关于钢结构交错桁架体系的一个观点.doc

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Erecting the Staggered-truss System:A View from the FieldARTHUR E. HASSLERThe staggered-steel truss system has been used as the major supporting element for many years. Projects using this truss system are economical, easy to fabricate and erect. and as a result beat out all other framing systems. This paper deals with a building recently completed which used the staggered steel truss system. The project is the Resorts International Hotel in Atlantic City, N.J.FABRICATIONFabrication of this type of structure can readily be accomplished by any shop engaged in structural steel fabrication, provided they possess certified welders and are welding-oriented. In addition, the building must contain overhead cranes capable of lifting 10 to 15-ton trusses and columns for projects up to 20 stories.Fabrication involves the following components: (1)Columns, (2) Spandrel Beams, (3) Trusses, (4) Secondary Columns and Beams and (5) Floor System.1—ColumnsColumn fabrication is not complicated for buildings up to 20-stories high. They will be rolled wide-flange sections up to W14 × 720 with the longest being approximately 25 ft in length and with a total weight of 9 to 10 tons each (base plates are shipped loose).Columns contained in buildings from 20 to 30 stories will probably be reinforced with flange plates as in Fig. 1. Above 30 stories will be built-up type columns consisting of three plates of varying thickness. On the Resorts International Hotel project, we had to fabricate (Fig. 2) columns with 8-in. thick flange plates and 6-in. thick web plates. This type of column requires more sophisticated fabrication equipment such as preheat devices, submerged arc welding equipment and heavy lifting capacity of 25 to 40 tons. In other words,Figure 1Figure 2shops having the capacity to handle heavy plates and plate fabrication. As in Fig. 3, these columns can also contain heavy bracing connections in the bottom tier which transfers the wind load to the concrete foundation. As an alternate for transfer of this wind stress, a bracing truss (Fig. 4) can be used at the bottom if architectural features are not obstructed.2—Spandrel BeamsSpandrel beams are designed to resist the wind moment imposed on the end walls in the longitudinal direction (Figs.5, 6). To resist this force, they can be steel beams moment connected (Fig. 7) either by end plate connections or flange plates top and bottom with a web-shear plate. Fabrication of this type of beam presents no problem. However, due to rolling tolerances, the matching holes on the column flange plates or the flange width dimension when using end plates will have to be checked and matched with the beam to insure correct center-to-center of columns. This beam controls the total length of the building. In a building with 9 or 10 bays a deviation of + ¼ in. per bay (within the rolling tolerance) increases the overall length by 2 to 2½ in. The beam will also require a 2-hour fire rating (spray on) in accordance with most building codes.Figure 3Figure 4Figure 5Figure 6Figure 7Figure 8As a result of this, and with the input of the architect on the projects we have completed, a concrete spandrel beam was developed (Fig. 8) to not only provide an increased lever arm to resist the applied moment, but also to eliminate fireproofing and at the same time serve as part of the exterior facade. This distance between connections on the column produced a considerable decrease in the quantity of field bolts. In addition, by using concrete, the beam can also function as part of the exterior facade. For the Resorts Hotel this beam was a structural component only