EXPERIMENTAL SAFETY EVALUATION OF ROOFS
Dr.-Ing. Marc Gutermann, Hochschule Bremen, Institut für Experimentelle Statik,
Neustadtswall 30, D-28199 Bremen, Germany; [email protected]
Prof. Dr.-Ing. Klaus Steffens, Prof. Dr.-Ing. Steffens Ingenieurgesellschaft mbH,
Neustadtswall 30, D-28199 Bremen, Germany; [email protected]
ABSTRACT
In January 2006 the roof of the German ice-skating rink in Bad Reichenhall near Munich collapsed. At this calamity 15 persons
have been killed. Later investigations showed that heavy snow fall as well as massive deficiencies in the construction of the
gluelam-girders had lead to the disaster, which started a national discussion about the structural safety of existing buildings.
This article shall show some examples how calculation and experiment can complement each other in analyzing the load
capacity of roofs, although a computed proof had failed.
Introduction
Structures are planned for a defined use and a limited lifetime. When damages occur, increased deflections are visible or the
service loads shall be increased due to changed customer’s requirements, the serviceability and load carrying safety must be
proofed again. If a calculation is not sufficient an experimental approach can be an economical alternative to a conventional
strengthening or demolition and rebuilding. An experimental safety evaluation yields a better result as computed methods as a
rule since a regular erected structure has load carrying reserves, which can be proofed only by experiment.
In such cases it is sometimes worth carrying out a load test in situ on the existing structure. A prerequisite is that it is carried
out without causing damage which might impair the load-bearing capacity or the durability of the object. The application of a
reliable, economical and rapidly deployable, mobile on-site measurement system is required as well.
Based on the experience of over 300 projects carried out by the PSI in collaboration with the Institute for Experimental Static
(University of Applied Sciences, Bremen), the following qualitative conclusions can be made:
Most structures can be verified by using appropriate conventional computational methods after exact preliminary examinations.
If not, few structures may not be tested experimentally because the costs of an experimental assessment do not form a
favorable ratio to the expected results. In these cases a reconstruction with conventional repair methods or a replacement may
be chosen. But the remaining structures (about 10%) are suitable for experimental safety evaluations which are showing
positive technical as well as economical results as a rule. Only few load tests (< 1%) turn out to be negative due to undetected,
severe building faults. This low level of negative results is based on careful preliminary examination/selection of the projects
and must not be attributed to the systematic superiority of load tests compared to static computation.
Methods
Load tests are as old as civilization and they are following a simple principle: Load is applied on the structure and reactions are
analyzed. But an experimental safety evaluation is more complicated since there are several preconditions which have to be
fulfilled to ensure a non-destructive execution:
•
•
•
•
The structure must be suitable in principle (ductile behavior ; rigid behavior only as exception)
The load device must allow an infinitely variable and controllable loading
The online measurement system must ensure a simultaneous monitoring and analysis of the reactions
Experienced crew
The test-set-up should be planned in preliminary examinations. It should be optimized according to basic questions as
following:
•
•
•
•
•
Why may an experiment deliver better results than a computed analysis?
What’s the magnitude of the maximum load (target load)?
How and where may the load be applied?
What and where has to be measured?
Which are the limit criteria which have to be kept to ensure the integrity of the structure (no damage / no destruction)?
These questions must be answered for each experiment - with different results each time. However, the detailed discussion is
not part of this article. A technical recommendation has been released in September 2000 in Germany (DAfStb [1]) as well as
several solutions and experience gained in practice are published (e.g. Gutermann [2], Steffens [3]).
The method and several solutions to assess experimentally the serviceability and load carrying safety of roofs shall be shown
with some examples which have taken place in Germany last year. The basic questions mentioned above shall be answered
for standard practice in doing so.
Why do experiments deliver better results than computed analysis?
•
•
•
The effective material properties are better than the allowed computed values
The structure has to be simplified for calculation (solvable!) which contains reserves
An experiment can proof the structural safety with reduced partial safety factors since imponderables are reduced
due to the object-related verification
What’s the magnitude of the external applied maximum load (target load)?
During a load test the existing structural component with an (unidentified) effective load-bearing resistance is assessed.
Though the load effects on the structure (e.g. deformations) can be measured, the magnitude of the present dead load as well
as the ultimate strength is unknown.
