采用力和变形测量技术对宫殿进行维修
The original, historic load transfer for the cupola was no longer effective and instead the load was redistributed to the ceilings and joists of the floors below, resulting in these areas becoming overloaded. The plan that emerged was to restore the historical load transfer of the cupola load. This involved developing a concept that centered on a complex redistribution of the cupola load from the posts to the struts of the truss frame.
1. 介绍和问题
位于Thuringia 最大的宫殿式为了纪念三十周年战争(1618–1648) 建立的，是德国最大的巴洛克宫殿。
安德烈鲁道夫，来自马格德堡要塞建设者，此建筑于1654年完成。
图 1: Schloss Friedenstein 俯瞰图
(照片来自: Ingenieurbüro Hirsch, Erfurt)
宫殿主要用于住所，办公用途。以及巴洛克时期的皇家事务。目前，其是 Gotha 的州立图书馆。宫殿的西南角有一个四十米高的塔楼，长度为26米。
此塔楼建于 1681 年, 被认为装配古代舞台装置的的最古老巴洛克剧院。
塔楼的屋顶是一个八角形的穹顶（一圆顶形建筑帐篷状结构）。原有的建筑结构将穹顶的负载直接作用到砖石塔上，因此非常宽敞并且免于维护。整个结构采用桁架帧结构，支撑穹顶的负载。
图 2: 塔楼
图 3: 塔楼的穹顶
日积月累，整个结构遭到了破坏支撑点已经转移到砖石的外部，导致天花板塌陷。原有的穹窿结构已经失效，导致部分区域负载过重。 图 4 显示出由于支撑点转移导致力留的变化。
Path taken by the force as originally planned
Actual path taken by the force
Fig. 4: Comparison of the paths taken by the force (force flow) in the truss frame area
2. Renovation concept
无论是规划者还是重修人员都面临以下限制：
- 找到一个合适的解决方案
- 避免使用额外的结构来加固
- 重修过程中，建筑依然要使用
- 保留建筑的历史结构
- 任何的加固必须保留在原位
- 节省维修费用
面对以上这些限制，权衡各种方案，最终的计划是恢复原有的穹顶负载结构。这涉及到将穹顶中心定位，并恢复原有桁架结构。
为完成此目的，需要加强木质的桁架结构，并将8角形力测量部件置入到对角支柱中。 To activate the truss frames, the previous force flow had to be redirected. A diagram of the procedure to be adopted is shown in Fig. 5.
穹顶结构的负责北暂时加载到8对液压机上，这样基础和墙体就不会承载穹顶的负载。在每个压力承受的部分都需要进行力/位移测量，以监控负载变形。这样就避免了额外的负载被施加到墙体上，因为其已经严重超载。当液压设备被拆除后，整个穹顶结构的负载将重新转移到外部的砖石结构上。为了做到这一点，每个桁架的负载都需要重新定义。
只有力重新分配完成后，才能进行重修工作。作用在每个桁架上的力和变形都必须在现场进行高精度测量。
Fig. 5: Redirection of the force flow
diagram at the top, with the on-site construction situation below
重修工作完成后，还需要对其进行力和变形的监控工作。
3. 测量技术的应用
在建筑重修过程中，测量技术扮演了重要角色。包括控制和监控力分配的复杂情况。在多个区域都需要应用。
3.1 Measuring the jacking forces on the truss frame posts
在负载重新分配过程中，作用在液压装置上的力必须被测量。采用了来自 HBM C6A 力传感器（量程 200 kN ） ，并连接到 Spider8 放大器，其带有了一个 SR55 扩展模块，将载波频率测量通道的数量增加到8个。 Force was only measured on one press of each of the press pairs as it could be assumed, from the hydraulic coupling of the two cylinders, that the load was evenly distributed. This type of measuring point is shown in Fig. 6.
By deliberately triggering the individual press pairs, it was considered possible to achieve an even distribution of load to the hydraulic cylinders. There were also specific limit loads that were not to be exceeded because this could overload the ceiling construction at certain points, resulting in the renovation work could be causing more damage than it repaired!
The working method developed was also intended to metrologically define the previously unknown original total loading of the cupola and make these forces available for any additional static calculations on the supporting structure.
The measurement data obtained from measuring the hydraulic force was made available in real time to the structural engineer on site.
Fig. 6: Hydraulic press with force transducer and displacement sensor on the base plate of a truss frame post
3.2 Measuring the amount of lift at truss frame posts
In parallel with measuring the hydraulic forces, the elevation of the truss frame posts relative to the underlying ceiling construction of the third floor was metrologically recorded. This was achieved using potentiometric displacement transducers connected to a separate eight-channel Spider8 amplifier. The measuring points were located next to the hydraulic presses as shown in Fig. 6.
The measurement also made it possible to determine how much of the total loading had been taken on by the hydraulic presses.
This was indicated by the truss frame posts becoming free of load from the underlying joists, which was reflected in the force-deformation characteristic of combined force/displacement measurement. The hydraulic presses were also controlled manually, from the evident displacement.