and contained inserts to secure the window wall facade. However, it did eliminate field-applied fireproofing. Shop work was not eliminated by this substitution of concrete for steel. To insure the accurate fit, as previously discussed, it was decided to develop a spandrel rebar cage to be fabricated in the shop and shipped to a precast contractor, who would set them in a steel form containing a shop supplied steel jig designed to maintain tolerances only a steel fabricator could provide.3—TrussesThe weight of a truss is determined by gravity loads, wind loads, penetrations due to corridor, doors, pipes and/or window openings (Fig. 9). The truss weight will vary from 10,000 lbs. in a 4-story building to 36 tons in a 43-story building on the Resort project. Truss fabrication is not difficult. They should be set up on jugs to maintain correct overall dimensions. However, qualified welders are required to follow a quality assurance program (Fig. 10).4—Secondary Supporting StructuresSecondary supporting structures are those necessary to support stair openings, elevator shafts and other framed openings required by the architectural design. This is important to overall costs. If the architect recognizes this type of structural solution will be used, he can design within its constraints and avoid unnecessary additional framing.(Figs. 11 and 12).The Resorts project and the Towers on the Park project (Figs. 13 and 14) require additional support steel due to architectural configurations. The added steel framing can be designed as conventional (i.e., simple shear-connected) or to resist applied wind moments requiring partial moment connections. This type of framing is familiar to most fabrication shops.5—Floor SystemsFloor systems can be the following types, however all must be designed to deliver the wind load as a diaphram (shear load) to the supporting trusses. For the steel fabricator they pose no problems. Architecturally, depth of construction and mechanical requirements can and will dictate the type of system to be used. As an example, the 8-in, hollow core floor deck provides a floor-to-floor height equal to a flat-plate concrete system. Therefore, total height of the building will not be affected.Figure 9Figure 10Figure 11Figure 12Figure 13FIELD ERECTIONThis type of structure has to be erected on a floor-by-floor sequence because of the instability caused by a floor system not in place. Also, depending on the height of the building, a tower crane has to be used to reach over the spandrel beams. Therefore, a crane must have the range (reach) and capacity (picking load) to cover the entire floor area if the building is to remain stable. Using guy cables or temporary bracing interferes with the erection process. Guy derricks (those attached to and climbing up on columns) and internal climbing cranes (supported on the floor structure) cannot be used, unless the design provides support areas—such as an elevator core which can be reinforced to support the additional loads.For buildings up to about 20 stories with a floor height of 8 ft-9 in. (a total height of about 190 ft, a tower crane, selfsupported, truck or cat-mounted (similar to a Manitowoc 4100W), can be used. Above 20 stories, use an external climbing crane (similar to Link Belt TG-1900) supported on steel tower units and connected to the structure at intervals of 75 ft, with the initial tie-in at 160 ft. Figures 15, 16, 17 and 18 show crane location, range etc. for erection of Hillsd1. Set columns2. Set spandrel beams to tie columns along strong axis3. Set trussesFigure 14Note: Connection of the bottom chord should be bolted tight after the dead load is imposed from the floor system. The