Hence a prior assessment of the actual condition in combination with calculation must yield to a target load Ftarget as well as a
limit load Flim. The latter is defined as a load level that won’t cause any serious damage that may reduce the remaining time of
use of the structure (recommendations of limit criteria are published in DAfStb [1]).
The target load Ftarget is calculated based on the valid load assumptions multiplied with their appropriate partial safety factors
(DAfStb [1] and equation 1):
ext Ft arg et = ∑ γg, j ⋅ Gk, j + γq,1 ⋅ Qk,1 ≤ ext Flim
(1)
j >1
characteristic value of permanent loads in load tests
characteristic value of (additional) permanent loads after load tests
characteristic value of service load
partial safety factor of permanent loads G
partial safety factor of variable loads Q
with
Gk,1
Gk,j
Qk,1
γg,j
γq,1
and
0.35 Gk,1 ≤ ext Ftarget ≤ ext Flim
The latter term ensures that structures with high permanent loads and low live loads are externally loaded with the design
value of permanent loads including the partial safety factor (γg,1 = 1.35) at least.
If the preliminary calculated target load is reached during a test without violating a limit criteria the load carrying safety is
proofed as a direct result.
The serviceability is proved if the external experimental service loads ext FQ (equation 2) causes reversible reactions of the
structure without exceeding the criteria of serviceability [1]:
ext FQ = ∑ Gk, j + Qk,1
j >1
After the experimental target load has been reached, the serviceability has to be proved again to detect possible structural
changes.
(2)
Where and how may the load be applied?
There are two different possibilities to apply a variable test load in principle: The external load can be produced effectively
through internal force circulation, e.g. using mobile steel equipment (Fig. 1). This solution enables high controllable test loads,
is versatile in use and failure-safe due to use of hydraulic jacks.
An alternative is the load generation using dead weight as a counter force on condition that the masses won’t risk a sudden
collapse. For this reason hydraulic jacks have to be used (Fig. 2).
The loading positions have to be calculated prior to the test. As a result the structure must be stressed similarly (magnitude
and place) as the theoretically approach would have done.
tension rod
load distribution
load cell
hydraulic jack
sensor
load cell
load generation
L/4
L/4
L/4
L/4
Figure 1. Force circulation
Figure 2. Dead weight used as counter force
What and where has to be measured?
Measurement devices which are used normally for loading tests are:
•
•
•
Displacement transducers to measure deflections and deformations, indirect used for strain measurement (∆ℓ/ℓ)
Load cells to measure the test loads
Devices to measure temperature and wind speed. Thus abnormal measurement values can be identified as results of
special ambient conditions (e.g. gust of wind)
In special cases additional equipment can be necessary:
•
•
•
•
Inclination sensors (e.g. to identify boundary conditions or continuous systems)
Acceleration measurement (e.g. detection of eigenfrequency)
Acoustic emission analysis to observe the development of cracks and structural changes (failure prediction)
Laser-Distance-Measurement (e.g. to measure positions of crossing vehicles)
All equipment must work electronically and has to be integrated in the online measurement chain to ensure that the critical
load level is definitely identified. The choice of the applied devices results from a static pre-calculation, which has to identify
the critical structural parts, where the reactions get maximal. Symmetries should be used for control of measurement
redundancy and not to reduce the number of sensors. The repeatable resolution of the sensors must ensure a safe
interpretation (reference value ≤ 5% of the calculated maximum reaction). Normally the reliability is proved by a “measurement
base test”.
Which are the limit criteria that have to be kept to ensure the integrity of the structure (no damage / no destruction)?
Experimental limit load criteria according to DAfStb [1]
•
Strain
•
Residual deformations
•
Crack depth, crack width, changes in crack width
•
Abutment displacements, settlements in exceptional cases
Experimental serviceability load criteria
•
Deformations
•
Crack depth, crack width, changes in crack width
•
Abutment displacements, settlements in exceptional cases
Application
The collapse of the roof of the ice-skating rink in Bad Reichenhall near Munich started a national discussion about the
structural safety of existing buildings in Germany and lead to an extensive inspection of roofs (Fig. 3).
