St. Jean Engineering, LLC Structural, Marine & Civil Engineering Licensed In: Rhode Island Massachusetts Connecticut Maine U.S. Virgin Islands New Jersey PRELIMINARY STUDY GIFFY BARRELS TENT BALLASTING SYSTEM Prepared for: Giffy Tent Barrels tm, Inc. Date: December 27th, 2014 Prepared By: St. Jean Engineering, LLC 1145 Middle Road East Greenwich, Rhode Island Preparers Note: The attached computations were developed to evaluate a "Framed Tent" ballasting system using Giffy Barrels. The review was limited to a 20 foot by 20 foot framed tent structure using 2 inch diameter by 1/8" wall aluminum pipe frame members with a fabric roof. The reviewer is cautioned that any conclusions drawn from this report should not be extrapolated to other tent ballasting or framing systems. 1145 Middle Road East Greenwich, Rhode Island 02818 Office/Fax (401)398-0999 e-mail: st.jean.engineering@verizon.net
Foreword: Numerous methods have been devised by framed tent installers to quickly and efficiently anchor temporary tent structures. Many building codes have attempted to address the practice of erecting temporary structures (tents, construction trailers, vendor booths, ) by placing a limit on the time of service for such a structure to one year. At the completion of the year a new permit is required or the temporary structure is required to be removed. Event tents are a of subset of these structures. The service duration of the installation is much shorter, typically a few days to a few weeks in duration. Building codes and the American Society of Civil Engineers provide design guidance to engineers for determining applicable gravity, wind and other loads that may be placed on a structure due to its regional location and occupancy or use. For permanent buildings severe wind and seismic loads are based on the probability of their occurrence over a period of years, usually once in 50 years or once in 100 years. Seismic loads are not generally considered for temporary structures unless specifically required by the local jurisdiction. Expected maximum design wind speeds are mapped by location and presented in the local building code or in reference 1 for possible adoption in the local building code. The probability that a structure will be exposed to a severe wind event diminishes as the length of time the structure is left in place diminishes. A permanent structure that has an lifetime expectancy of 100 years has a one percent probability of experiencing a 100 year event and a two percent probability of experiencing a 50 year event over its lifespan. A temporary structure that will only be in place for a few days or weeks has a much smaller percent chance of experiencing the 50 or 100 year event that is prescribed for permanent structures. As a result it is often times accepted practice to design temporary structures for environmental loads that are less than for permanent structures. Tents that are only erected for several days to a couple of weeks have the can be erected during a preferable season of the year, there typically is some knowledge of the anticipated weather, there is the ability to cancel an event should unexpected weather occur, and there is the capability to prevent occupation during extreme weather events. Staking is generally the preferred method by most tent manufacturers for anchoring their products. Staking is a time proven method for anchoring tents providing a direct load path to the ground to resist wind loads which produce uplift and lateral forces on the structure. Although staking is an efficient method for anchoring most types of tents there are times when staking a tent is not possible or preferred. Staking may not be possible if the temporary structure is to be erected on a concrete or asphalt surface where holes left from the stakes are undesirable. Staking can be inefficient in soft soils where the soils are not capable of resisting the pullout and lateral forces applied by the tent structure under wind loading. Since the soil conditions at each site can vary considerably there is always a certain degree of uncertainty as to the capacity of the stakes installed at a particular site. A survey of underground utilities should also be undertaken prior to driving stakes to prevent damage to buried utilities. Several reports have been written regarding ballasting versus staking to anchor framed tents. For the most part the reports have concluded that ballasting, particularly with standard water barrels tie off at the top of the barrel, is an inefficient method for anchoring a framed tent. The preparer is not in disagreement with these findings. Standard barrels have a tendency to either tip over, if the attachment point is at the top of the barrel and the load is applied at some upward angle, or to slide if the attachment point is placed below the center of gravity of the barrel and the barrel is Page 2 of 30
