing three-story panels for an office building, a slab such as this may not work. The same concerns lie in areas where high winds are common. Even short panels can exhibit significant loads on a slab in high winds. Always be sure to consult with your bracing engineer and coordinate with the building structural engineer to specify the right slab.
Pourback strips can be a touchy subject to some but I try to eliminate them if possible, especially with office buildings and other facilities that will be occupied in a similar manner. Why? Often times a pourback strip is placed over compacted soil that, quite honestly, is not compacted to the same specification as the main slab. Settlement of this area is a common result, thus an obtrusive ridge occurs at the concrete joint between the pourback and the main slab, something similar to a slab that has curled. This nuisance is unacceptable during tenant fit out as office personnel start locating furniture in this area. I would suggest deepening your footing so that the top of the footing is within 12 inches of the floor slab. Set your slab edge form off the inside panel edge and place your slab accordingly. Erect panels right up to this slab edge and caulk the joint. This will provide a solid, continuous slab to the panel alleviating the pourback strip.
Lastly, understand what work is to be installed below the slab. Typically, a speculative warehouse will have minimal below slab plumbing. This is most often not a real schedule concern when planning slab placement or panel construction. But, if there is significant underground work, alternate plans may be in order. For example, our firm is utilizing the Tilt-Up system for a design-build ambulatory care center that has many individual exam rooms with bathrooms scattered throughout the facility. As we evaluated the schedule, we realized that because of the extensive amount of underground plumbing it would be in our best interest to install waste slabs all around the building to cast panels. Although we will spend money for these casting slabs, we cut time off our schedule because we don’t have to wait for the main slab to be completed before starting panels.
PANEL DESIGN
All too often, designers have a tendency to design panel widths that are dictated more by precast limitations or masonry design than that which is possible with Tilt-Up. Depending on crane size availability in your area, Tilt-Up is far more flexible to expand to wider dimensions compared to prefabricated and transported precast panels. Be careful to consult with your panel erector on the maximum size panel their rig can lift on that specific project while taking into consideration layout and access. For example, I recently requested that an architect and engineer make single-story office panels 44 feet wide just to minimize the quantity of panels and their related forming. The result was less panel joints, which made the architect happy and less cost, which made the owner happy.
Another piece of advice when designing panels is to evaluate consistency in panel sizes. Be sure you’re not out of balance in terms of panel weights. If most of the panels on a given project weigh 65-75 Kips and a couple of panels weigh almost 125 Kips, try splitting them into smaller panels if the architect will allow it. Don’t let those panels dictate the entire project’s crane size or you’ll end up spending unnecessary money in erection costs.
SCHEDULING
While this was briefly discussed in part one, it is worth restating. Careful pre-planning of a Tilt-Up project is imperative for success. Many of the components of a Tilt-Up building need to be engineered for fabrication much earlier in the process than other building types. A detailed pre-construction schedule can assist in determining when shop drawings and procurement of materials need to commence. In fact, I find it beneficial to work the construction schedule backwards to determine these dates. Oftentimes, an owner will issue final construction documents for bidding, award the project, and then release you for construction. If there is minor earthwork to get your foundations and slab in, it will likely result in an empty slab waiting for panel shop drawings, rebar, inserts and embeds to get the panels built. Obviously, it is important to avoid this scenario if you want repeat business and a solid reputation. Proper pre-planning of procuring these shop drawings and materials will make your life a lot easier.
In summary, there are many things to consider when designing and building a Tilt-Up project, but most important, understand that Tilt-Up is very much a different system and has its own idiosyncrasies. Proper planning, site investigation, consultation with the erector and accurate cost estimating are critical components to successful Tilt-Up projects. Further, if you are savvy with Tilt-Up, then share your knowledge with others. Partner with your architect and engineer to help them understand how to design a better and less expensive, contractor-friendly Tilt-Up building. Chances are you’ll work together again and they’ll remember you.