Figure 3. Collapse of the German ice-skating rink in Bad Reichenhall (Photo: ZDF)
Several selected examples shall show how the safety can be proved experimentally even if a computed verification has failed.
•
•
•
•
Precast light-weight concrete planks
Prestressed precast girders made of high-alumina cement
Truss of square-shaped timber
Reinforced concrete roof of a football stadium (T-beam-slab system)
Precast light-weight concrete planks (“Deutschlandhalle”, Berlin)
One of the exhibitions halls of the fair in Berlin is the “Deutschlandhalle”: It has been built in 1935 for the Olympic Games and
was converted to a ice skating rink in 2001. The roof has been erected with precast light-weight concrete planks
(b/l = 0.50/2.00 m). The longitudinal joint has been closed on-site with reinforced concrete. An inspection in the year 2004
showed cracks as well as spelling of several planks.
The planks were classified in four categories to identify a sample of over 4.500 examples. As a result 24 planks of three
different areas have been chosen for an experimental assessment.
The extreme load case was detected as the repair load of a single person (1 kN). The test set-up has been developed to test
two planks each with one installation and to provide the proof of bending moment and shear force at the same time (Fig. 4).
plank
1
12
2
plank
11
5
6
0
plank
load introduction area
71
72
plank
3
9
4
13
7
14
8
Figure 4: Load application and measurement device (deflection ∆, load □, inclination Ο, change of crack width ◊)
Since the height of the hall was more than 30 metres a force circulation has been chosen for load application (Fig. 1). During
the tests none of the measurement values (deflections, inclinations, change of crack width) exceeded a limit criteria. The
serviceability and load carrying safety was proved as a result, which is obvious mainly assessed due to
•
•
Load distribution in lateral direction despite the joint (detected by displacments)
Slight continuous effect in longitudinal direction (detected by inclinations)
Truss of square-shaped timber (Community Centre, Schneverdingen)
The roof structure has been designed by 5 trusses of square-shaped timber with a span of l = 15.0 m. Upper and lower flange
were built with two parts, all other members were one-piece (Fig. 5). The junctions were made by bolt and lay-in connector.
When a deflection of approximately 15 cm was assessed, an investigation identified several causes:
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•
•
•
Water pond due to inefficient dewatering (water depth ≥ 6 cm)
Timber of minor quality (humidity, fiber direction)
Eccentric execution of junctions
Few bolts were under-tightened
All deficiencies in the construction lead to a major deflection, which effected additional load as well as deformation caused by
standing water. As a results serviceability and load carrying safety were no longer existent and an emergency support has
been erected.
Because of the serious deficiencies only an experimental assessment would have been not enough. The deficiencies must be
remedied in parallel as well. The following procedure had been chosen:
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•
•
Exchange of bad bars and bolts
Retightening of all bolts
Experimental assessment of two trusses to identify so far undetected deficiencies
The following test set-up has been chosen for load tests (Fig. 5):
•
•
•
•
Dead weight used as counter-force (concrete blocks)
Load applied in each vertical bar
Scaffold used as collapse protection
Hydraulic load generation, simultaneous measurement of load and deflections as well as change of crack width
upper flange
lower flange
3
4
5
dead weight (counter weight)
Figure 5: Test set-up (deflection ∆, load □)
The load tests showed that all major cracks caused by dehumidifying had to be grouted supplementary using special twocomponent glue. The affected areas were strengthened additionally with bolts and screws before re-testing the structures (Fig.
6). These measures lead to a noticeable increase of stiffness and a successful experimental proof of serviceability and load
carrying safety. The measured deformations were less than one-third of the tolerated values.
Figure 6. Redeveloped truss of square-shaped timber
Prestressed precast girders made of high-alumina cement (factory floor, Worms)
The roof of the factory floor has been erected in 1960 using prefabricated girders, stringers and planks. The prestressed
precast girders and stringers were made of high-alumnia cement, which causes a serious reduction of the compression
strength of the concrete. Preliminary investigations had shown a reduction up to 50% of the original value (core hole and
rebound tester assessment).
A load test of the girders and stringers should prove the actual load carrying safety. Therefore two girders and three stringers
in a storage area had been chosen, which on the one hand have worse condition and on the other hand can be tested without
constricting factory operation.