placed on a smooth surface. There are numerous studies and tests demonstrating these particular modes of failure. In an attempt to correct the inadequacies of anchoring tents with barrels some installers have resorted to using heavy concrete blocks. Concrete blocks have the advantage of increased weight, a low center of gravity to reduce the tendency for tipping, and an increase in the coefficient of friction between the concrete and the surface it's placed on to combat sliding. Several of the disadvantages of concrete blocks are the effort required to deliver and retrieve the blocks from the installation site, the need for larger equipment to transport and set the blocks, the amount of obstructed area around the framed tent due to the presence of the blocks, and storage of the blocks for future use. If several tents are erected in close proximity there may not be enough room to place all the required blocks needed and provide clear pedestrian access. From a structural engineer's point of view, the primary difference between staking a framed tent and ballasting a framed tent is the way external forces are distributed through the tent frame. Commercial tent frames typically consist of 2 inch diameter schedule 40 aluminum pipe having a wall thickness of 1/8" inch. Connections are made through a series of rigid welded fittings that the 2 inch pipe members slip over and are pinned together with bolts or quick release pins. The assembled frame is capable of resisting some degree of dead, live and wind loading providing the tent is held down and the allowable stress in any pipe member, or welded connection is not exceeded. A tent held down only at the base of the supporting legs resists lateral loads primarily through the bending of the slender support legs. A staked tent captures lateral roof loads at the eave of the tent. The lateral loads are transferred through the guys and into the ground stake. Guys can accept only tension loads so one opposing side of guys are engaged at any one time depending on the direction of the wind and the other side may go slack. The amount of deflection of the tent frame under load is direct correlation of the amount of forces and stress in the tent frame members. The following report examines the mechanics of a ballasted framed tent using the Giffy Barrel system. Page 3 of 30
References: The following references were reviewed by the preparer to form a baseline for comparison of typical tent anchorage methods. The preparer makes no warranty as to the accuracy of information contained in Manufacturer's installation instructions or IFAI publications. The reviewer is referred to the publications listed to draw their own conclusions as to the validity of the information for their particular tent installation. 1. ASCE 7-10, Minimum Design Loads for Buildings and Other Structures, published by the American Society of Civil Engineers 2. 2013 State of Rhode Island Building Code SBC-1, copyright 2013, International Code Council, Inc. and National Association of State Contractors Licensing Agencies (NASCLA) 3. Aluminum Design Manual, 2010 by the Aluminum Association. 4. Multiframe Advanced structural analysis software, Version 16.01, by Bentley Systems, 2012, release March 14, 2013 5. Install Instructions, Tectrac tm SK, Aztec Tents 6. TRD Tent Rental Division IFAI, The IFAI Procedural Handbook For the Safe Installation & Maintenance of Tentage 7. Pullout Capacity of Tent Stakes, Pocket Guide, by TRD Tent Rental Division IFAI 8. What's Wrong with Water Barrels?, by Maura Paternoster, re-printed from ARA Event Pros, September 2008, page 4. Page 4 of 30
