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业界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,就能轻而易举地制造出这种结构.此外,建筑还必须具备吊车,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14×720,最长近25英尺, 每块总重量达9至10吨 (底板分散安装).20层到30层的建筑物的柱子需要用翼缘板加固，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.这座国际度假酒店,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.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凝土基座上.支撑桁架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山墙上的风矩.为抵抗这股力量拱侧梁可以是由板的连接件来连接的,或者带有焊剪板的翼缘板顶和底连接起来的钢板梁.这种梁的制造不会出现任何问题.但是,由于辗轧延性不同,在使用尾板时,柱上的匹配孔或者是翼缘板必须经过校对,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小时的耐火级别。
因此，再加上建筑设计师的努力，一根混凝土拱侧梁就不仅是提供了一根杠杆来抗拒外力矩，并能省去防火装置，同时还可作为建筑外表面面的一部分。柱上的这种连接距离促使螺栓的使用大大减少。此外，通过使用混凝土，此梁还可以作为外表面的一部分。拱侧梁只是度假酒店的一个构件，它包含了一些填充物以确保玻璃幕墙。但是，它确实消除了防火装置的使用。用混凝土代替钢板并不能消除车间的工作。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模板，并将它运送给预制承包商。承包商会将它安入一个钢模板，此模板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透光率所决定的。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 壶状物上以确保总面积大小。但是，合格的焊接工必须遵循质量保证程序，。
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，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，建设的难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（等同于混凝土楼盖）。因此，建筑物的总体高度不会受到影响。
建筑物的施工
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。它也取决于建筑物的高度，因为高楼起重机的使用会超过拱侧梁。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8.9英尺的建筑物来说（总高度大约为190英尺）高楼起重机、自我支撑物、卡车或锚具拖架（类似于一个maintowoc4100w）可以被使用。20层以上，可使用一个塔式起重机（类似于Link Belt TG1900），它由塔式起重臂支撑，起重半径在75英尺到160英尺间。对于Hillsdale大厦，Hackettstown大厦以及公圆的塔来说，建筑物的顺序是十分重要的。因为先前就提到过结构的稳定性问题。它可以按照下面的程序进行：
1）建柱
2）建立拱侧梁用来沿着强轴系柱状物
3）建立桁架
4）使用高强度螺栓锚固
5）把空心板锚固到桁架上，这将在桁架间提供如同空心板等直接有效的受力系。尽管如此，主要节点的焊接必须在受力过程中分担一部分。
注意：
在静荷载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\16英寸。
建筑成本
正如以前显示的那样，这栋建筑物包括43个的支撑体系，大约1,116,000 平方英尺; 420 英尺高，这项工程的总成本，基于对比价$22，100，000（包括执行合同和销售税），总计$19.80/平方英尺。
注意:
建筑物始于1985,3,18,完工于1986,3,31。这项工程估计历时1年，使用2台起重机，包括节假日、天气和延误花费的时间
结论
公寓和酒店使用这种体系已经得到了展示，并且既经济又有效率。一个结构框架包括外部装修在内，可以在展示的两个月之内完成，并且它的成本等同于一个结构支撑框架所能提供的，比如说平板、混凝土。材料成本基本上与整个国家的相似。
但是，由于工厂折扣和运费等，它们可以不断变化。同时由于其它因素，比方说工会和非工会，边缘利益，保险等等。工厂产量和领域劳动的成本也可以作一定的变化。局部性的研究分析这种体系的影响，在转化为当地工资问题上，需要更多的管理与关注。

文献：《新泽西州大西洋市国际度假酒店》

《工程报》172期/美国钢材建设研究所

Erecting the Staggered-truss System:
A View from the Field

The staggered-steel truss system has been used as the major supporting ele ment 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.

FABRICATION
Fabrication 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—Columns
Column 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 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,shops having the capacity to handle heavy plates and plate fabrication. 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 can be used at the bottom if architectural features are not obstructed.

2—Spandrel Beams
Spandrel beams are designed to resist the wind moment imposed on the end walls in the longitudinal direction . To resist this force, they can be steel beams moment connected 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.
As a result of this, and with the input of the architect on the projects we have completed, a concrete spandrel beam was developed 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—Trusses
The weight of a truss is determined by gravity loads, wind loads, penetrations due to corridor, doors, pipes and/or window openings . 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.

4—Secondary Supporting Structures
Secondary 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.
The Resorts project and the Towers on the Park project require additional support steel due to architectural configurations. The added steel framing can be designed as conventional or to resist applied wind moments requiring partial moment connections. This type of framing is familiar to most fabrication shops.
5—Floor Systems
Floor 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.

FIELD ERECTION
This 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. For erection of Hillsdale Senior Citizens Building, Hackettstown Senior Citizens Building and Towers on the Park respectively. The erection sequence is important because of the previously mentioned stability of the structure. It should proceed as follows:
1. Set columns
2. Set spandrel beams to tie columns along strong axis
3. Set trusses
Note:
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 bolts
5. The 8-in. hollow-core planks are bolted (in lieu of weld plates) to the truss . 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.

BUILDING COSTS
As previously shown, this building contained 43 supported levels approx. 1,116,000 sq ft; 420 ft high.
Total 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:
Field erection commenced on March 18, 1985, with the final lift on March 31, 1986. Project was estimated to take one year using two cranes, and including holidays, weather and down time.

SUMMARY
It 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.
REFERENCE:Resorts International Atlantic City, New Jersey
172 ENGINEERINGJOURNAL / AMERICAN INSTITUTE OF STEEL CONSTRUCTION

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