3.3 Determining strut forces by measuring the strain on the truss frame struts
Another important part of the metrological aspect of the renovation work was measuring the compressive forces in the truss frame struts. This was because the planner had called for a specific load distribution to be realized between the individual struts of the supporting structure. This was made possible by selective stressing during fine adjustment. To prevent overloading, it was also essential to adhere to a specified, safe limit load. The load situation in the truss frame was required to be monitored for a longer period after the renovation.
Fig. 7: Diagram of the combined hanging truss/truss frame with tensioning elements in the diagonal struts
In view of these requirements, a combined measuring and tensioning element (MSE) was designed, and installed in the force flow of each truss frame strut. A diagram of this is shown in Fig. 7. The length of the element could be varied by a thread, allowing the effect on the amount of load on the struts to be controlled. This was done manually.
Fig. 8: Tensioning element with a strain gage full bridge test object at the top, installation in the truss frame below
The same element was also used when measuring force, as the strain was measured by means of a foil strain gage and a relevant force calculation was performed using the linear stress-to-strain ratio. The measuring points were two HBM type 6/120 XY 31 T-rosettes sited on opposite sides of the perimeter. The strain gages were electrically connected to the full bridge. The intention here was to compensate for changes in the signal caused by the variable temperature and humidity. In addition to this, the chosen strain gage arrangement and connection also mutually offset the unavoidable amount of bending caused by the considerable dead weight of the strut and its inclination.
The measuring points were first prepared with Z70 adhesive. The multi-layer system recommended by HBM was used as the covering agent, so that the strain gages were first covered with PU120 polyurethane paint and then with SG250 silicone rubber. ABM75 covering tape provided extra covering for the measuring point once curing was complete. This tape comprises 0.05 mm thick aluminum foil with an underlying 3 mm thick layer of kneading compound. It provides a highly effective barrier against moisture penetration.
Fig. 9: Test device for calibrating the tensioning elements
A measuring and tensioning element like this is shown in Figure 8. A model with identical construction to the measuring and tensioning element was calibrated experimentally in the laboratory. The load frame shown in Fig. 9 was made so that part of the tensioning device, with a strain gage measuring point already attached, could be subjected to a real load by a hydraulic system. A force transducer was inserted so that the force could be measured. From the measured values obtained at different load levels, it was possible to establish a sensitivity to determine the strut force from the bridge output voltage. The measured values of the various load levels show that there is a very good approximation to a linear correlation between these quantities across the entire measuring range.
3.4 Accompanying geodetic measurements
During the redirection, the relative measurements of the online measurement system were supplemented by geodetic methods of measurement. Although the accuracy of these measurements must be given a far lower rating, they do have the advantage of an absolute deformation reference to the solid exterior masonry. This allows the electronic measurements to be checked independently. The following quantities were recorded
- The altitude of the truss frame posts in relation to the exterior masonry
- The altitude of the main runner of the third floor ceiling in relation to the exterior masonry
- The deflection curves of the ceilings at selected points on the second and third floors
The fixed installation of selected geodetic measuring points also allowed changes in deformation to be recorded later on.
3.5 Measured value processing and visualization
The need to visualize the measurement signals on site posed a particular problem. Data from 32 electronic measuring points had to be prepared and summarized to provide visual clarity, so that it was possible for the planner to quickly identify and assess the load and deformation states in the supporting structure at any time during force redirection. The planner also needed to immediately detect when any prescribed limit values were exceeded.
Electrical measured values had to be converted into their corresponding physical quantities in real time and different results were to be determined, such as force totals derived from calculations. A relevant visualization concept was identified in advance of measurement, agreed with the competent supporting structure planner and then implemented.
HBM’s multi-channel Spider 8 amplifier and its associated catman® 5 software were the heart of measured value processing and realization. The combination of catman® and the hardware configuration made it possible to achieve measurement acquisition, data storage and real-time realization of results. A data rate of 1 Hz was considered adequate.
Visualization took place on several monitors, and was simultaneously projected onto screens. Fig. 10 shows the layout of a typical screenshot. The measured press forces and the prescribed limit loads are in the center, and the color of the bars changes if a limit value is exceeded. The associated displacements of the truss frame posts can be seen above and below, and their extent, graded in multiple colors, can be seen at a glance. There are additional calculated values on the right-hand edge of the screen.
Fig. 10:On-site realization of measured values in real time; a Catman online document, projected onto a projector screen
4. Continuous monitoring
The load state of the truss frame struts will now being monitored continuously for a period of two years following the successful force redirection. Data is being acquired and stored by electronically, with a measurement cycle of two hours. The measurement data are regularly transferred and evaluated by remote data transmission, so that they are readily available for assessing the status of the supporting structure.
Seasonal climatic influences are a very important factor in the assessment of the measurement data from long-duration measurements, as changes in the measured value caused by temperature in particular can have a considerable effect on the measurement result. To enable influences of this type to be correctly assessed, both temperatures and relative humidity are also recorded in the course of continuous monitoring.
The measurements that are currently running have already shown that it takes a certain amount of time to establish a stable load state. After a year of continuous monitoring, the tensioning elements were adjusted as defined by the project planner to correct the truss frames loads. It was felt that a complete year was needed following the force redirection to determine the existing stable load state.