Objective: The following study was performed to develop an understanding of the mechanics of the Giffy Barrel tm tent ballasting system when employed on a typical "framed" tent installation. The study is intended to evaluate the capacity of the system to restrain a framed tent structure from displacement at its base during moderate winds. Effects on the tent frame were reviewed to establish a baseline for limiting stress in the framing system members to within established acceptable levels and to estimate frame deformation levels associated with applied lateral loads. The Giffy Barrel tm ballasting system is unique to other ballasting systems in that it integrates the supporting tent pole with the ballast by direct connection of the barrel to the tent pole at the base and a strap from the bottom of the barrel to the top of the post, forming a triangular support member. This greatly eliminates the ability of the barrel to tip over since the strap is connected below the center of gravity of the barrel and is attached to the tent pole at the top. Additionally, even if the barrel were to ride up on edge all the weight of the ballast is still effective in anchoring the tent since the tent post pole would have to also lift. Definitions: Framed Tent: Pipe Connectors: Ballast: Kiting: Sliding: (The following brief definitions refer to terms used in this study) As taken from reference no. 6: "A shelter composed of a framework of standardized pipe/tubing, joined together with connection fittings, be they steel or aluminum, which becomes the supporting element for fabric roof. The framing supports the roof structure, allowing unrestricted ground space within the structure." Pipe connectors are the pre-fabricated welded tubular joints which form the connections that the pipes, forming the tent structure slip over or into. Pipe connectors are considered in this report to be capable of preventing translation and some degree of rotation of the ends of the main members. Dead weight used to anchor the tent against uplift and lateral movement due to applied forces on the tent structure. Common types of ballast used to anchor a tent are barrels filled with water, sand, or concrete blocks. Is another term for uplift. Uplift is caused by wind flowing over and under the tent roof structure or by internal pressure within the tent. Lateral translation of the tent structure at its base caused by wind forces pushing horizontally on the roof and side walls (if any) of the tent. Sliding occurs when there is an imbalance of applied horizontal forces which exceed the frictional forces at the interface between the dead weight anchoring the tent and the surface the dead weight is placed on. Page 5 of 30
Typical Frame Tent Structure Typical Corner Connections Design Parameters and Assumptions use for Review: Live Load: Dead Load: Snow Load: Assumed live load was negligible (tent is placed on the ground) Tent weight plus fabric Not considered Wind Exposure "C": Open terrain with scattered obstructions having heights generally less than 30 ft. This category includes flat open country and grasslands. Basic Wind Speed: Wind Direction: 3 second gust at 33 feet above ground In the direction of any one of the four sides Tent Parameters: Size: 20 ft. x 20 ft. with posts on 10' centers Roof: Hip Roof with 6" rise over 12" run Members: 2" O.D. x 1/8" wall Aluminum Pipe Connections: Standard commercially available welded tees type Fabric: Assume fabric is strong enough to sustain wind pressure without tearing. Page 6 of 30
Basic Wind Velocity Pressure from Reference No. 1 qh = 0.00256 * Kz * Kzt * Kd * Velocity^2 qh = Velocity Pressure at Height h Kz = Velocity Pressure Exposure, taken as 0.85 for mean roof height of 15 ft. and wind exposure C Kzt = Topographic Factor taken as 1.0 Kd = Wind Directionality Factor taken as 0.85 Basic Wind Speed (miles per hour) Velocity Pressure (qh) lbs / sf 10 0.18 20 0.74 30 1.66 40 2.96 50 4.62 60 6.66 70 9.06 80 11.84 90 14.98 100 18.50 110 22.38 120 26.63 Page 7 of 30
Wind Pressures Evaluated Case 1 Case 2 Applied Wind Cases Evaluated (from Ref. 1) Page 8 of 30
Tent Arrangement Evaluated - No Side Walls Typical Cross Section Computation of Wind Forces on Open Building (no sidewalls) Page 9 of 30
From Reference No.1 Pressure = qh G Cn For roof angle of 26.6 Roof Angle 26.6 Wind Direction 0, 180 Load Case Clear Wind Flow Obstructed Wind Flow Cnw Cnl Cnw Cnl A +1.2 +0.2-0.95-0.95 B -0.1-0.85-0.5-1.4 Load Combinations Evaluated for Wind Speeds of 40 mph to 70 mph: 1. Case A: Pressure on Windward and Leeward Roof 2. Case A: Pressure on Sides of Roof 3. Case B: Pressure on Windward and Leeward Roof 4. Case B: Pressure on Sides of Roof 5. Case A: 0.75 Windward + 0.75 Leeward + 0.75 Sides 6. Case B: 0.75 Windward + 0.75 Leeward + 0.75 Sides Wind Speed (mph) 40 Wind Speed (mph) 50 Wind Speed (mph) 60 Wind Direction 0, 180 Load Case Clear Wind Flow Obstructed Wind Flow P Windward P Leeward P Windward P Leeward A +3.02 +0.50-2.39-2.39 B -0.25-2.14-1.26-3.52 Wind Direction 0, 180 Load Case Clear Wind Flow Obstructed Wind Flow P Windward P Leeward P Windward P Leeward A +4.71 +0.79-3.73-3.73 B -0.39-3.34-1.96-5.50 Wind Direction 0, 180 Load Case Clear Wind Flow Obstructed Wind Flow P Windward P Leeward P Windward P Leeward A +6.79 +1.13-5.38-5.38 B -0.57-4.81-2.83-7.93 Wind Speed (mph) 70 Wind Direction 0, 180 Load Case Clear Wind Flow Obstructed Wind Flow P Windward P Leeward P Windward P Leeward A +9.24 +1.54-7.32-7.32 B -0.77-6.55-3.85-10.78 Page 10 of 30