bottom chord should be shortened approximately 3/16 in. to allow for camber reduction and subsequent chord lengthening)4. Bolt up and torque high-tension bolts5. The 8-in. hollow-core planks are bolted (in lieu of weld plates) to the truss (Fig. 19). This provides an immediate bracing system, working platform etc. between trusses. However, grouting the key joints must proceed to provide the shear distribution previously mentioned.Note: Due to the inherent stiffness of the trusses, concrete spandrel beams and hollow-core floor planks, these structures can safely be erected for about 8–10 floors before grouting has to be in place. During extreme weather conditions—cold, rain etc.—erection can proceed even though concrete cannot be poured.Figure 15Figure 16Figure 17Figure 18Figure 19BUILDING COSTSUsing a building containing 13 supported levels, 9 bays @ 25 ft c. to c. long and 56 ft-1 in. c. to c. main building columns containing about 170,000 sq ft of supported area using 6-in, thick concrete spandrel beams acting also as the exterior facade and 4-in. thick precaBuilding Cost Breakdown1. Structural steel2. Precast concrete elements3. Miscellaneous items4. Erection5. Unit price per sq ftItem 1—Structural steelMaterial costs Weight (lbs.)1. Anchor bolts 5002. Columns & loose base plates 600,0003. Trusses 580,0004. Miscellaneous beamsA. Elevator divider beamsB. End spandrel beamsC. Roof beamsD. Penthouse & elevator mach. room beams 167,5005. Field bolts 12,0006. Paint (if required)7. ShippingTotal cost: 1,360,000@ $.30/lb. average 1,360,000 lbs. (8 lb./sq ft) = $ 408,000.Note: No material discounts have been appliedShop drafting—1,360 hrs. @= 27,2 $20/hr. 00.Shop labor—Shop labor averaged 15 hrs./ton 680 tons × 15 H/T= 10,200 hrs. @ $15/hr. = 153,000.Total item 1—steel furnished & delivered $ 588,200.Item 2Precast concrete components1. Spandrel beams(a) Rebar cages 234 pcs. @ $400 = $ 93,600(b) Casting 22,340 sq ft @ $9.50 = 212,230Total delivered price $305,830 = $ 350,830.2. 4-in. precast end walls, 12,826 sq ft @ $6.50/sq ft = 83,369.3. 8-in. hollow-core plank, 165,000 sq ft @ $3.50/sq ft = 577,500.4. Connection angles, plank & spandrel ties, field bolts = 10,000. $ 976,699.5. Grout precast planks(a) Plank joints(b) Plank ends, 165,000 sq ft @ $.50/sq ft = 82,500.6. CaulkingExterior joints Interior plank joints = 25,000.Total Item 2—precast concrete $1,084,199.Item 3Miscellaneous items(a) Field expenses(b) Shop & field testing = 25.000.Item 4Field labor—Equipment crane Manitowoc 4100W(a) 1. Crane set up & removal2. Shipping to & from site3. 70-ton truck crane & crew to assemble = 40,000.Hoisting(b) For this size building—should complete about 13,000 sq ft/floor in four days, includes a 10% down factor for weather and maintenance.Crane—3 months rentalCrane crew—2 men × 60 days = 120 man-daysHoist " —6 men × 60 days = 360 " "Bolt " —6 men × 60 days = 360 " "Stone " —4 men × 60 days = 240 " "Surveys —1 man × 60 days = 60 " "Super —1 man × 60 days = 60 " "Total 1,200 " " @ $315/day = 378,000.Total Item 4—Field labor 418,000.Total Items 1, 2, 3, 4 Total cost $2,115,399.× O&P 15% 317,310.Total $2,432,709. + Sales tax if requiredItem 5Unit price per square foot(a) Structural frame consisting of:1. Structural steel2. Concrete spandrel beams3. 8-in. hollow-core plank, including grouting(b) Architectural component(c) 4-in. Precast end walls, including caulking1. Total cost of the supported frame = $2,284,343 = $13.44 sq ft2. " " " "exterior facade = 148,266 = .87Total $2,432,609 = $14.31 sq ftErection allowance—approx. $31,760COST FOR FRAMING THE RESORTS INTERNATIONAL HOTELBuilding DescriptionAs previously shown, this building contained 43 supported levels approx. 