Balau Compression Strength – Testing Consideration

In this post, I share my personal observation & experience related to specification & testing method of Balau timber. I collect all data from the project which I involved before. Sorry, I can’t tell you more detail but I guarantee that all data are valid. Testing was done by international-reputation-testing-company. My intention doing this is just sharing the knowledge. Otherwise this lesson is just kept in my brain, forgotten and you’ll do the same thing, spend more money which not required.

Statistically, it may not a good example because it taken from one case. Timber used on the project was sourced from Malaysia company (which taken from Indonesian Forest… ). But still I’m thinking it’s a good case and worth to share. It’s a good lesson learn indeed.

Technical Specification

Project specified that timber runner shall be Balau Grade HS or equivalent and shall meet the criteria detailed in Table 14 of BS 5268.

Also, minimum compression perpendicular to the grain shall be 5.3 N/mm² (as Table 14 BS 5268). And, timbers shall be tested to ensure that a crushing stress of 60 N/mm² acting perpendicular to grain of full width timber will produce a maximum indentation of 12mm.

Crush Test Configuration

Other requirements are:

• The grain direction shall run parallel to the longitudinal axis of the timber with a maximum allowable slope of 1:12.
• The timber shall be cut such that the tree heart is well “boxed” in the section.
• The timber shall be free from shakes, splits and all other irregularities.
• The moisture content shall not exceed 24% but shall be equal to, or greater than, 20% at a level of 8mm below the finished surface.

First Test (and Failed!)

In the first opportunity, specimens were tested to satisfy client spec. Specimens were cut from three different timber logs and marked with specific number. Here are the results:

Timber Data	Specimen			Remarks
	1	2	3
Load Strip Area (25 x 300 mm), in mm2	7500	7500	7500	All specimens have 300 mm D x 300 mm W x 150 mm L
Crushing Load, KN	490	425	425
Crushing Stress, MPa	65.3	56.7	56.7
Load @ 12mm indentation	415	385	370
Compressive Stress @ Strip area during 12mm indentation, MPa	55.3	51.3	49.3
Avg. Moisture Content, % (3 sides)	10.7	11.1	9.8
Summary:All samples were not meeting Project Spec. Only 1 speciment (#1) which achieve 65.3 MPa (>60 MPa) for crushing load but not meeting max. deflection.

 
Same data was treated other way by looking compressive strength at base area. Bigger area can produce smaller compressive area and they would be compared to Table 14 of BS 5268.

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Timber Data	Specimen			Remarks
	1	2	3
Timber Base (150 x 300), in mm2	45000	45000	45000
Crushing Load, KN	490	425	425
Crushing Stress, MPa	10.8	9.4	9.4
Load @ 12mm indentation	415	385	370
Compressive Stress @ Strip area during 12mm indentation, MPa	9.2	8.5	8.2
Summary:Regionally available Balau HS timber is meeting BS5268-2 strength requirement, Min compressive strength perpendicular to grain, 5.3 MPa.

 Second and third test – A different approach (Full compression area).

Second test was a trial and using full area compression test. If we’re talking about compression strength (not considering buckling), bigger area can take more load compared to smaller ones by using same material grade. The results proved that bigger area can take more load. All passed the BS 5268 standard.

However the max compression strength is decreasing if using bigger area. It’s unknown whether these samples were taken from same timber log or 3 different logs. So, let’s assume that it’s because heterogeneous materials.

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Timber Data	Specimen
	1	2	3
Timber Size	300Wx300Dx150L	300Wx300Dx300L	300Wx300Dx450L
Max. Load, KN	440	635	920
Max. Compression Stress, MPa	9.78	7.05	6.81[1]
Weight before test, Kg	15.71	32.27	49.18
Weight after test, Kg	15.42	31.84	47.98
Moisture Content before test, % [2]	18.18 | 16.52 | 17.40	14.71 | 21.32 | 18.80	21.28 | 16.99 | 19.04
Moisture Content after Test, %[3]	21.7	23.1	23.2
Note: 1.    Specimen #3 was tested against 600KN compression stress for 1 hour holding time. 2.    Measured at 3 locations. 3.    Measured at center half of cut out block.

Third test was performed with bigger samples to resemble actual condition. They’re 5 specimens block 300x300x300, assembled with stud bolt & bearing plate. Maximum bearing force was found 3350 KN (max. stress = 7.44 MPa > min compression perpendicular to grain of Balau).

By looking all test results, it’s justified that regional timber could not meet the project requirement. Modification is needed in order to finish job on schedule but still meeting the engineering requirement.

Modification

Previously it’s been known from engineering analysis that maximum distributed stress, 5.1 MPa at extreme condition, was not more than minimum compressive strength, 5.3 MPa as specified on BS5268. Also above testing results confirmed that actual compressive stress has more than enough factor of safety.

We proposed to :

1. Select 36 balau timber logs, cut the test sample 300mm long from separate timber and send sample for testing. Ensure the timber is free from crack, shakes and wane.
2. Prepare traceability record of selected timber and mark the direction of testing on the sample and identify the location of these logs on shop drawings with a specific piece mark numbers. (Note: during the testing, some samples were tested with 3 directional holding loads and all met the minimum compressive strength as BS5268 to cover miss orientation during assembly phase).
3. Measure & record moisture content at 3 locations before testing. After testing, split the specimen into half, measure and record at center of the timber.
4. Select only logs which satisfied the BS 5268 requirement. Once they satisfied the requirement, they will be put on highest compressive nodes.
5. Also additional 8 timber logs were taken randomly for the remaining quantity. (We’re dealing with over 200 timber logs).
6. Using kempas to cover the remaining quantity. Kempas timber will be mixed with Balau and put on the area where compressive stress is less. (Vendor hardly fulfilled total quantity of Balau because the timber are rare and considered as protected ones, so quantity is limited and more limited due to project requirement)

Tagged: Consideration, Construction, Engineering, Inspection, Procurement, Quality, Testing, Timber, Timber Runner

Posted in: Materials, Quality