Wind Forces on Sides of Roof (wind direction parallel to face) Roof Angle 0 < 45 Wind Speed (mph) 40 Wind Speed (mph) 50 Wind Speed (mph) 60 Wind Speed (mph) 70 Wind Direction 0, 180 Load Case Clear Wind Flow Obstructed Wind Flow Cn Cn A -0.8-1.2 B +0.8 +0.5 Wind Direction 0, 180 Load Case Clear Wind Flow Obstructed Wind Flow P P A -2.01-3.02 B +2.01 +1.26 Wind Direction 0, 180 Load Case Clear Wind Flow Obstructed Wind Flow P P A -3.14-4.71 B +3.14 +1.96 Wind Direction 0, 180 Load Case Clear Wind Flow Obstructed Wind Flow P P A -4.53-6.79 B +4.53 ++2.83 Wind Direction 0, 180 Load Case Clear Wind Flow Obstructed Wind Flow P P A -6.16-9.24 B +6.16 +3.85 The following 3 dimensional structural model was assembled to evaluate the tent response under dead and wind loading. The perimeter tube at the eave was modeled with fixed ends since it is connected with a welded tee and guyed to the barrel. The rafters were modeled with the ends pinned to account for any looseness in the fittings. The guy wires were modeled as "tension only" members. The uprights (8 tent posts) where pinned at the bottom to allow rotation in all three axies. The ballast (barrel weight) was placed as a 660 lb. load at the bottom end of the guy wire and kept off the ground 1 inch so as not to treat it as a support. This was considered appropriate since, as modeled, it doesn't come into play with preventing the tent from sliding and has only a downward force component which can be overcome as the wind load increases. Summing the horizontal reactions at the base of the tent posts can be used as a check to see if the barrels slide considering varying coefficients of friction. Checking the vertical reactions at the post is useful in determining if the tent could overturn, which is unlikely due to the weight of the ballast. Page 11 of 30
Graphic of Structural Model Evaluated The following 3 dimensional computer model of the tent illustrates the overall wind forces applied to the structure for each load case: Page 12 of 30
Plus and minus signs signify pressures acting towards and away from the top of roof surfaces, respectively. The horizontal area of each roof section taken at 100 sf. The vertical area of each roof section taken as 50 sf. Note: Horizontal wind forces acting on sides of tent are equal and act in opposite directions so they cancel out with regard to the reactions of the free body diagram. Clear Wind Flow Case Clear Wind Flow - 40 mph Load Vertical Forces (lbs) Horizontal Forces (lbs) Combinations Windward Leeward Sides Total Windward Leeward Total 1 +302 +50-352 151 25 126 2 - - -201 201 3-25 -214-239 13 107 94 4 - - +201 201 5 +227 +38-151 114 114 19 95 6-19 -161 +151 29 10 80 90 Clear Wind Flow - 50 mph Load Vertical Forces (lbs) Horizontal Forces (lbs) Combinations Windward Leeward Sides Total Windward Leeward Total 1 +471 +79-550 236 40 196 2 - - -314 314 3-39 -334-373 20 167 147 4 - - +314 314 5 +353 +59-236 176 177 30 147 6-29 -251 +236 44 15 125 140 Clear Wind Flow - 60 mph Page 13 of 30
Load Vertical Forces (lbs) Horizontal Forces (lbs) Combinations Windward Leeward Sides Total Windward Leeward Total 1 +679 +113-792 340 57 283 2 - - -453 453 3-57 -481-538 29 241 212 4 - - +453 453 5 +509 +85-340 254 255 43 212 6-43 -361 +340 64 22 181 203 Obstructed Wind Flow Case Obstructed Wind Flow - 40 mph Load Vertical Forces (lbs) Horizontal Forces (lbs) Combinations Windward Leeward Sides Total Windward Leeward Total 1-239 -239 478 120 120 0 2 - - -302 302 - - 3-126 -352 478 63 176 113 4 - - +126 126 - - 5-179 -179-227 585 90 90 0 6-95 -264 +95 264 22 132 112 Obstructed Wind Flow - 50 mph Load Vertical Forces (lbs) Horizontal Forces (lbs) Combinations Windward Leeward Sides Total Windward Leeward Total 1-373 -373-746 187 187 0 2 - - -471 471 - - 3-196 -550-746 98 275 177 4 - - +196 196 - - 5-280 -280-353 913 140 140 0 6-147 -413 +147 413 74 206 132 Obstructed Wind Flow - 60 mph Load Vertical Forces (lbs) Horizontal Forces (lbs) Combinations Windward Leeward Sides Total Windward Leeward Total 1-539 -539-1078 269 296 0 2 - - -679 679 - - 3-283 -793-1076 141 397 256 4 - - +283 283 - - 5-404 -404-509 1317 202 222 0 6-212 -595 +212 595 106 298 192 Maximum Uplift force on tent is 1,317 lbs. at 60 mph wind speed based on the model. Maximum Lateral force on tent is 283 lbs. at 60 mph wind speed based on the model. Page 14 of 30