1,116,000 sq ft; 420 ft high.Center (elev. core) — 35 ft × 114 ft Two adjacent wings — 68 ft × 157 ft-6 inTotal steel weight — 13,700 tons, 24.6 lbs./sq ft 0Total cost of this project, based on a contract price of $22,100,000 (including performance bond and sales tax), amounted to $19.80/sq ft.Notes:1. Field erection commenced on March 18, 1985, with the final lift on March 31, 1986. Project wasestimated to take one year using two cranes, and including holidays, weather and down time.2. The structure is plumb within 5/8 in. from top floor to ground level—quite an achievement (Fig. 20).SUMMARYIt has been shown the use of this system for apartments and hotels is fast and economical. A structural frame, including the exterior facade, can be completed as shown in two months for a cost equal to what would provide only a structural support frame, such as flat-plate concrete. Material costs are basically the same throughout the country. However, they can vary due to mill discounts, freight etc.The cost of shop production and field labor can vary considerably due to factors such as union vs. non-union, fringe benefits, insurance etc. To analyze the impact of this system locally, care should be exercised in converting to local wages.Resorts International Atlantic City, New JerseyFigure 20172 ENGINEERING JOURNAL / AMERICAN INSTITUTE OF STEEL CONSTRUCTION关于钢结构交错桁架体系的一个观点钢结构交错桁架体系成为主要的支撑体系已经很多年了，工程中使用这种桁架体系是经济的，易于制造和安装，因此优于所有其他的支撑体系。这篇论文研究了一栋近期建造的完全使用了交错桁架建筑体系，该工程是新泽西州,大西洋城的一家国际度假酒店。制造任何从事结构钢生产的企业,只要他们拥有合格的焊工及焊接方面的技术,特们就能轻而易举地制造出这种结构.此外,建筑还必须具备架空吊车,能够将10至15吨重的桁架及柱吊至20层高处.该结构的制造包括下列构件(1)柱 (2)拱侧梁 (3)桁架 (4)次柱及次梁(5)楼盖1-柱高达20层的建筑物,其柱子的制造并不复杂.它们被轧制成宽翼缘截面大可达W14×720,最长近25英尺,总重量达9至10吨每块(底版分散安装).30层以上建筑物的柱子是组合式柱,由3块厚度不一的板组成.我们所建的国际度假酒店,其翼缘板厚8英寸,腹板厚6英寸.这种柱需要精密的制造设备,如,预热装置,水下焊接设备以及具备25-40吨的携重能力.换句话说,企业必须具有处理重板及板面制造的能力.如图3,这些柱子底层具有重型支撑连接,此连接可将风荷转移到混泥土基座上.支撑桁架(如图4)作为一种转移风压的方法,如果对建筑特色不造成影响,可以用于底部.2-拱侧梁　 拱侧梁是用于抵抗衡附压在纵向尾墙上的风矩(见图5\6).为抵抗这股力量拱侧梁可以是钢板梁矩,由尾板的连接部分来连接的,或者带有焊剪板的翼缘板顶和底连接起来的.这种梁的制造不会出现任何问题.但是,由于辗轧耐度不同,在使用尾板时,柱上的匹配孔,翼缘板,或者是翼缘宽围必须经过校对,与梁相匹配,以确保柱心与柱心之间无误.这种梁控制着整个建筑物的长度.一栋建筑物有九个或十个壁洞(偏差为±1\4英寸),每个壁洞将使总长度从2英尺增加到2.5英尺.根据大多数建筑物的规则,梁也要求有2小时的耐火定额度。　 因此，再加上建筑设计师的努力，一根混泥土拱侧梁就不仅是提供了一根杠杆来抗拒应用矩，并能根除防火装置的使用，同时还可作为外用面的一部分。柱上的这种连接距离促使域螺栓的使用大大减少。此外，通过使用混泥土，此梁还可以作为外正面的一部分。拱侧梁只是度假酒店的一个构件，它包含了一些插入件以确保窗墙的正面。但是，它确实消除了防火装置的使用。用混泥土代替钢板并不能消除车间工作。为确保能够精确匹配，如前所述，决定在车间内研制一个拱侧箱，并将它运送给预制承包商。承包商会将它安入一个钢板格7，此钢板格具有工厂提供的用来保持韧性的钻模（只有钢板生产商才能提供）。3—桁架　 桁架的重量是由物荷、风荷以及由于走廊、门、管道及窗户的敞开而引起的穿透率所决定的（如图9）。其大小变化可以从4层楼建筑的10，000Ibs变到43层的度假酒店工程的36吨。桁架制造并不困难。桁架应该是竖立在　壶状物上以保持准确的总面积大小。但是，合格的焊接工必须遵循质量保证程序，（如图10）4—次支撑结构　 次支撑结构就是那些在建筑设计上要求用来支撑楼梯接隙、电梯轴、以及其它框架物的附属物。次支撑结构对总成本至关重要。如果设计师意识到这种结构方法会被使用到，他的设计就能保持在限度范围之内，从而避免不必要的附加设计。度假酒店工程及公园工程中的塔（如图13、14），由于建筑设计构造都要求附加支撑钢板。附加支撑钢板结构可以设计成传统式的或者设计成抗风矩的。这种框架与大多数生产厂里的是相似的。5—楼盖　 楼盖可以是如下几种类型，但是设计上都必须能作为一种隔板传递风荷带支撑桁架上。这对钢板制造者并不会产生任何影响，从建筑上说，建设的深度及机械要求能够也会决定所使用的楼盖种类。例如，那块8英寸的空心楼板就能提供一种楼层与楼层间的高度（等同于混泥土楼盖）。因此，建筑物的总体高度不会受到影响。领域建筑物这种类型的结构必须按照一层接一层的顺序建筑，因为不适合的楼盖会造成不稳定所以必须这么做。也取决于建筑物的高度。一因为高楼起重机的使用，所以会超过拱侧梁。因此，如果想保持建筑物的稳定，一台起重机必须要有覆盖整个搂层面积的范围和能力。使用钢索电缆成临时的支撑装置介入建筑物进程。钢索起重机（那些附属的和向上的柱状物）以及内部上升起重机（支撑搂层结构）不能使用，除非设计提供支撑区域---比方说电梯中心，它能支撑额外的负荷。 对于高达20层，每层高度为8ft-9in的建筑物来说（总高度大约为190ft）高楼起重机、自我支撑物、卡车或猫形拖架（类似于一个maintowoc4100w）可以被使用。20层以上，可使用一个外部上升起重机（类似于Link Belt TG1900），它由钢筋塔单元支撑，在间隔为75ft原1）建柱2）建立拱侧梁用来沿着强轴系柱状物3）建立构架注意：在冷负荷从楼层体系中被加附以后，底部弦应该用螺栓紧紧连接。考虑到木材，底部弦应缩短到大约3\16in。建筑成本建筑一个包含13支撑程度，9堤@25ftc to c长和56ft-1 建筑成本的分解1 钢材2 预制混泥土成分3 各种条款4 建筑物5 每个sqft的单价条款1—钢材材料花费重量（1bs）1 猫形螺栓5002 柱及松木板600，0003 构架580，0004各种梁A 电梯隔离梁 B 端面拱侧梁 C屋梁 D 电梯机器房＆电梯机器房梁167，5005 领域螺栓12，0006 油漆（如果有要求）7 运输总成本：1，360，000＠＄.301 lb平均 1,360,000 lbs (8 lb./sq ft) = $ 408,000.注意：没有任何材料折扣可供应用工厂制图----1,360 hrs. @= 27,2 $20