In the case of the girders test load was applied in the quarter points and has been generated hydraulically using concrete
blocks as counter force and scaffolds as collapse protection (Fig. 7 and 8). The measurement device was complemented with
strain measurement and acoustic emission to identify reliably the moment of decompression as well as critical shear strain
(Fig. 8). In both cases the moment of decompression was reached and clearly identified near the service load level (Fig. 9).
The reactions of the girders reached nearly critical values at the target load level so that the sufficient load carrying safety had
been proved indeed but the remaining time of use should be limited. A correlation between the actual state of concrete
compression strength and the load carrying behavior had been detected as an additional result. An exchange and rebuilding of
the roof in sections is already planned and shall start presumably in 2008.
Figure 7. Prestressed precast girders made of high-alumina cement
5
15
16
7
11
6
12
13
11,50 m
2
1
4
3
Figure 8. Load application and measurement device (deflection ∆, load □, elongation/compression
Σ F [kN]
200
180
elongation
compression
160
140
state of
decompression
120
100
80
60
[05] upper flange
40
[06] lower flange
[07] lower flange
20
0
-500 -400
inital load
-300 -200
-100
residual elongation
0
100
200
300
400
500
Figure 9. Load-Strain-Curve
ε [µm/m]
600
700
800
900
)
Reinforced concrete roof of a football stadium (Koblenz)
When the soccer team TUS Koblenz went up to the second league, the stadium had to fulfill additional requirements. One of
the requirements was that a camera must be placed on the roof of the grandstand. An investigation should clarify if the T-beam
-slab-structure is sustainable even for additional loads. The assessment showed serious deficiencies due to bad waterproofing
(corrosion of reinforcement, concrete spelling; Fig. 10). An analytical approach gave insufficient results.
Figure 10. Roof of football stadium Koblenz with damaged tee-beams
A load test of two adjacent T-beams should evaluate sufficient load carrying safety for dead load, snow as well as additional
loads (camera). The hydraulic load generation was solved by an internal force circulation using the dead weight of the
grandstand as counter weight (Fig. 11). Two steel rods were stuck into the lower girder of the tribune and transferred the test
load trough boreholes above roof where the load was applied into the T-beams by girders. The load position and the
magnitude of the test load were chosen to stress the construction in the same way as the analytical approach would have
done. During the controlled and variable loading, structure behavior was measured (deflection, elongation/compression,
incline; Fig. 11). A preliminary calculation was used to estimate the expected deflection (Table 1). The results have shown a
good correlation between estimated and measured values if three dimensional load transfer is considered. Thus the load test
proved not only sufficient load carrying safety but identified also the existing load transfer behavior. The deficiencies were
remedied and the stadium is in use.
Table 1: Expected and measured deflection of the tee-beam at the end of the cantilever (F = 70 kN)
Experiment
Calculation
Cross section
Rectangle
Rectangle
T-beam
Dimension
Averaged
(B / H = 45 / 75 cm)
Variable
(B / H = 45 / 32-118 cm)
Exact (variable)
including slab
Physical reality
Deflection [mm]
13,5
11,9
5,6
5,0
load introduction
01
71
72
73
02
~ 8,0 m
00
81
hydraulic jack
anchorage (stuck rod)
Figure 11. Cross section and test set-up (deflection ∆, load □, inclination Ο, elongation/compression
)
Conclusions
If computational verification does not give realistic results due to inadequate or missing building documentation, complex loadcarrying behavior or obvious or hidden defects experimental assessments of load-bearing capacity can supply information
about the real structural behavior including all existing conditions. In these cases the experimental investigations usually
produce more favorable results than the static computation, as has clearly been shown using some examples. The method of
in-situ experimental safety evaluation opens up chances of retaining building structures, particularly for buildings subject to
historical preservation.
References
1.
2.
3.
DAfStb (Ed.), Richtlinie für Belastungsversuche an Betonbauwerken, Beuth, Berlin, Germany Sept. 2000
Gutermann, M., An article on experimental assessment of structural safety of solid bridges. In Proceedings of ICEM1212th International Conference on Experimental Mechanics. 29. August – 2. September, 2004 Politecnico di Bari, Italy
Steffens, K., Practical Building Examples; In: Monitoring and Assessment of Structures, Spon Press, Juli 2001, S. 138-165