By: Arthur D. Moeller, PE, Owner/President of Moeller Engineering, LLC, Ankeny, IA
A demolition project can develop as the result of an aging structure, a need to make room for progress or rebuilding from extensive damage due to storm or other incident. There is an art and science to silo demolition and this presentation will address both by explaining the structural surveys, permits, authority notifications, safety considerations, demolition plan development and execution, waste management and salvage. Lastly, the roles and responsibilities of both the contractor and the owner will also be explained.
Why should the Agricultural industry care about the details of Demolition Engineering?
In the United States throughout the 1950’s roughly 3 billion bushels of grain storage was constructed, raising the total storage capacity to 5.5 billion bushels by 1962. Since then the total U.S. Off-Farm storage has only risen by another 5 billion bushels. That means that half of the total grain storage structures are more than 50 years old. Most of the concrete storage built during that time had a life expectancy of 50 years, therefore these silos are living on borrowed time.
Of the total storage built, the ratio of flat to upright structures is roughly 1:1. Of that upright storage, concrete facilities account for an estimated 60 percent. During the 50’s, one could assume an average concrete silo held 25,000 bu, based on that size there were approximately 36,000 concrete silos built during that decade that are now past their designed life expectancy, that volume accounts for nearly 10% of our total grain storage infrastructure. These aging structures will either need to be repaired or removed to make way for new.
Know if your silos are on this “Greater than 50 list”.
It is important to identify the silos that are at risk on your facility. These aging silos could have the potential for sudden and catastrophic failure, most commonly occurring in a sidewall. The walls of a silo are designed to resist extreme outward forces from the grain loading, which over time, can weaken due to stress, corrosion and fatigue. By identify these at risk silos, before such a failure, you will have options on what to do to preserve, repair or remove this structure.
Is Demolition the best option?
If you are thinking demolition, one should seek help to analyze the cost/benefit ratio of having the structure removed vs. having it refurbished. If the decision has been made to remove the structure, there are a number of steps to follow to insure the silo can be brought down safely and with no negative impact on the environment.
Any demolition job shall start with a survey of the structure as required by OSHA 1926.850. This type of survey should be done by a competent demolition engineer or expert. The survey is intended to determine the condition of the structure and identify any potential for an unplanned collapse. The results of this inspection can also help the contractor determine the best means of wrecking the structure down. This inspection should also include any surrounding buildings, looking for any potential for structural failures that would expose employees or the adjacent community to harm. This inspection should include the identification of site runoff potential and identifying the possibility of air contamination. This potential air contamination and how to control it will be covered in the development and implementation of a dust control plan. Inspections to identify these issues shall periodically continue throughout the entire razing project.
In addition to the structural survey, a hazardous substance survey shall also be conducted to determine what, if any, abatement will be required prior to demolition. Asbestos is always a concern when tearing down older structures. All building utilities will also need to be located and removed or capped and protected before the demolition work begins. The respective utility companies will need to be contacted and can prove quite helpful in the protection of any electric, gas, water, steam, sewer or other service line. If power or other service is required during the demolition, a temporary service or relocation of the existing will need to be figured into the scope of work.
Each State, City and/or local jurisdiction will require the demolition contractor to complete a demolition notification form/permit. Part of the requirements of this form is the verification of an asbestos inspection and a waste disposal plan.
If the structure is near a rail line you will also want to notify railroad of your plans. There will be safe guards that the railroad will want built into the demolition plan to insure the rail service will not be interrupted. Some examples of these safe guards are keeping a crane far enough from the tracks that the boom could not cross the tracks in a failure, ceasing the demo process while a train passes the site and posting a flag man is another frequently used safe guard.
In addition to contacting the utility companies it is also important to notify the local authorities such as the police and fire departments of your plan so they can be prepared in case of an emergency. The contractor should work in conjunction with the fire department to also develop an Emergency Response Plan that will include personnel roles and responsibilities, activating emergency alarms, an evacuation plan with routes, reporting emergencies, first aid and a fire prevention plan.
Demolition activities can create a hazardous work environment for the personnel performing the work. Identifying and controlling these hazards before the job begins will help reduce the possibility for work place injuries, this is done by means of a Job Hazard Analysis (JHA). The JHA will list the hazards associated with the project activity as well as hazard control strategies. This is also when the Personal Protective Equipment (PPE) can be identified for each task being performed on the demolition site.
Structural Engineering Razing Plan:
The first step in this section of the overall demolition plan is to determine the method in which the structure will be razed. There are three main methods used in razing silos. Two of those are classified as mechanical, the wrecking ball and high-reach excavators, and the third is implosion. Depending on the type and size of structure, its location with respect to surrounding buildings, the nearby population and many other factors are used to determine the means in which the structure will removed.
Next in this razing plan is the need to determine the sequence is which the building will be brought down. If a ball is used, you need to position the crane, make room for a debris pile, keep enough space to separate the steel and concrete and yet have enough room for the trucks to drive in, be loaded and head back off the site. There can be a lot of coordination for this effort and the position and size of crane, the building location and available property will make a big difference in how well this works.
Creating that final plan for how the site will be left once the structure has been removed can be one of those elements frequently overlooked but it is still very important to have this final site plan be part of the overall demolition. Regardless if you are planning to leave the property vacant or rebuild, the location of where the foundation was will need to be filled to reestablish grade and compacted to restore the soil stability.
Demolition of large structures especially those that have become structurally compromised is a job for professional people that have a unique expertise for this type of work. As shown here there is a lot that goes into insuring the safe removal of these structures. Taking the necessary time to develop a clear and complete plan will help reduce the owner’s liability and risk associate with razing a structure.