Input and Results for the 50 mph Clear Wind Flow Case: Load Case 1 - CWF Load Case 1 Results - Deflections and Reactions (units: inches, pounds) Page 15 of 30
Load Case 2 - CWF Load Case 2 Results - Deflections and Reactions (units: inches, pounds) Page 16 of 30
Load Case 3 - CWF Load Case 3 Results - Deflections and Reactions (units: inches, pounds) Page 17 of 30
Load Case 4 - CWF Load Case 4 Results - Deflections and Reactions (units: inches, pounds) Page 18 of 30
Load Case 5 - CWF Load Case 5 Results - Deflections and Reactions (units: inches, pounds) Page 19 of 30
Load Case 6 - CWF Load Case 6 Results - Deflections and Reactions (units: inches, pounds) Page 20 of 30
Input and Results for the 50 mph Obstructed Wind Flow Case: Load Case 1 - OWF Load Case 1 Results - Deflections and Reactions (units: inches, pounds) Page 21 of 30
Load Case 2 - OWF Load Case 2 Results - Deflections and Reactions (units: inches, pounds) Page 22 of 30
Load Case 3 - OWF Load Case 3 Results - Deflections and Reactions (units: inches, pounds) Page 23 of 30
Load Case 4 - OWF Load Case 4 Results - Deflections and Reactions (units: inches, pounds) Page 24 of 30
Load Case 5 - OWF Load Case 5 Results - Deflections and Reactions (units: inches, pounds) Page 25 of 30
Load Case 6 - OWF Load Case 6 Results - Deflections and Reactions (units: inches, pounds) Page 26 of 30
As can be seen from the above results the maximum structure deflection and member stresses occur for the Case 1 condition, clear wind flow. Review of the member stresses indicated that two of the interior columns were stressed to 71% of their allowable and two hip framing members were stressed to 75% of their allowable during a 50 mph wind speed. The 60 mph wind speed noted an over stress in the same members of approximately 6%, which is still below their yield point where permanent deformation would occur. The following diagram indicates stress levels in the tent structure during Case1 - Clear Wind Flow: Load Case 1 Stresses - Clear Wind Flow - 50 mph Due to the weight of the ballast (660 lbs) and the method which the Giffy Barrel is attached to the uprights sliding is not a factor for the wind speeds evaluated. This is borne out in the model and the summation of the loads where the maximum horizontal force is approximately 397 lbs (60 mph case) on the leeward side for the open roof condition. Total uplift for this case is 1,076 lbs (60 mph case). All barrels are securely attached to the tent frame by means of a ratchet strap to the top of the frame and a steel plate secured bolted to the post and the bottom of the Giffy Barrel. For the tent to lift off the ground a dead weight force of 5,280 lbs ( 8 barrels @ 660 lbs. per barrel) would have to be overcome. The tent frame is not strong enough and would collapse before completely lifting off the ground. Page 27 of 30
Conservatively checking the potential for sliding of the leeward barrels only (50 mph case): All lateral force (275 lbs for the 50 mph wind case) is taken out in only the 3 leeward barrels, the contribution of the other 5 barrels are neglected The 746 lbs uplift force is taken out by only the 3 barrels Neglecting the weight of the tent The sliding coefficient between the barrel and ground surface taken as 0.4 Yields: Weight per barrel = 660 lbs Minus 746 lbs (uplift from wind) 3 barrels = 249 lbs Net Weight = 411 lbs 3 barrels x 0.4 x 411 lbs = 493 lbs Factor of Safety against sliding 493 lbs / 275 lbs = 1.79 What isn't obvious in the free body diagrams and summation of loads, but can be seen in the computer model, is the way the barrel interacts with the tent posts. As lateral force is applied to the tent structure causing deflection at the top of the tent an upward force is applied through the guy to the bottom of the barrel. This is similar to picking up a package with a hand truck. As you tilt the top of hand truck the weight of the package is transferred to the wheels of the hand truck which act as the fulcrum of the lever. The wheels not only see the full weight of the package but also the weight of the hand truck. The free body diagram below illustrates what is occurring at the tent post as the Giffy Barrel tm is lifted. This is contrary to traditional 55 gallon barrels or concrete block ballasting systems. Both can slide or tilt when tension is applied to the guy. The windward guys become taut and the leeward guys become slack and lose their effectiveness against lateral loads. 55 gallon barrels pose an additional concern in that they may tilt or tip over lose their effectiveness in anchoring the tent. This separates the Giffy Barrel tm from other ballast systems in that once it is attached to the tent Page 28 of 30
post and the guy (ratchet strap) is tensioned it becomes an integral part of the structure. The Giffy Barrel tm provides a constant 660 lbs of downward anchoring force at each post. Summary: Based on the information provided in the study a 20 ft. by 20 ft. open tent (as described in the above study) can be safely anchored in up to 50 mph winds by the Giffy Barrel tm system. An important assumption in the study is that the tent structure, consisting of 2 inch diameter by 1/8" wall pipes members is in good condition and that the rigid welded connectors are properly welded and fit snugly into or around the pipe members. As with all anchoring methods it is the Tent Supplier's and Installer's sole responsibility to thoroughly inspect the tent frame and connections prior to delivery and installation for any defects in workmanship or defects due to wear. Any defective framing members or connections should be replaced prior to installation. Depending on the installation, the Giffy Barrel tm framed tent anchorage system may be capable of anchoring a framed tent structure well beyond the inherent structural capacity of the framing members comprising the framed tent. Based on the preliminary structural analysis performed for this study, and considering the stated assumptions made herein, the tent frame members are the weakest link for the Giffy Barrel tm ballasted frame tent reviewed. This is due to the way the Giffy Barrels are integrated into the tent frame to supply anchorage. Staked tents transfer most of the applied lateral load to the ground surface whereas many ballasting systems rely somewhat on the structural integrity of the tent frame to provide internal lateral resistance. Full scale lateral load field tests were conducted on a 20 ft x 20 ft. Giffy Barrel tm ballasted tent to supplement the above study. Lateral loads in excess of 700 lbs were applied to the Giffy Barrel tm ballasted tent with no lateral movement of any of the 8 Giffy Barrels tm anchoring the tent. The edge of the Giffy Barrels tm on the windward side did lift slightly but came back to rest in its original position once the load was relaxed. A lateral load of this magnitude represents an applied wind load in excess of 70 mph. The testing was stopped at 700 lbs. due to fear of damaging the tent framing members. Page 29 of 30
Supplemental Information: As previously mentioned a tent structure with sides was not considered in this particular study. Reference 1 requires a different method to analyze an enclosed structure. Internal pressures which contribute to uplift are dependent on how "closed" the structure is during the wind event. Buildings are classified by the codes as closed, partially closed and open. If there are significant openings in the walls or roof the building can experience positive or negative pressure, like blowing up or sucking air out of a plastic bag. This causes internal pressures that can exert forces on the roof and walls. Another consideration is the lateral forces on the exterior of the walls, which offer a flat plane for the wind to act on. Structural engineers typically take these loads out through diagonal bracing in the exterior walls or other shear walls. As a curiosity we looked at adding diagonal bracing, in the form of cross cables on diagonal corners, to the open roof tent structure being studied. The following show the results the model with diagonal cross cables: Load Case 1 - CWF 50 mph The analysis illustrates the benefits of cross bracing. The maximum tent deflections were reduced by a factor of 162 (4.38" to 0.027") in the case examined. The hip rafters are still stressed to 75 % of their capacity but the stresses in most of the other members were reduced substantially. Two of the corners were braced and the opposing corners left open for access to the inside of the tent. The reason for including these results in the study is to offer a method for reducing stresses and deflections in the tent structure if side walls are scheduled for the installation. Obviously, additional analysis is required to proof this for all the load cases required for an enclosed building. Page 30 of 30